Tag Archives: Chemical Physics

Theory of Gas Phase Scattering and Reactivity for Astrochemistry [IMA]

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http://arxiv.org/abs/1610.00438


Because of the very peculiar conditions of chemistry in many astrophysical gases (low densities, mostly low temperatures, kinetics-dominated chemical evolution), great efforts have been devoted to study molecular signatures and chemical evolution. While experiments are being performed in many laboratories, it appears that the efforts directed towards theoretical works are not as strong.
This report deals with the present status of chemical physics/physical chemistry theory, for the qualitative and quantitative understanding of kinetics of molecular scattering, being it reactive or inelastic. By gathering several types of expertise, from applied mathematics to physical chemistry, dialog is made possible, as a step towards new and more adapted theoretical frameworks, capable of meeting the theoretical, methodological and numerical challenges of kinetics-dominated gas phase chemistry in astrophysical environments.
A state of the art panorama is presented, alongside present-day strengths and shortcomings. However, coverage is not complete, being limited in this report to actual attendance of the workshop. Some paths towards relevant progress are proposed.

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L. Wiesenfeld, W. Thi, P. Caselli, et. al.
Tue, 4 Oct 16
41/81

Comments: 24 pages, 7 figures. White paper reporting a workshop held in Garching, Nov 23-Dec 4, 2015

Accurate Enthalpies of Formation of Astromolecules: Energy, Stability and Abundance [GA]

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http://arxiv.org/abs/1609.09589


Accurate enthalpies of formation are reported for known and potential astromolecules using high level ab initio quantum chemical calculations. A total of 130 molecules comprising of 31 isomeric groups and 24 cyanide/isocyanide pairs with atoms ranging from 3 to 12 have been considered. The results show an interesting, surprisingly not well explored, relationship between energy, stability and abundance (ESA) existing among these molecules. Among the isomeric species, isomers with lower enthalpies of formation are more easily observed in the interstellar medium compared to their counterparts with higher enthalpies of formation. Available data in literature confirm the high abundance of the most stable isomer over other isomers in the different groups considered. Potential for interstellar hydrogen bonding accounts for the few exceptions observed. Thus, in general, it suffices to say that the interstellar abundances of related species are directly proportional to their stabilities. The immediate consequences of this relationship in addressing some of the whys and wherefores among astromolecules and in predicting some possible candidates for future astronomical observations are discussed. Our comprehensive results on 130 molecules indicate that the available experimental enthalpy of formation for some molecules, such as NaCN, may be less reliable and new measurements may be needed.

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E. Etim and E. Arunan
Mon, 3 Oct 16
5/47

Comments: 40 pages, 11 Tables and 12 Figures

High-resolution IR absorption spectroscopy of polycyclic aromatic hydrocarbons in the 3 μm region: Role of periphery [GA]

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http://arxiv.org/abs/1609.09325


In this work we report on high-resolution IR absorption studies that provide a detailed view on how the peripheral structure of irregular polycyclic aromatic hydrocarbons (PAHs) affects the shape and position of their 3 {\mu}m absorption band. To this purpose we present mass-selected, high-resolution absorption spectra of cold and isolated phenanthrene, pyrene, benz[a]antracene, chrysene, triphenylene, and perylene molecules in the 2950-3150 cm-1 range. The experimental spectra are compared with standard harmonic calculations, and anharmonic calculations using a modified version of the SPECTRO program that incorporates a Fermi resonance treatment utilizing intensity redistribution. We show that the 3 {\mu}m region is dominated by the effects of anharmonicity, resulting in many more bands than would have been expected in a purely harmonic approximation. Importantly, we find that anharmonic spectra as calculated by SPECTRO are in good agreement with the experimental spectra. Together with previously reported high-resolution spectra of linear acenes, the present spectra provide us with an extensive dataset of spectra of PAHs with a varying number of aromatic rings, with geometries that range from open to highly-condensed structures, and featuring CH groups in all possible edge configurations. We discuss the astrophysical implications of the comparison of these spectra on the interpretation of the appearance of the aromatic infrared 3 {\mu}m band, and on features such as the two-component emission character of this band and the 3 {\mu}m emission plateau.

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E. Maltseva, A. Petrignani, A. Candian, et. al.
Fri, 30 Sep 16
11/75

Comments: Accepted for publication on The Astrophysical Journal

ExoMol line lists XVIII. The high temperature spectrum of VO [SSA]

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http://arxiv.org/abs/1609.06120


An accurate line list, VOMYT, of spectroscopic transitions is presented for hot VO. The 13 lowest electronic states are considered. Curves and couplings are based on initial {\it ab initio} electronic structure calculations and then tuned using available experimental data. Dipole moment curves, used to obtain transition intensities, are computed using high levels of theory (e.g. MRCI/aug-cc-pVQZ using state-specific or minimal-state CAS for dipole moments). This line list contains over 277 million transitions between almost 640,000 energy levels. It covers the wavelengths longer than 0.29 $\mu$m and includes all transitions from energy levels within the lowest nine electronic states which have energies less than 20,000 \cm{} to upper states within the lowest 13 electronic states which have energies below 50,000 \cm{}. The line lists give significantly increased absorption at infrared wavelengths compared to currently available VO line lists. The full line lists is made available in electronic form via the CDS database and at www.exomol.com.

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L. McKemmish, S. Yurchenko and J. Tennyson
Wed, 21 Sep 16
4/53

Comments: MNRAS in press

Radiative cooling of H3O+ and its deuterated isotopologues [CL]

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http://arxiv.org/abs/1609.05659


In conjunction with ab initio potential energy and dipole moment surfaces for the electronic ground state, we have made a theoretical study of the radiative lifetimes for the hydronium ion H$_3$O$^{+}$ and its deuterated isotopologues. We compute the ro-vibrational energy levels and their associated wavefunctions together with Einstein coefficients for the electric dipole transitions. A detailed analysis of the stability of the ro-vibrational states have been carried out and the longest-living states of the hydronium ions have been identified. We report estimated radiative lifetimes and cooling functions for temperatures $<$ 200 K. A number of long-living meta-stable states are identified, capable of population trapping.

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V. Melnikov, S. Yurchenko, J. Tennyson, et. al.
Tue, 20 Sep 16
13/74

Comments: Phys. Chem. Chem. Phys., 2016

A Comprehensive Study of Hydrogen Adsorbing to Amorphous Water-Ice: Defining Adsorption in Classical Molecular Dynamics [GA]

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http://arxiv.org/abs/1608.05158


Gas-grain and gas-phase reactions dominate the formation of molecules in the interstellar medium (ISM). Gas-grain reactions require a substrate (e.g. a dust or ice grain) on which the reaction is able to occur. The formation of molecular hydrogen (H$_2$) in the ISM is the prototypical example of a gas-grain reaction. In these reactions, an atom of hydrogen will strike a surface, stick to it, and diffuse across it. When it encounters another adsorbed hydrogen atom, the two can react to form molecular hydrogen and then be ejected from the surface by the energy released in the reaction. We perform in-depth classical molecular dynamics (MD) simulations of hydrogen atoms interacting with an amorphous water-ice surface. This study focuses on the first step in the formation process; the sticking of the hydrogen atom to the substrate. We find that careful attention must be paid in dealing with the ambiguities in defining a sticking event. The technical definition of a sticking event will affect the computed sticking probabilities and coefficients. Here, using our new definition of a sticking event, we report sticking probabilities and sticking coefficients for nine different incident kinetic energies of hydrogen atoms [5 K – 400 K] across seven different temperatures of dust grains [10 K – 70 K]. We find that probabilities and coefficients vary both as a function of grain temperature and incident kinetic energy over the range of 0.99 – 0.22.

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J. Dupuy, S. Lewis and P. Stancil
Fri, 19 Aug 16
11/45

Comments: 30 pages, 10 figures, 2 tables

Kinetics and mechanisms of the acid-base reaction between NH$_3$ and HCOOH in interstellar ice analogs [IMA]

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http://arxiv.org/abs/1608.00010


Interstellar complex organic molecules (COMs) are commonly observed during star formation, and are proposed to form through radical chemistry in icy grain mantles. Reactions between ions and neutral molecules in ices may provide an alternative cold channel to complexity, as ion-neutral reactions are thought to have low or even no energy barriers. Here we present a study of a the kinetics and mechanisms of a potential ion-generating acid-base reaction between NH$_{3}$ and HCOOH to form the salt NH$_{4}^{+}$HCOO$^{-}$. We observe salt growth at temperatures as low as 15K, indicating that this reaction is feasible in cold environments. The kinetics of salt growth are best fit by a two-step model involving a slow “pre-reaction” step followed by a fast reaction step. The reaction energy barrier is determined to be 70 $\pm$ 30K with a pre-exponential factor 1.4 $\pm$ 0.4 x 10$^{-3}$ s$^{-1}$. The pre-reaction rate varies under different experimental conditions and likely represents a combination of diffusion and orientation of reactant molecules. For a diffusion-limited case, the pre-reaction barrier is 770 $\pm$ 110K with a pre-exponential factor of $\sim$7.6 x 10$^{-3}$ s$^{-1}$. Acid-base chemistry of common ice constituents is thus a potential cold pathway to generating ions in interstellar ices.

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J. Bergner, K. Oberg, M. Rajappan, et. al.
Tue, 2 Aug 16
51/80

Comments: N/A

Partition functions 1: Improved partition functions and thermodynamic quantities for normal, equilibrium, and ortho and para molecular hydrogen [CL]

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http://arxiv.org/abs/1607.04479


Aims. In this work we rigorously show the shortcomings of various simplifications that are used to calculate the total internal partition function. These shortcomings can lead to errors of up to 40 percent or more in the estimated partition function. These errors carry on to calculations of thermodynamic quantities. Therefore a more complicated approach has to be taken.
Methods. Seven possible simplifications of various complexity are described, together with advantages and disadvantages of direct summation of experimental values. These were compared to what we consider the most accurate and most complete treatment (case 8). Dunham coefficients were determined from experimental and theoretical energy levels of a number of electronically excited states of H$_2$ . Both equilibrium and normal hydrogen was taken into consideration.
Results. Various shortcomings in existing calculations are demonstrated, and the reasons for them are explained. New partition functions for equilibrium, normal, and ortho and para hydrogen are calculated and thermodynamic quantities are reported for the temperature range 1 – 20000 K. Our results are compared to previous estimates in the literature. The calculations are not limited to the ground electronic state, but include all bound and quasi-bound levels of excited electronic states. Dunham coefficients of these states of H$_2$ are also reported.
Conclusions. For most of the relevant astrophysical cases it is strongly advised to avoid using simplifications, such as a harmonic oscillator and rigid rotor or ad hoc summation limits of the eigenstates to estimate accurate partition functions and to be particularly careful when using polynomial fits to the computed values. Reported internal partition functions and thermodynamic quantities in the present work are shown to be more accurate than previously available data.

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A. Popovas and U. Jorgensen
Mon, 18 Jul 16
36/50

Comments: Accepted for publication in Astronomy & Astrophysics, 22 pages, 10 figures, 28 tables

Molecular Polymorphism: Microwave Spectra, Equilibrium Structures, and an Astronomical Investigation of the HNCS Isomeric Family [GA]

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http://arxiv.org/abs/1607.03799


The rotational spectra of thioisocyanic acid (HNCS), and its three energetic isomers (HSCN, HCNS, and HSNC) have been observed at high spectral resolution by a combination of chirped-pulse and Fabry-P\'{e}rot Fourier-transform microwave spectroscopy between 6 and 40~GHz in a pulsed-jet discharge expansion. Two isomers, thiofulminic acid (HCNS) and isothiofulminic acid (HSNC), calculated here to be 35-37~kcal/mol less stable than the ground state isomer HNCS, have been detected for the first time. Precise rotational, centrifugal distortion, and nitrogen hyperfine coupling constants have been determined for the normal and rare isotopic species of both molecules; all are in good agreement with theoretical predictions obtained at the coupled cluster level of theory. On the basis of isotopic spectroscopy, precise molecular structures have been derived for all four isomers by correcting experimental rotational constants for the effects of rotation-vibration calculated theoretically. Formation and isomerization pathways have also been investigated; the high abundance of HSCN relative to ground state HNCS, and the detection of strong lines of SH using CH$_3$CN and H$_2$S, suggest that HSCN is preferentially produced by the radical-radical reaction HS + CN. A radio astronomical search for HSCN and its isomers has been undertaken toward the high-mass star-forming region Sgr B2(N) in the Galactic Center with the 100 m Green Bank Telescope. While we find clear evidence for HSCN, only a tentative detection of HNCS is proposed, and there is no indication of HCNS or HSNC at the same rms noise level. HSCN, and tentatively HNCS, displays clear deviations from a single-excitation temperature model, suggesting weak masing may be occurring in some transitions in this source.

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B. McGuire, M. Martin-Drumel, S. Thorwirth, et. al.
Thu, 14 Jul 16
24/72

Comments: Accepted in Physical Chemistry Chemical Physics

Non-thermal production and escape of OH from the upper atmosphere of Mars [EPA]

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http://arxiv.org/abs/1607.03602


We present a theoretical analysis of formation and kinetics of hot OH molecules in the upper atmosphere of Mars produced in reactions of thermal molecular hydrogen and energetic oxygen atoms. Two major sources of energetic O considered are the photochemical production, via dissociative recombination of O$_{2}^{+}$ ions, and energizing collisions with fast atoms produced by the precipitating Solar Wind (SW) ions, mostly H$^+$ and He$^{2+}$, and energetic neutral atoms (ENAs) originating in the charge-exchange collisions between the SW ions and atmospheric gases. Energizing collisions of O with atmospheric secondary hot atoms, induced by precipitating SW ions and ENAs, are also included in our consideration. The non-thermal reaction O + H$_2(v,j) \rightarrow$ H + OH$(v’,j’)$ is described using recent quantum-mechanical state-to-state cross sections, which allow us to predict non-equilibrium distributions of excited rotational and vibrational states $(v’,j’)$ of OH and expected emission spectra. A fraction of produced translationally hot OH is sufficiently energetic to overcome Mars’ gravitational potential and escape into space, contributing to the hot corona. We estimate the total escape flux from dayside of Mars for low solar activity conditions at about $5\times10^{22}$ s$^{-1}$, or about 0.1\% of the total escape rate of atomic O and H. The described non-thermal OH production mechanism is general and expected to contribute to the evolution of atmospheres of the planets, satellites, and exoplanets with similar atmospheric compositions.

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M. Gacesa, N. Lewkow and V. Kharchenko
Thu, 14 Jul 16
62/72

Comments: 18 pages, 6 figures; submitted to Icarus; comments & suggestions welcome

ExoMol molecular line lists – XVII The rotation-vibration spectrum of hot SO$_3$ [EPA]

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http://arxiv.org/abs/1607.01242


Sulphur trioxide (SO$_3$) is a trace species in the atmospheres of the Earth and Venus, as well as well as being an industrial product and an environmental pollutant. A variational line list for $^{32}$S$^{16}$O$_{3}$, named UYT2, is presented containing 21 billion vibration-rotation transitions. UYT2 can be used to model infrared spectra of SO$_3$ at wavelengths longwards of 2 $\mu$m ($\nu < 5000$ cm$^{-1}$) for temperatures up to 800 K. Infrared absorption cross sections are also recorded at 300 and 500 C are used to validate the UYT2 line list. The intensities in UYT2 are scaled to match the measured cross sections. The line list is made available in electronic form as supplementary data to this article and at \url{www.exomol.com}.

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D. Underwood, S. Yurchenko, J. Tennyson, et. al.
Wed, 6 Jul 16
13/58

Comments: 15 pages, 10 figures, 9 tables MNRAS submitted

Temperature Dependent Product Yields for the Spin Forbidden Singlet Channel of the C(3P) + C2H2 Reaction [GA]

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http://arxiv.org/abs/1607.01175


The atomic hydrogen formation channels of the C + C2H2 reaction have been investigated using a continuous supersonic flow reactor over the 52 K 296 K temperature range. H-atoms were detected directly at 121.567 nm by vacuum ultraviolet laser induced fluorescence. Absolute H-atom yields were determined by comparison with the H-atom signal generated by the C + C2H4 reaction. The product yields agree with earlier crossed beam experiments employing universal detection methods. Incorporating these branching ratios in a gas-grain model of dense interstellar clouds increases the cC3H abundance. This reaction is a minor source of C3 containing molecules in the present simulations.

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K. Hickson, J. Loison and V. Wakelam
Wed, 6 Jul 16
58/58

Comments: 15 pages, 3 figures

ExoMol line lists XV: A new hot line list for hydrogen peroxide [EPA]

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http://arxiv.org/abs/1607.00498


A computed line list for hydrogen peroxide, H$_2{}^{16}$O$_2$, applicable to temperatures up to $T=1250$~K is presented. A semi-empirical high accuracy potential energy surface is constructed and used with an {\it ab initio} dipole moment surface as input TROVE to compute 7.5 million rotational-vibrational states and around 20 billion transitions with associated Einstein-$A$ coefficients for rotational excitations up to $J=85$. The resulting APTY line list is complete for wavenumbers below 6~000 cm$^{-1}$ ($\lambda < 1.67$~$\mu$m) and temperatures up to 1250~K. Room-temperature spectra are compared with laboratory measurements and data currently available in the HITRAN database and literature. Our rms with line positions from the literature is 0.152 \cm\ and our absolute intensities agree better than 10\%. The full line list is available from the CDS databas

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A. Al-Refaie, O. Polyansky, R. Ovsyannikov, et. al.
Tue, 5 Jul 16
73/80

Comments: 14 pages, 8 figures, 7 tables

Predissociation of methyl cyanoformate: The HCN and HNC channels [CL]

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http://arxiv.org/abs/1606.06230


We present a combined experimental and theoretical investigation of the 193 nm photolysis of the cyano-ester, methyl cyanoformate (MCF). Specifically, nanosecond time-resolved infrared emission spectroscopy was used to monitor the ro-vibrationally excited photoproducts generated in the photolysis reaction. The signal-to-noise of all time-resolved spectra were enhanced using the recently developed algorithm, spectral reconstruction analysis, which allowed observation of previously obscured minor resonances, and revealed new dissociation channels producing HCN and HNC. Spectral fit analysis of the nascent HCN and electronically excited CN($A^2\Pi_1$) resonances yield a lower bound estimate for the HCN quantum yield of ca. 0.42$\pm$0.24%. Multi-configuration self-consistent field calculations were used to interrogate the ground and four lowest energy singlet excited states of MCF. At 193 nm, dissociation is predicted to occur predominantly on the repulsive S$_2$ state. Nevertheless, minor pathways leading to the production of highly excited ground state MCF are available via cascading internal conversion from nascent S$_2$ population. An automated transition-state search algorithm was employed to identify the corresponding ground state dissociation channels, and Rice-Ramsperger-Kassel-Marcus and Kinetic Monte Carlo kinetic simulations were used to calculate the associated branching ratios. The proposed mechanism was validated by direct comparison of the experimentally measured and quasi-classical trajectory deduced nascent internal energy distribution of HCN, which were found to be in near perfect agreement. The propensity for cyano containing hydrocarbons to act as photolytic sources for ro-vibrationally excited HCN and HNC, as well as their significance to astrophysical environments, are discussed.

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M. Wilhelm, E. Martinez-Nunez, J. Gonzalez-Vazquez, et. al.
Tue, 21 Jun 16
35/75

Comments: 10 pages, 8 figures

Quadrupole association and dissociation of hydrogen in the early Universe [GA]

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http://arxiv.org/abs/1605.09651


Radiative association and photodissociation rates are calculated for quadrupole transitions of H2. A complete set of bound and unbound states are included in a self-consistent master equation to obtain steady-state concentrations for a dilute system of hydrogen atoms and molecules. Phenomenological rate constants computed from the steady-state concentrations satisfy detailed balance for any combination of matter and radiation temperature. Simple formulas are derived for expressing the steady-state distributions in terms of equilibrium distributions. The rate constant for radiative association is found to be generally small for all temperature combinations. The photodissociation rate constant for quadrupole transitions is found to dominate the rate constants for other H2 photodestruction mechanisms for radiation temperatures less than or equal to 3000 K. Implications for the formation and destruction of H2 in the early universe are discussed.

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R. Forrey
Wed, 1 Jun 16
25/59

Comments: N/A

Reaction Rates and Kinetic Isotope Effects of H$_2$ + OH $\rightarrow$ H$_2$O + H [CL]

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http://arxiv.org/abs/1605.08776


We calculated reaction rate constants including atom tunneling of the reaction of dihydrogen with the hydroxy radical down to a temperature of 50 K. Instanton theory and canonical variational theory with microcanonical optimized multidimensional tunneling (CVT/$\mu$OMT) were applied using a fitted potential energy surface [J. Chem. Phys. 138, 154301 (2013)]. All possible protium/deuterium isotopologues were considered. Atom tunneling increases at about 250 K (200 K for deuterium transfer). Even at 50 K the rate constants of all isotopologues remain in the interval $ 4 \cdot 10^{-20}$ to $4 \cdot 10^{-17}$ cm$^3$ s$^{-1}$ , demonstrating that even deuterated versions of the title reaction are possibly relevant to astrochemical processes in molecular clouds. The transferred hydrogen atom dominates the kinetic isotope effect at all temperatures.

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J. Meisner and J. Kastner
Tue, 31 May 16
69/70

Comments: title page, 16 pages, followed by 6 pages of SI

Collisional excitation of HC3N by para- and ortho-H2 [GA]

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http://arxiv.org/abs/1605.03786


New calculations for rotational excitation of cyanoacetylene by collisions with hydrogen molecules are performed to include the lowest 38 rotational levels of HC3N and kinetic temperatures to 300 K. Calculations are based on the interaction potential of Wernli et al. A&A, 464, 1147 (2007) whose accuracy is checked against spectroscopic measurements of the HC3N-H2 complex. The quantum coupled-channel approach is employed and complemented by quasi-classical trajectory calculations. Rate coefficients for ortho-H2 are provided for the first time. Hyperfine resolved rate coefficients are also deduced. Collisional propensity rules are discussed and comparisons between quantum and classical rate coefficients are presented. This collisional data should prove useful in interpreting HC3N observations in the cold and warm ISM, as well as in protoplanetary disks.

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A. Faure, F. Lique and L. Wiesenfeld
Fri, 13 May 16
2/63

Comments: 8 pages, 2 tables, 4 figures, accepted for publication in MNRAS

Experimental energy levels and partition function of the $^{12}$C$_2$ molecule [GA]

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http://arxiv.org/abs/1605.01504


The carbon dimer, the $^{12}$C$_2$ molecule, is ubiquitous in astronomical environments. Experimental-quality rovibronic energy levels are reported for $^{12}$C$_2$, based on rovibronic transitions measured for and among its singlet, triplet, and quintet electronic states, reported in 42 publications. The determination utilizes the Measured Active Rotational-Vibrational Energy Levels (MARVEL) technique. The 23,343 transitions measured experimentally and validated within this study determine 5,699 rovibronic energy levels, 1,325, 4,309, and 65 levels for the singlet, triplet, and quintet states investigated, respectively. The MARVEL analysis provides rovibronic energies for six singlet, six triplet, and two quintet electronic states. For example, the lowest measurable energy level of the \astate\ state, corresponding to the $J=2$ total angular momentum quantum number and the $F_1$ spin-multiplet component, is 603.817(5) \cm. This well-determined energy difference should facilitate observations of singlet–triplet intercombination lines which are thought to occur in the interstellar medium and comets. The large number of highly accurate and clearly labeled transitions that can be derived by combining MARVEL energy levels with computed temperature-dependent intensities should help a number of astrophysical observations as well as corresponding laboratory measurements. The experimental rovibronic energy levels, augmented, where needed, with {\it ab initio} variational ones based on empirically adjusted and spin-orbit coupled potential energy curves obtained using the \Duo\ code, are used to obtain a highly accurate partition function, and related thermodynamic data, for $^{12}$C$_2$ up to 4,000 K.

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T. Furtenbacher, I. Szabo, A. Csaszar, et. al.
Fri, 6 May 16
13/60

Comments: ApJ Supplements (in press), 48 pages

The C(3P) + NH3 reaction in interstellar chemistry: I. Investigation of the product formation channels [GA]

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http://arxiv.org/abs/1603.08257


The product formation channels of ground state carbon atoms, C(3P), reacting with ammonia, NH3, have been investigated using two complementary experiments and electronic structure calculations. Reaction products are detected in a gas flow tube experiment (330 K, 4 Torr) using tunable VUV photoionization coupled with time of flight mass spectrometry. Temporal profiles of the species formed and photoionization spectra are used to identify primary products of the C + NH3 reaction. In addition, H-atom formation is monitored by VUV laser induced fluorescence from room temperature to 50 K in a supersonic gas flow generated by the Laval nozzle technique. Electronic structure calculations are performed to derive intermediates, transition states and complexes formed along the reaction coordinate. The combination of photoionization and laser induced fluorescence experiments supported by theoretical calculations indicate that in the temperature and pressure range investigated, the H + H2CN production channel represents 100% of the product yield for this reaction. Kinetics measurements of the title reaction down to 50 K and the effect of the new rate constants on interstellar nitrogen hydride abundances using a model of dense interstellar clouds are reported in paper II.

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J. Bourgalais, M. Capron, R. Kailasanathan, et. al.
Tue, 29 Mar 16
47/53

Comments: N/A

The ExoMol database: molecular line lists for exoplanet and other hot atmospheres [GA]

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http://arxiv.org/abs/1603.05890


The ExoMol database (www.exomol.com) provides extensive line lists of molecular transitions which are valid over extended temperatures ranges. The status of the current release of the database is reviewed and a new data structure is specified. This structure augments the provision of energy levels (and hence transition frequencies) and Einstein $A$ coefficients with other key properties, including lifetimes of individual states, temperature-dependent cooling functions, Land\’e $g$-factors, partition functions, cross sections, $k$-coefficients and transition dipoles with phase relations. Particular attention is paid to the treatment of pressure broadening parameters. The new data structure includes a definition file which provides the necessary information for utilities accessing ExoMol through its application programming interface (API). Prospects for the inclusion of new species into the database are discussed.

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J. Tennyson, S. Yurchenko, A. Al-Refaie, et. al.
Mon, 21 Mar 16
23/50

Comments: 68 pages, 24 Tables, 1 figure. Submitted to J. Molec. Spectrosc. special issue on spectroscopic databases

Analytical Models of Exoplanetary Atmospheres. III. Gaseous C-H-O-N Chemistry with 9 Molecules [EPA]

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http://arxiv.org/abs/1603.05418


We present novel, analytical, equilibrium-chemistry formulae for the abundances of molecules in hot exoplanetary atmospheres that include the carbon, oxygen and nitrogen networks. Our hydrogen-dominated solutions involve acetylene (C$_2$H$_2$), ammonia (NH$_3$), carbon dioxide (CO$_2$), carbon monoxide (CO), ethylene (C$_2$H$_4$), hydrogen cyanide (HCN), methane (CH$_4$), molecular nitrogen (N$_2$) and water (H$_2$O). By considering only the gaseous phase, we prove that the mixing ratio of carbon monoxide is governed by a decic equation (polynomial equation of degree 10). We validate our solutions against numerical calculations of equilibrium chemistry that perform Gibbs free energy minimization and demonstrate that they are accurate for temperatures from 500–3000 K. In hydrogen-dominated atmospheres, the ratio of abundances of HCN to CH$_4$ is nearly constant across a wide range of carbon-to-oxygen ratios, which makes it a robust diagnostic of the metallicity in the gas phase. Our validated formulae allow for the convenient benchmarking of chemical kinetics codes and provide an efficient way of enforcing chemical equilibrium in atmospheric retrieval calculations.

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K. Heng and S. Tsai
Fri, 18 Mar 16
45/53

Comments: 6 pages, 4 figures

Nitrogen Substituted Polycyclic Aromatic Hydrocarbon As Capable Interstellar Infrared Spectrum Source Considering Astronomical Chemical Evolution Step To Biological Organic Purine And Adenine [CL]

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http://arxiv.org/abs/1603.03399


In order to find out capable chemical evolution step from astronomically created organic in interstellar space to biological organic on the earth, infrared spectrum of nitrogen substituted carbon pentagon-hexagon coupled polycyclic aromatic hydrocarbon was analyzed by the density functional theory. Ionization was modeled from neutral to tri-cation. Among one nitrogen and two nitrogen substituted NPAH, we could find good examples showing similar IR behavior with astronomically well observed one as like C8H6N1, C7H5N2, and C7H5N2. We can imagine that such ionized NPAH may be created in interstellar space by attacks of high energy nitrogen and photon. Whereas, in case of three and four nitrogen substituted cases as like C6H4N3 and C5H3N4, there were no candidate showing similar behavior with observed one. Also, IR of typical biological organic with four and five nitrogen substituted one as like purine and adenine resulted no good similarity with observed one. By such theoretical comparison, one capable story of chemical evolution of PAH in interstellar space was that one and two nitrogen substituted carbon pentagon-hexagon molecules may have a potential to be created in interstellar space, whereas more than three nitrogen substituted molecules including biological organic purine and adenine may not be synthesized in space, possibly on the earth.

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N. Ota
Fri, 11 Mar 16
12/59

Comments: 17pages, 16Figures, 6 Tables

Methylacetylene (CH3CCH) and propene (C3H6) formation in cold dense clouds: a case of dust grain chemistry [GA]

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http://arxiv.org/abs/1603.02703


We present an extensive review of gas phase reactions producing methylacetylene and propene showing that these relatively abundant unsaturated hydrocarbons cannot be synthesized through gas-phase reactions. We explain the formation of propene and methylacetylene through surface hydrogenation of C3 depleted onto interstellar ices, C3 being a very abundant species in the gas phase.

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K. Hickson, V. Wakelam and J. Loison
Thu, 10 Mar 16
24/49

Comments: N/A

Direct measurement of desorption and diffusion energies of O and N atoms physisorbed on amorphous surfaces [SSA]

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http://arxiv.org/abs/1603.02897


Physisorbed atoms on the surface of interstellar dust grains play a central role in solid state astrochemistry. Their surface reactivity is one source of the observed molecular complexity in space. In experimental astrophysics, the high reactivity of atoms also constitutes an obstacle to measuring two of the fundamental properties in surface physics, namely desorption and diffusion energies, and so far direct measurements are non-existent for O and N atoms. We investigated the diffusion and desorption processes of O and N atoms on cold surfaces in order to give boundary conditions to astrochemical models. Here we propose a new technique for directly measuring the N- and O-atom mass signals. Including the experimental results in a simple model allows us to almost directly derive the desorption and diffusion barriers of N atoms on amorphous solid water ice (ASW) and O atoms on ASW and oxidized graphite. We find a strong constraint on the values of desorption and thermal diffusion energy barriers. The measured barriers for O atoms are consistent with recent independent estimations and prove to be much higher than previously believed (E$_{des}=1410_{-160}^{+290}$; E$_{dif}=990_{-360}^{+530}$ K on ASW). As for oxygen atoms, we propose that the combination E$_{des}$-E$_{dif}$=1320-750 K is a sensible choice among the possible pairs of solutions. Also, we managed to measure the desorption and diffusion energy of N atoms for the first time (E$_{des}=720_{-80}^{+160}$; E$_{dif}=525_{-200}^{+260}$ K on ASW) in the thermal hopping regime and propose that the combination E$_{des}$-E$_{dif}$=720-400 K can be reasonably adopted in models. The value of E$_{dif}$ for N atoms is slightly lower than previously suggested, which implies that the N chemistry on dust grains might be richer.

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M. Minissale, E. Congiu and F. Dulieu
Thu, 10 Mar 16
32/49

Comments: N/A

The sequence to hydrogenate coronene cations: A journey guided by magic numbers [CL]

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http://arxiv.org/abs/1603.02593


The understanding of hydrogen attachment to carbonaceous surfaces is essential to a wide variety of research fields and technologies such as hydrogen storage for transportation, precise localization of hydrogen in electronic devices and the formation of cosmic H2. For coronene cations as prototypical Polycyclic Aromatic Hydrocarbon (PAH) molecules, the existence of magic numbers upon hydrogenation was uncovered experimentally. Quantum chemistry calculations show that hydrogenation follows a site-specific sequence leading to the appearance of cations having 5, 11, or 17 hydrogen atoms attached, exactly the magic numbers found in the experiments. For these closed-shell cations, further hydrogenation requires appreciable structural changes associated with a high transition barrier. Controlling specific hydrogenation pathways would provide the possibility to tune the location of hydrogen attachment and the stability of the system. The sequence to hydrogenate PAHs, leading to PAHs with magic numbers of H atoms attached, provides clues to understand that carbon in space is mostly aromatic and partially aliphatic in PAHs. PAH hydrogenation is fundamental to assess the contribution of PAHs to the formation of cosmic H2.

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S. Cazaux, L. Boschman, N. Rougeau, et. al.
Wed, 9 Mar 16
6/71

Comments: Published in Scientific reports from Nature publishing group

On the formation of molecules and solid-state compounds from the AGB to the PN phases [SSA]

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http://arxiv.org/abs/1603.00723


During the asymptoyic giant branch (AGB) phase, different elements are dredge-up to the stellar surface depending on progenitor mass and metallicity. When the mass loss increases at the end of the AGB, a circumstellar dust shell is formed, where different (C-rich or O-rich) molecules and solid-state compounds are formed. These are further processed in the transition phase between AGB stars and planetary nebulae (PNe) to create more complex organic molecules and inorganic solid-state compounds (e.g., polycyclic aromatic hydrocarbons, fullerenes, and graphene precursors in C-rich environments and oxides and crystalline silicates in O-rich ones). We present an observational review of the different molecules and solid-state materials that are formed from the AGB to the PN phases. We focus on the formation routes of complex fullerene (and fullerene-based) molecules as well as on the level of dust processing depending on metallicity.

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D. Garcia-Hernandez and A. Manchado
Thu, 3 Mar 16
51/75

Comments: 8 pages, 4 figures, Invited Review in “The 11th Pacific Rim Conference on Stellar Astrophysics: Physics and Chemistry of the Late Stages of Stellar Evolution”, IOP Publishing Ltd. (in press)

VUV photo-processing of PAH cations: quantitative study on the ionization versus fragmentation processes [GA]

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http://arxiv.org/abs/1602.05015


Interstellar polycyclic aromatic hydrocarbons (PAHs) are strongly affected by the absorption of vacuum ultraviolet (VUV) photons in the interstellar medium (ISM), yet the branching ratio between ionization and fragmentation is poorly studied. This is crucial for the stability and charge state of PAHs in the ISM in different environments, affecting in turn the chemistry, the energy balance, and the contribution of PAHs to the extinction and emission curves. We studied the interaction of PAH cations with VUV photons in the 7-20 eV range from the synchrotron SOLEIL beamline, DESIRS. We recorded by action spectroscopy the relative intensities of photo-fragmentation and photo-ionization for a set of eight PAH cations ranging in size from 14 to 24 carbon atoms, with different structures. At photon energies below ~13.6 eV fragmentation dominates for the smaller species, while for larger species ionization is immediately competitive after the second ionization potential (IP). At higher photon energies, all species behave similarly, the ionization yield gradually increases, leveling off between 0.8 and 0.9 at ~18 eV. Among isomers, PAH structure appears to mainly affect the fragmentation cross section, but not the ionization cross section. We also measured the second IP for all species and the third IP for two of them, all are in good agreement with theoretical ones confirming that PAH cations can be further ionized in the diffuse ISM. Determining actual PAH dication abundances in the ISM will require detailed modeling. Our measured photo-ionization yields for several PAH cations provide a necessary ingredient for such models.

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J. Zhen, S. Castillo, C. Joblin, et. al.
Wed, 17 Feb 16
50/55

Comments: Accepted for publication in The Astrophysical Journal

Radiative lifetimes and cooling functions for astrophysically important molecules [SSA]

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http://arxiv.org/abs/1601.07997


Extensive line lists generated as part of the ExoMol project are used to compute lifetimes for individual rotational, rovibrational and rovibronic excited states, and temperature-dependent cooling functions by summing over all dipole-allowed transitions for the states concerned. Results are presented for SiO, CaH, AlO, ScH, H$_2$O and methane. The results for CH$_4$ are particularly unusual with 4 excited states with no dipole-allowed decay route and several others where these decays lead to exceptionally long lifetimes. These lifetime data should be useful in models of masers and estimates of critical densities, and can provide a link with laboratory measurements. Cooling functions are important in stellar and planet formation.

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J. Tennyson, K. Hulme, O. Naim, et. al.
Mon, 1 Feb 16
33/42

Comments: 16 pages, 10 figures

Single and Double Photoionization and Photodissociation of Toluene by Soft X-rays in Circumstellar Environment [SSA]

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http://arxiv.org/abs/1601.06856


The formation of polycyclic aromatic hydrocarbons (PAHs) and their methyl derivatives occurs mainly in the dust shells of asymptotic giant branch (AGB) stars. The bands at 3.3 and 3.4 $\mu$m, observed in infrared emission spectra of several objects, are attributed C-H vibrational modes in aromatic and aliphatic structures, respectively. In general, the feature at 3.3 $\mu$m is more intense than the 3.4 $\mu$m. Photoionization and photodissociation processes of toluene, the precursor of methylated PAHs, were studied using synchrotron radiation at soft X-ray energies around the carbon K edge with time-of-flight mass spectrometry. Partial ion yields of a large number of ionic fragments were extracted from single and 2D-spectra, where electron-ion coincidences have revealed the doubly charged parent-molecule and several doubly charged fragments containing seven carbon atoms with considerable abundance. \textit{Ab initio} calculations based on density functional theory were performed to elucidate the chemical structure of these stable dicationic species. The survival of the dications subjected to hard inner shell ionization suggests that they could be observed in the interstellar medium, especially in regions where PAHs are detected. The ionization and destruction of toluene induced by X-rays were examined in the T Dra conditions, a carbon-rich AGB star. In this context, a minimum photodissociation radius and the half-life of toluene subjected to the incidence of the soft X-ray flux emitted from a companion white dwarf star were determined.

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T. Monfredini, F. Fantuzzi, M. Nascimento, et. al.
Wed, 27 Jan 16
36/57

Comments: 11 pages, 4 figures, accept for publication in ApJ

Exploring molecular complexity with ALMA (EMoCA): Alkanethiols and alkanols in Sagittarius B2(N2) [GA]

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http://arxiv.org/abs/1512.05301


Over the past five decades, radio astronomy has shown that molecular complexity is a natural outcome of interstellar chemistry, in particular in star forming regions. However, the pathways that lead to the formation of complex molecules are not completely understood and the depth of chemical complexity has not been entirely revealed. In addition, the sulfur chemistry in the dense interstellar medium is not well understood.
We want to know the relative abundances of alkanethiols and alkanols in the Galactic Center source Sagittarius B2(N2), the northern hot molecular core in Sgr B2(N), whose relatively small line widths are favorable for studying the molecular complexity in space.
We investigated spectroscopic parameter sets that were able to reproduce published laboratory rotational spectra of ethanethiol and studied effects that modify intensities in the predicted rotational spectrum of ethanol. We used the Atacama Large Millimeter Array (ALMA) in its Cycles~0 and 1 for a spectral line survey of Sagittarius B2(N) between 84 and 114.4 GHz. These data were analyzed by assuming local thermodynamic equilibrium (LTE) for each molecule. Our observations are supplemented by astrochemical modeling; a new network is used for the first time that includes reaction pathways for alkanethiols.
The column density ratios involving methanol, ethanol, and methanethiol in Sgr B2(N2) are similar to values reported for Orion KL, but those involving ethanethiol are significantly different and suggest that the detection of ethanethiol reported toward Orion KL is uncertain. Our chemical model presently does not permit the prediction of sufficiently accurate column densities of alkanethiols or their ratios among alkanethiols and alkanols. Therefore, additional observational results are required to establish the level of C2H5SH in the dense and warm interstellar medium with certainty.

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H. Muller, A. Belloche, L. Xu, et. al.
Thu, 17 Dec 15
7/55

Comments: Astron. Astrophys., accepted; 14 pages of manuscript with figure and tables plus 23 pages of additional online figures. Abstract here shortened considerably

Rotational spectra of isotopic species of methyl cyanide, CH3CN, in their v8 = 1 excited vibrational states [GA]

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http://arxiv.org/abs/1512.05271


Methyl cyanide is an important trace molecule in space, especially in star-forming regions where it is one of the more common molecules used to derive kinetic temperatures.
We want to obtain accurate spectroscopic parameters of minor isotopologs of methyl cyanide in their lowest excited v8 = 1 vibrational states to support astronomical observations, in particular, with interferometers such as ALMA.
The laboratory rotational spectrum of methyl cyanide in natural isotopic composition has been recorded from the millimeter to the terahertz regions.
Transitions with good signal-to-noise ratios could be identified for the three isotopic species CH3(13)CN, (13)CH3CN, and CH3C(15)N up to about 1.2 THz (J” <= 66). Accurate spectroscopic parameters were obtained for all three species.
The present data were already instrumental in identifying v8 = 1 lines of methyl cyanide with one (13)C in IRAM 30 m and ALMA data toward Sagittarius B2(N).

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H. Muller, B. Drouin, J. Pearson, et. al.
Thu, 17 Dec 15
45/55

Comments: 6 pages, 1 figure, accepted at A&A

Threshold Energies for Single Carbon Knockout from Polycyclic Aromatic Hydrocarbons [CL]

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http://arxiv.org/abs/1511.05742


We have measured absolute cross sections for ultrafast (fs) single-carbon knockout from Polycyclic Aromatic Hydrocarbon (PAH) cations as functions of He-PAH center-of-mass collision energy in the range 10-200 eV. Classical Molecular Dynamics (MD) simulations cover this range and extend up to 10$^5$ eV. The shapes of the knockout cross sections are well described by a simple analytical expression yielding experimental and MD threshold energies of $E_{th}^{Exp}=32.5\pm 0.4$ eV and $E_{th}^{MD}=41.0\pm 0.3$ eV, respectively. These are the first measurements of knockout threshold energies for molecules isolated \emph{in vacuo}. We further deduce semi-empirical (SE) and MD displacement energies — \emph{i.e.} the energy transfers to the PAH molecules at the threshold energies for knockout — of $T_{disp}^{SE}=23.3\pm 0.3$ eV and $T_{disp}^{MD}=27.0\pm 0.3$ eV. The semi-empirical results compare favorably with measured displacement energies for graphene $T_{disp}=23.6$ eV [Meyer \emph{et al.} Phys. Rev Lett. \textbf{108} 196102 (2012) and \textbf{110} 239902 (2013)].

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M. Stockett, M. Gatchell, T. Chen, et. al.
Thu, 19 Nov 15
67/73

Comments: N/A

Quantum Calculation of Inelastic CO Collisions with H. II. Pure Rotational Quenching of High Rotational Levels [GA]

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http://arxiv.org/abs/1511.04981


Carbon monoxide is a simple molecule present in many astrophysical environments, and collisional excitation rate coefficients due to the dominant collision partners are necessary to accurately predict spectral line intensities and extract astrophysical parameters. We report new quantum scattering calculations for rotational deexcitation transitions of CO induced by H using the three-dimensional potential energy surface~(PES) of Song et al. (2015). State-to-state cross sections for collision energies from 10$^{-5}$ to 15,000~cm$^{-1}$ and rate coefficients for temperatures ranging from 1 to 3000~K are obtained for CO($v=0$, $j$) deexcitation from $j=1-45$ to all lower $j’$ levels, where $j$ is the rotational quantum number. Close-coupling and coupled-states calculations were performed in full-dimension for $j$=1-5, 10, 15, 20, 25, 30, 35, 40, and 45 while scaling approaches were used to estimate rate coefficients for all other intermediate rotational states. The current rate coefficients are compared with previous scattering results using earlier PESs. Astrophysical applications of the current results are briefly discussed.

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K. Walker, L. Song, B. Yang, et. al.
Tue, 17 Nov 15
71/87

Comments: 8 figures, 1 table

Potential formation of three pyrimidine bases in interstellar regions [CL]

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http://arxiv.org/abs/1511.04343


Work on the chemical evolution of pre-biotic molecules remains incomplete since the major obstacle is the lack of adequate knowledge of rate coefficients of various reactions which take place in interstellar conditions. In this work, we study the possibility of forming three pyrimidine bases, namely, cytosine, uracil and thymine in interstellar regions. Our study reveals that the synthesis of uracil from cytosine and water is quite impossible under interstellar circumstances. For the synthesis of thymine, reaction between uracil and :CH2 is investigated. Since no other relevant pathways for the formation of uracil and thymine were available in the literature, we consider a large gas-grain chemical network to study the chemical evolution of cytosine in gas and ice phases. Our modeling result shows that cytosine would be produced in cold, dense interstellar conditions. However, presence of cytosine is yet to be established. We propose that a new molecule, namely, C4N3OH5 could be observable in the interstellar region. C4N3OH5 is a precursor (Z isomer of cytosine) of cytosine and far more abundant than cytosine. We hope that observation of this precursor molecule would enable us to estimate the abundance of cytosine in interstellar regions. We also carry out quantum chemical calculations to find out the vibrational as well as rotational transitions of this precursor molecule along with three pyrimidine bases.

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L. Majumdar, P. Gorai, A. Das, et. al.
Mon, 16 Nov 15
5/57

Comments: 38 pages, 7 figures, Accepted in Astrophysics and Space Science

Microhydration effect on structural, energetic and light scattering properties of first branched interstellar molecule ( i-PrCN) [CL]

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http://arxiv.org/abs/1511.04272


In this work, we have focused on microsolvation of isopropyl cyanide (i-PrCN) as isopropyl cyanide has been recently detected in interstellar space and is of great importance from the astrochemical and bio-chemical point of view for its branching carbon chains. Such branches are needed for many molecules crucial to life, such as the amino acids that build proteins. The phenomenon of the formation of hydrogen bond affects structure, energetic and electric properties of microhydrated isopropyl cyanide and this has been explored by using three different quantum chemical models. It is observed that the structural parameters calculated by the three models display similarities, however model dependence is evident from equilibrium electronic energies of the clusters. Presence of water molecule has a significant effect on the values of dipole moments and polarizabilities. Rayleigh intensities which are calculated using mean polarizibility and polarizibility anisotropy are increased much due to the formation of hydrogen bonding. For CN stretching vibration of isopropyl cyanide, intensification of Raman scattering activities are observed upon complexation.

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S. Chakraborty, S. Routh and M. Krishnappa
Mon, 16 Nov 15
8/57

Comments: N/A

Low-temperature chemistry between water and hydroxyl radicals: H/D isotopic effects [SSA]

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http://arxiv.org/abs/1511.02010


Sets of systematic laboratory experiments are presented — combining Ultra High Vacuum cryogenic and plasma-line deposition techniques — that allow us to compare H/D isotopic effects in the reaction of H2O (D2O) ice with the hydroxyl radical OD (OH). The latter is known to play a key role as intermediate species in the solid-state formation of water on icy grains in space. The main finding of our work is that the reaction H2O + OD –> OH + HDO occurs and that this may affect the HDO/H2O abundances in space. The opposite reaction D2O + OH –> OD + HDO is much less effective, and also given the lower D2O abundances in space not expected to be of astronomical relevance. The experimental results are extended to the other four possible reactions between hydroxyl and water isotopes and are subsequently used as input for Kinetic Monte Carlo simulations. This way we interpret our findings in an astronomical context, qualitatively testing the influence of the reaction rates.

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T. Lamberts, G. Fedoseev, F. Puletti, et. al.
Mon, 9 Nov 15
5/55

Comments: Accepted for publication in MNRAS

Failure of hydrogenation in protecting polycyclic aromatic hydrocarbons from fragmentation [GA]

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http://arxiv.org/abs/1510.07820


A recent study of soft X-ray absorption in native and hydrogenated coronene cations, C$_{24}$H$_{12+m}^+$ $m=0-7$, led to the conclusion that additional hydrogen atoms protect (interstellar) Polycyclic Aromatic Hydrocarbon (PAH) molecules from fragmentation [Reitsma et al., Phys. Rev. Lett. 113, 053002 (2014)]. The present experiment with collisions between fast (30-200 eV) He atoms and pyrene (C$_{16}$H$_{10+m}^+$, $m=0$, 6, and 16) and simulations without reference to the excitation method suggests the opposite. We find that the absolute carbon-backbone fragmentation cross section does not decrease but increases with the degree of hydrogenation for pyrene molecules.

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M. Gatchell, M. Stockett, N. Ruette, et. al.
Wed, 28 Oct 15
47/79

Comments: 10 pages, 5 figures

Asymmetric polycyclic aromatic hydrocarbon as a capable source of astronomically observed interstellar infrared spectrum [GA]

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http://arxiv.org/abs/1510.07403


In order to find out capable molecular source of astronomically well observed infrared (IR) spectrum, asymmetric molecular configuration polycyclic aromatic hydrocarbon (PAH) was analyzed by the density functional theory (DFT) analysis. Starting molecules were benzene C6H6, naphthalene C10H8 and 1H-phenalene C13H9. In interstellar space, those molecules will be attacked by high energy photon and proton, which may bring cationic molecules as like C6H6n+ (n=0~3 in calculation), C10H8n+, and C13H9n+, also CH lacked molecules C5H5n+, C9H7n+, and C12H8n+. IR spectra of those molecules were analyzed based on DFT based Gaussian program. Results suggested that symmetrical configuration molecules as like benzene, naphthalene , 1H-phenalene and those cation ( +, 2+, and 3+) show little resemblance with observed IR. Contrast to such symmetrical molecules, several cases among cationic and asymmetric configuration molecules show fairly good IR tendency. One typical example was C12H83+, of which calculated harmonic IR wavelength were 3.2, 6.3, 7.5, 7.8, 8.7, 11.3, and 12.8 micro meter, which correspond well to astronomically observed wavelength of 3.3, 6.2, 7.6, 7.8, 8.6, 11.2, and 12.7 micro meter. It was amazing agreement. Also, some cases like C5H5+, C9H7+, C9H72+, C9H73+ and C12H82+ show fairly good coincidence. Such results suggest that asymmetric and cationic PAH may be capable source of interstellar dust.

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N. Ota
Tue, 27 Oct 15
70/76

Comments: 12 pages, 10 figures, 5 tables

High-resolution IR absorption spectroscopy of polycyclic aromatic hydrocarbons: the realm of anharmonicity [GA]

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http://arxiv.org/abs/1510.04948


We report on an experimental and theoretical investigation of the importance of anharmonicity in the 3 micron CH stretching region of Polycyclic Aromatic Hydrocarbon (PAH) molecules. We present mass-resolved, high-resolution spectra of the gas-phase cold (~4K) linear PAH molecules naphthalene, anthracene, and tetracene. The measured IR spectra show a surprisingly high number of strong vibrational bands. For naphthalene, the observed bands are well separated and limited by the rotational contour, revealing the band symmetries. Comparisons are made to the harmonic and anharmonic approaches of the widely used Gaussian software. We also present calculated spectra of these acenes using the computational program SPECTRO, providing anharmonic predictions enhanced with a Fermi-resonance treatment that utilises intensity redistribution. We demonstrate that the anharmonicity of the investigated acenes is strong, dominated by Fermi resonances between the fundamental and double combination modes, with triple combination bands as possible candidates to resolve remaining discrepancies. The anharmonic spectra as calculated with SPECTRO lead to predictions of the main modes that fall within 0.5% of the experimental frequencies. The implications for the Aromatic Infrared Bands, specifically the 3 micron band are discussed.

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E. Maltseva, A. Petrignani, A. Candian, et. al.
Mon, 19 Oct 15
11/44

Comments: 13 pages, 4 figures. Accepted for publication in The Astrophysical Journal

Rare isotopic species of sulphur monoxide: the rotational spectrum in the THz region [EPA]

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http://arxiv.org/abs/1509.06566


Many sulphur-bearing species have been detected in different astronomical environments and have allowed to derive important information about the chemical and physical composition of interstellar regions. In particular, these species have also been showed to trace and probe hot-core environment time evolution. Among the most prominent sulphur-bearing molecules, SO, sulphur monoxide radical, is one of the more ubiquitous and abundant, observed also in its isotopic substituted species such as $^{34}$SO and S$^{18}$O. Due to the importance of this simple diatomic system and to face the challenge of modern radioastronomical facilities, an extension to THz range of the rare isotopologues of sulphur monoxide has been performed. High-resolution rotational molecular spectroscopy has been employed to extend the available dataset of four isotopic species, SO, $^{34}$SO, S$^{17}$O, and S$^{18}$O up to the 1.5 THz region. The frequency coverage and the spectral resolution of our measurements allowed a better constraint of the molecular constants of the four species considered, focusing especially for the two oxygen substituted isotopologues. Our measurements were also employed in an isotopically invariant fit including all available pure rotational and ro-vibrational transitions for all SO isotopologues, thus enabling accurate predictions for rotational transitions at higher frequencies. Comparison with recent works performed on the same system are also provided, showing the quality of our experiment and the improvement of the datasets for all the species here considered. Transition frequencies for this system can now be used with confidence by the astronomical community well into the THz spectral region.

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V. Lattanzi, G. Cazzoli and C. Puzzarini
Wed, 23 Sep 15
59/63

Comments: 18 pages, 3 figures, to be published in ApJ

State of the art for ab initio vs empirical potentials for predicting 6e$^{-}$ excited state molecular energies: Application to Li$_{2}\left(b,1^{3}Π_{u}\right)$ [CL]

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http://arxiv.org/abs/1508.07184


We build the first analytic empirical potential for the most deeply bound $\mbox{Li}_{2}$ state: $b\left(1^{3}\Pi_{u}\right)$. Our potential is based on experimental energy transitions covering $v=0-34$, and very high precision theoretical long-range constants. It provides high accuracy predictions up to $v=100$ which pave the way for high-precision long-range measurements, and hopefully an eventual resolution of the age old discrepancy between experiment and theory for the $\mbox{Li}\left(2^{2}S\right)+\mbox{Li}\left(2^{2}P\right)$ $C_{3}$ value. State of the art ab initio calculations predict vibrational energy spacings that are all in at most 0.8 cm$^{-1}$ disagreement with the empirical potential.

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N. Dattani and R. Roy
Mon, 31 Aug 15
28/63

Comments: Feedback encouraged. 16 pages, 3 figures

Electronic and Rovibrational Quantum Chemical Analysis of C$_3$P$^-$: The Next Interstellar Anion? [CL]

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http://arxiv.org/abs/1508.02386


C$_3$P$^-$ is analogous to the known interstellar anion C$_3$N$^-$ with phosphorus replacing the nitrogen in a simple step down the periodic table. In this work, it is shown that C$_3$P$^-$ is likely to possess a dipole-bound excited state. It has been hypothesized and observationally supported that dipole-bound excited states are an avenue through which anions could be formed in the interstellar medium. Additionally, C$_3$P$^-$ has a valence excited state that may lead to further stabilization of this molecule, and C$_3$P$^-$ has a larger dipole moment than neutral C$_3$P ($\sim 6$ D vs. $\sim 4$ D). As such, C$_3$P$^-$ is probably a more detectable astromolecule than even its corresponding neutral radical. Highly-accurate quantum chemical quartic force fields are also applied to C$_3$P$^-$ and its singly $^{13}$C substituted isotopologues in order to provide structures, vibrational frequencies, and spectroscopic constants that may aid in its detection.

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R. Fortenberry and J. Lukemire
Wed, 12 Aug 15
25/50

Comments: 7 pages, 4 tables, Accepted for publication in MNRAS

Dissociative photoionization of polycyclic aromatic hydrocarbon molecules carrying an ethynyl group [GA]

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http://arxiv.org/abs/1508.00259


The life cycle of the population of interstellar polycyclic aromatic hydrocarbon (PAH) molecules depends partly on the photostability of the individual species. We have studied the dissociative photoionization of two ethynyl-substituted PAH species, namely, 9-ethynylphenanthrene and 1-ethynylpyrene. Their adiabatic ionization energy and the appearance energy of fragment ions have been measured with the photoelectron photoion coincidence (PEPICO) spectroscopy technique. The adiabatic ionization energy has been found at 7.84 +/- 0.02 eV for 9-ethynylphenanthrene and at 7.41 +/- 0.02 eV for 1-ethynylpyrene. These values are similar to those determined for the corresponding non-substituted PAH molecules phenanthrene and pyrene. The appearance energy of the fragment ion indicative of the loss of a H atom following photoionization is also similar for either ethynyl-substituted PAH molecule and its non-substituted counterpart. The measurements are used to estimate the critical energy for the loss of a H atom by the PAH cations and the stability of ethynyl-substituted PAH molecules upon photoionization. We conclude that these PAH derivatives are as photostable as the non-substituted species in HI regions. If present in the interstellar medium, they may play an important role in the growth of interstellar PAH molecules.

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G. Rouille, S. Krasnokutski, D. Fulvio, et. al.
Tue, 4 Aug 15
28/54

Comments: Accepted for publication in Astrophys. J. on 1 August 2015, 19 pages, 6 figures, 1 table

On the basis-set extrapolation [CL]

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http://arxiv.org/abs/1507.08684


A possible solution for the problem of memory-size and computer-time, is the extrapolation of basis-set$^1$. This extrapolation has two exponents $\alpha$ and $\beta$, corresponding to the HF (reference energy) and the energy of correlations (EC), respectively. For a given system, the exponents are taken as constant$^2$, and potential energy surfaces (PES) are generated. We have found that the values of $\alpha$ and $\beta$ are not constant, but vary from position to position in the system. How to deal with such situation and get very accurate PES, is discussed.

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S. Chandra and M. Sharma
Mon, 3 Aug 15
33/43

Comments: 4 pages, 4 figures

On partition function in Astronomy \& Astrophysics [GA]

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http://arxiv.org/abs/1507.04890


In order to analyze spectrum from the interstellar medium (ISM), spectrum of the molecule of interest is recorded in a laboratory, and accurate rotational and centrifugal distortion constants are derived. By using these constants, one can calculate accurate partition function. However, in the same paper, where these constants are derived, the partition function is calculated by using a semi-empirical expression.
We have looked into the details of this semi-empirical expression and compared the values, obtained from it, with the accurate ones. As an example, we have considered the case of Methanimine (CH$_2$NH) which is detected in a number of cosmic objects. It is found that for the kinetic temperature $T > 120$ K, the semi-empirical expression gives large value as compared to the accurate one. The deviation becomes about 25\% larger than the accurate one at the kinetic temperature of 400 K.

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M. Sharma, M. Sharma and S. Chandra
Mon, 20 Jul 15
17/52

Comments: 7 pages, 1 figure, 1 table

Some observations about the MOLSCAT [IMA]

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http://arxiv.org/abs/1507.04882


For calculation of cross sections for collisional transitions between rotational levels in a molecule, a computer code, MOLSCAT has been developed by Hutson \& Green (1994). For the transitions between rotational levels in H$_2$CS due to collisions with He atom, we have calculated cross sections under the CS approximation. In the MOLSCAT, there is provision to input more than one values of total energies. Here, for example, we are interested in the cross sections for total energy 11 cm$^{-1}$. The calculations have been done for the single energy 11 cm$^{-1}$ and for eight combinations, having energies (11, 12), (12, 11), (10, 11), (11, 10), (11, 12, 13), (9, 10, 11), (10, 11, 12), (9, 10, 11, 12, 13) cm$^{-1}$. We have found that the cross sections for 11 cm$^{-1}$, in general, differ from one another in all the 9 calculations. The reason for the difference in the results appears that the MOLSCAT uses the intermediate data of calculations for one energy, in the calculations for other energies. Under such circumstances, the possible suggestion can be to run the MOLSCAT for a single energy at a time.

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M. Sharma, M. Sharma and S. Chandra
Mon, 20 Jul 15
25/52

Comments: 9 pages

Gas phase formation of the prebiotic molecule formamide: insights from new quantum computations [GA]

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http://arxiv.org/abs/1507.03741


New insights into the formation of interstellar formamide, a species of great relevance in prebiotic chemistry, are provided by electronic structure and kinetic calculations for the reaction NH2 + H2CO -> NH2CHO + H. Contrarily to what previously suggested, this reaction is essentially barrierless and can, therefore, occur under the low temperature conditions of interstellar objects thus providing a facile formation route of formamide. The rate coefficient parameters for the reaction channel leading to NH2CHO + H have been calculated to be A = 2.6×10^{-12} cm^3 s^{-1}, beta = -2.1 and gamma = 26.9 K in the range of temperatures 10-300 K. Including these new kinetic data in a refined astrochemical model, we show that the proposed mechanism can well reproduce the abundances of formamide observed in two very different interstellar objects: the cold envelope of the Sun-like protostar IRAS16293-2422 and the molecular shock L1551-B2. Therefore, the major conclusion of this Letter is that there is no need to invoke grain-surface chemistry to explain the presence of formamide provided that its precursors, NH2 and H2CO, are available in the gas-phase.

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V. Barone, C. Latouche, D. Skouteris, et. al.
Wed, 15 Jul 15
59/65

Comments: MNRAS Letters, in press

Carbon Dioxide in Exoplanetary Atmospheres: Rarely Dominant Compared to Carbon Monoxide and Water [EPA]

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http://arxiv.org/abs/1507.01944


We present a comprehensive study of the abundance of carbon dioxide in exoplanetary atmospheres. We construct analytical models of systems in chemical equilibrium that include carbon monoxide, carbon dioxide, water, methane and acetylene and relate the equilibrium constants of the chemical reactions to temperature and pressure via the tabulated Gibbs free energies. We prove that such chemical systems may be described by a quintic equation for the mixing ratio of methane. By examining the abundances of these molecules across a broad range of temperatures (spanning equilibrium temperatures from 600 to 2500 K), pressures (via temperature-pressure profiles that explore albedo and opacity variations) and carbon-to-oxygen ratios (from 0.1 to 100), we conclude that carbon dioxide is subdominant compared to carbon monoxide and water. Atmospheric mixing does not alter this conclusion if carbon dioxide is subdominant everywhere in the atmosphere. Carbon dioxide and carbon monoxide may attain comparable abundances if the metallicity is greatly enhanced, but this property is negated by temperatures above 1000 K. For hydrogen-dominated atmospheres, our generic result has the implication that retrieval studies need to set the subdominance of carbon dioxide as a prior of the calculation and not let its abundance completely roam free as a fitting parameter, because it directly affects the inferred value of the carbon-to-oxygen ratio and may produce unphysical conclusions. We discuss the relevance of these implications for the hot Jupiter WASP-12b and suggest that some of the previous results are chemically impossible. The relative abundance of carbon dioxide to acetylene is potentially a sensitive diagnostic of the carbon-to-oxygen ratio.

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K. Heng and J. Lyons
Thu, 9 Jul 15
46/50

Comments: Submitted to ApJ. 10 pages, 8 figures, 2 tables

Two phase coexistence for the hydrogen-helium mixture [CL]

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http://arxiv.org/abs/1507.01809


We use our newly constructed quantum Gibbs ensemble Monte Carlo algorithm to perform computer experiments for the two phase coexistence of a hydrogen-helium mixture. Our results are in quantitative agreement with the experimental results of C. M. Sneed, W. B. Streett, R. E. Sonntag, and G. J. Van Wylen. The difference between our results and the experimental ones is in all cases less than 15% relative to the experiment, reducing to less than 5% in the low helium concentration phase. At the gravitational inversion between the vapor and the liquid phase, at low temperatures and high pressures, the quantum effects become relevant. At extremely low temperature and pressure the first component to show superfluidity is the helium in the vapor phase.

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R. Fantoni
Wed, 8 Jul 15
40/42

Comments: 7 pages, 4 figures, 4 tables

Electronic spectra of linear HC$_5$H and cumulene carbene H$_2$C$_5$ [CL]

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http://arxiv.org/abs/1506.08720


The $1 ^3\Sigma_u^- \leftarrow X^3\Sigma_g^-$ transition of linear HC$_5$H (A) has been observed in a neon matrix and gas phase. The assignment is based on mass-selective experiments, extrapolation of previous results of the longer HC$_{2n+1}$H homologues, and density functional and multi-state CASPT2 theoretical methods. Another band system starting at 303 nm in neon is assigned as the $1 ^1 A_1 \leftarrow X ^1 A_1$ transition of the cumulene carbene pentatetraenylidene H$_2$C$_5$ (B).

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M. Steglich, J. Fulara, S. Maity, et. al.
Tue, 30 Jun 15
56/75

Comments: 7 pages, 4 figures, 5 tables

Atmospheric Chemistry for Astrophysicists: A Self-consistent Formalism and Analytical Solutions for Arbitrary C/O [EPA]

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http://arxiv.org/abs/1506.05501


We present a self-consistent formalism for computing and understanding the atmospheric chemistry of exoplanets. Starting from the first law of thermodynamics, we demonstrate that the van’t Hoff equation (which describes the equilibrium constant), Arrhenius equation (which describes the rate coefficients) and procedures associated with the Gibbs free energy (minimisation, rescaling) have a common physical and mathematical origin. We correct an ambiguity associated with the equilibrium constant, which is used to relate the forward and reverse rate coefficients, and rigorously derive its two definitions. By necessity, one of the equilibrium constants must be dimensionless and equate to an exponential function involving the Gibbs free energy, while the other is a ratio of rate coefficients and must therefore possess physical units. To avoid confusion, we simply term them the dimensionless and dimensional equilibrium constants. We demonstrate that the Arrhenius equation takes on a functional form that is more general than previously thought without recourse to tagging on ad hoc functional forms. Our formulation of the evolution equations for chemical kinetics correctly enforces the book-keeping of elemental abundances, reproduces chemical equilibrium in the steady-state limit and is able to explain why photochemistry is an intrinsically disequilibrium effect. Finally, we derive analytical models of chemical systems with only hydrogen and with carbon, hydrogen and oxygen. For the latter, we include acetylene and are able to reproduce several key trends, versus temperature and carbon-to-oxygen ratio, published in the literature. The rich variety of behavior that mixing ratios exhibit as a function of the carbon-to-oxygen ratio is merely the outcome of stoichiometric book-keeping and not the direct consequence of temperature or pressure variations.

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K. Heng, J. Lyons and S. Tsai
Fri, 19 Jun 15
18/51

Comments: Submitted to ApJ. 7 pages, 3 figures

Modeling astronomically observed interstellar infrared spectra by ionized carbon pentagon-hexagon molecules (c9h7) n+ [CL]

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http://arxiv.org/abs/1506.05512


Modeling a promising carrier of the astronomically observed polycyclic aromatic hydrocarbon (PAH), infrared (IR) spectra of ionized molecules (C9H7) n+ were calculated based on density functional theory (DFT). In a previous study, it was found that void induced coronene C23H12++ could reproduce observed spectra from 3 to 15 micron, which has carbon two pentagons connected with five hexagons. In this paper, we tried to test the simplest model, that is, one pentagon connected with one hexagon, which is indene like molecule (C9H7) n+ (n=0 to 4). DFT based harmonic frequency analysis resulted that observed spectrum could be almost reproduced by a suitable sum of ionized C9H7n+ molecules. Typical example is C9H7++. Calculated peaks were 3.2, 7.4, 7.6, 8.4, and 12.7 micron, whereas observed one 3.3, 7.6, 7.8, 8.6 and 12.7 micron. By a combination of different degree of ionized molecules, we can expect to reproduce total spectrum. For a comparison, hexagon-hexagon molecule naphthalene (C10H8) n+ was studied. Unfortunately, ionized naphthalene shows little coincidence with observed one. Carbon pentagon- hexagon molecules may play an important role as interstellar molecular dust.

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N. Ota
Fri, 19 Jun 15
28/51

Comments: 7 pages, 4 figures, 2 tables

ExoMol molecular line lists X: The spectrum of sodium hydride [GA]

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http://arxiv.org/abs/1506.00174


Accurate and complete rotational, rotational-vibrational and rotational-vibrational-electronic line lists are calculated for sodium hydride: both the NaH and NaD isotopologues are considered. These line lists cover all ro-vibrational states of the ground ($X$~$^1\Sigma^+$) and first excited ($A$~$^1\Sigma^+$) electronic states. The calculations use available spectroscopically-determined potential energy curves and new high-quality, \textit{ab initio} dipole moment curves. Partition functions for both isotopologues are calculated and the effect of quasibound states is considered. The resulting line lists are suitable for temperatures up to about 7000~K and are designed for studies of exoplanet atmospheres, brown dwarfs and cool stars. In particular, the NaH $A-X$ band is found to show a broad absorption feature at about 385 nm which should provide a signature for the molecule. All partition functions, lines and transitions are available as Supplementary Information to this article and at \url{www.exomol.com}.

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T. Rivlin, L. Lodi, S. Yurchenko, et. al.
Tue, 2 Jun 15
34/57

Comments: 8 pages, 5 figures, 6 tables

ExoMol line lists VIII: A variationally computed line list for hot formaldehyde [GA]

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http://arxiv.org/abs/1506.00172


A computed line list for formaldehyde, H$_2{}^{12}$C$^{16}$O, applicable to temperatures up to $T=1500$~K is presented. An empirical potential energy and {\it ab initio} dipole moment surfaces are used as the input to nuclear motion program TROVE. The resulting line list, referred to as \textit{AYTY}, contains 10.3 million rotational-vibrational states and around 10 billion transition frequencies. Each transition includes associated Einstein-$A$ coefficients and absolute transition intensities, for wavenumbers below 10~000 cm\(^{-1}\) and rotational excitations up to \(J=70\). Room-temperature spectra are compared with laboratory measurements and data currently available in the HITRAN database. These spectra show excellent agreement with experimental spectra and highlight the gaps and limitations of the HITRAN data. The full line list is available from the CDS database as well as at \url{www.exomol.com}.

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A. Al-Refaie, A. Yachmenev, J. Tennyson, et. al.
Tue, 2 Jun 15
40/57

Comments: 14 pages, 8 figures, 7 tables

Accurate Prediction of the Ammonia Probes of a Variable Proton-to-Electron Mass Ratio [CEA]

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http://arxiv.org/abs/1505.05989


A comprehensive study of the mass sensitivity of the vibration-rotation-inversion transitions of $^{14}$NH$_3$, $^{15}$NH$_3$, $^{14}$ND$_3$, and $^{15}$ND$_3$ is carried out variationally using the TROVE approach. Variational calculations are robust and accurate, offering a new way to compute sensitivity coefficients. Particular attention is paid to the $\Delta k=\pm 3$ transitions between the accidentally coinciding rotation-inversion energy levels of the $\nu_2=0^+,0^-,1^+$ and $1^-$ states, and the inversion transitions in the $\nu_4=1$ state affected by the “giant” $l$-type doubling effect. These transitions exhibit highly anomalous sensitivities, thus appearing as promising probes of a possible cosmological variation of the proton-to-electron mass ratio $\mu$. Moreover, a simultaneous comparison of the calculated sensitivities reveals a sizeable isotopic dependence which could aid an exclusive ammonia detection.

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A. Owens, S. Yurchenko, W. Thiel, et. al.
Mon, 25 May 15
37/47

Comments: 18 pages, 2 figures, 10 tables

Laboratory Photo-chemistry of PAHs: Ionization versus Fragmentation [IMA]

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http://arxiv.org/abs/1505.00576


Interstellar polycyclic aromatic hydrocarbons (PAHs) are expected to be strongly processed by vacuum ultraviolet photons. Here, we report experimental studies on the ionization and fragmentation of coronene (C24H12), ovalene (C32H14) and hexa-peri-hexabenzocoronene (HBC; C42H18) cations by exposure to synchrotron radiation in the range of 8–40 eV. The results show that for small PAH cations such as coronene, fragmentation (H-loss) is more important than ionization. However, as the size increases, ionization becomes more and more important and for the HBC cation, ionization dominates. These results are discussed and it is concluded that, for large PAHs, fragmentation only becomes important when the photon energy has reached the highest ionization potential accessible. This implies that PAHs are even more photo-stable than previously thought. The implications of this experimental study for the photo-chemical evolution of PAHs in the interstellar medium are briefly discussed.

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J. Zhen, P. Castellanos, D. Paardekooper, et. al.
Tue, 5 May 15
12/51

Comments: N/A

Collisional quenching of highly rotationally excited HF [SSA]

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http://arxiv.org/abs/1504.07659


Collisional excitation rate coefficients play an important role in the dynamics of energy transfer in the interstellar medium. In particular, accurate rotational excitation rates are needed to interpret microwave and infrared observations of the interstellar gas for nonlocal thermodynamic equilibrium line formation. Theoretical cross sections and rate coefficients for collisional deexcitation of rotationally excited HF in the vibrational ground state are reported. The quantum-mechanical close-coupling approach implemented in the nonreactive scattering code MOLSCAT was applied in the cross section and rate coefficient calculations on an accurate 2D HF-He potential energy surface. Estimates of rate coefficients for H and H$_2$ colliders were obtained from the HF-He collisional data with a reduced-potential scaling approach. The calculation of state-to-state rotational quenching cross sections for HF due to He with initial rotational levels up to $j=20$ were performed for kinetic energies from 10$^{-5}$ to 15000 cm$^{-1}$. State-to-state rate coefficients for temperatures between 0.1 and 3000 K are also presented. The comparison of the present results with previous work for lowly-excited rotational levels reveals significant differences. In estimating HF-H$_2$ rate coefficients, the reduced-potential method is found to be more reliable than the standard reduced-mass approach. The current state-to-state rate coefficient calculations are the most comprehensive to date for HF-He collisions. We attribute the differences between previously reported and our results to differences in the adopted interaction potential energy surfaces. The new He rate coefficients can be used in a variety of applications. The estimated H$_2$ and H collision rates can also augment the smaller datasets previously developed for H$_2$ and electrons.

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B. Yang, K. Walker, R. Forrey, et. al.
Thu, 30 Apr 15
36/43

Comments: 26 pages, 14 figures, and 3 tables in A&A 2015

Ice chemistry in starless molecular cores [GA]

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http://arxiv.org/abs/1504.06065


Starless molecular cores are natural laboratories for interstellar molecular chemistry research. The chemistry of ices in such objects was investigated with a three-phase (gas, surface, and mantle) model. We considered the center part of five starless cores, with their physical conditions derived from observations. The ice chemistry of oxygen, nitrogen, sulfur, and complex organic molecules (COMs) was analyzed. We found that an ice-depth dimension, measured, e.g., in monolayers, is essential for modeling of chemistry in interstellar ices. Particularly, the H2O:CO:CO2:N2:NH3 ice abundance ratio regulates the production and destruction of minor species. It is suggested that photodesorption during core collapse period is responsible for high abundance of interstellar H2O2 and O2H, and other species synthesized on the surface. The calculated abundances of COMs in ice were compared to observed gas-phase values. Smaller activation barriers for CO and H2CO hydrogenation may help explain the production of a number of COMs. The observed abundance of methyl formate HCOOCH3 could be reproduced with a 1kyr, 20K temperature spike. Possible desorption mechanisms, relevant for COMs, are gas turbulence (ice exposure to interstellar photons) or a weak shock within the cloud core (grain collisions). To reproduce the observed COM abundances with the present 0D model, 1-10% of ice mass needs to be sublimated. We estimate that the lifetime for starless cores likely does not exceed 1Myr. Taurus cores are likely to be younger than their counterparts in most other clouds.

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J. Kalvans
Fri, 24 Apr 15
40/64

Comments: Accepted for publication in ApJ. 22 figures, 6 tables

The calculated rovibronic spectrum of scandium hydride, ScH [GA]

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http://arxiv.org/abs/1504.04051


The electronic structure of six low-lying electronic states of scandium hydride, $X\,{}^{1}\Sigma^+$, $a\,{}^{3}\Delta$, $b\,{}^{3}\Pi$, $A\,{}^{1}\Delta$ $c\,{}^{3}\Sigma^+$, and $B\,{}^{1}\Pi$, is studied using multi-reference configuration interaction as a function of bond length. Diagonal and off-diagonal dipole moment, spin-orbit coupling and electronic angular momentum curves are also computed. The results are benchmarked against experimental measurements and calculations on atomic scandium. The resulting curves are used to compute a line list of molecular ro-vibronic transitions for $^{45}$ScH.

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L. Lodi, S. Yurchenko and J. Tennyson%5C
Fri, 17 Apr 15
42/54

Comments: 22 pages, 7 figures, 9 tables In press

Exomol molecular line lists VI: A high temperature line list for Phosphorus Nitride [GA]

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http://arxiv.org/abs/1504.02804


Accurate rotational-vibrational line lists for $^{31}$P$^{14}$N and $^{31}$P$^{15}$N in their ground electronic states are computed. The line lists are produced using an empirical potential energy curve obtained by fitting to the experimental transition frequencies available in the literature in conjunction with an accurate, high level \textit{ab initio} dipole moment curve. In these calculations the programs DPotFit and LEVEL~8.0 were used. The new line lists reproduce the experimental wavenumbers with a root-mean-square error of 0.004~cm$^{-1}$. The line lists cover the frequency range 0–51000 cm$^{-1}$, contain almost 700~000 lines each and extend up to a maximum vibrational level of $v$=66 and a maximum rotational level of $J$=357. They should be applicable for a large range of temperature up to, at least, 5000~K. These new line lists are used to simulate spectra for PN at a range of temperatures and are deposited in the Strasbourg data centre. This work is performed as part of the ExoMol project.

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L. Yorke, S. Yurchenko, L. Lodi, et. al.
Tue, 14 Apr 15
3/87

Comments: 12 pages. 5 figures, 8 Tables

ExoMol molecular line lists: IX The spectrum of AlO [GA]

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http://arxiv.org/abs/1504.02938


Accurate line lists are calculated for aluminium monoxide covering the pure rotation, rotation-vibration and electronic (B — X blue-green and A — X infrared bands) spectrum. Line lists are presented for the main isotopologue, $^{27}$Al$^{16}$O, as well as for $^{27}$Al$^{17}$O, $^{27}$Al$^{18}$O and $^{26}$Al$^{16}$O. These line lists are suitable for high temperatures (up to 8000 K) including those relevant to exoplanetary atmospheres and cool stars. A combination of empirical and \textit{ab initio} methods is used: the potential energy curves were previously determined to high accuracy by fitting to extensive data from analysis of laboratory spectra; a high quality {\it ab initio} dipole moment curve is calculated using quadruple zeta basis set and the multi-reference configuration interaction (MRCI) method. Partition functions plus full line lists of transitions are made available in an electronic form as supplementary data to this article and at \url{www.exomol.com}.

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A. Patrascu, S. Yurchenko and J. Tennyson
Tue, 14 Apr 15
70/87

Comments: 8 pages, 7 figures, 6 tables

The electronic spectra of protonated PANH molecules [SSA]

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http://arxiv.org/abs/1503.03329


Aims. This study was designed to examine the viability of protonated nitrogen-substituted polycyclic aromatic hydrocarbons (H+PANHs) as candidates for the carriers of the diffuse interstellar bands (DIBs). Methods. We obtained the electronic spectra of two protonated PANH cations, protonated acridine and phenanthridine, using parent ion photo-fragment spectroscopy and generated theoretical electronic spectra using ab initio calculations. Results. We show that the spectra of the two species studied here do not correspond to known DIBs. However, based on the general properties derived from the spectra of these small protonated nitrogen-substituted PAHs, we propose that larger H+PANH cations represent good candidates for DIB carriers due to the expected positions of their electronic transitions in the UV-visible and their narrow spectral bands.

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J. Noble, C. Dedonder and C. Jouvet
Thu, 12 Mar 15
3/57

Comments: 7 pages, 2 figures, accepted for publication in A&A

A theoretical room-temperature line list for 15NH3 [EPA]

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http://arxiv.org/abs/1502.07975


A new room temperature line list for $^{15}$NH$_3$ is presented. This line list comprised of transition frequencies and Einstein coefficients has been generated using the `spectroscopic’ potential energy surface NH3-Y2010 and an ab initio dipole moment surface. The $^{15}$NH$_3$ line list is based on the same computational procedure used for the line list for $^{14}$NH$_3$ BYTe reported recently and should be as accurate. Comparisons with experimental frequencies and intensities are presented. The synthetic spectra show excellent agreement with experimental spectra.

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S. Yurchenko
Mon, 2 Mar 15
3/39

Comments: 22 pages, 2 figures

CO diffusion into amorphous H2O ices [GA]

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http://arxiv.org/abs/1502.07772


The mobility of atoms, molecules and radicals in icy grain mantles regulate ice restructuring, desorption, and chemistry in astrophysical environments. Interstellar ices are dominated by H2O, and diffusion on external and internal (pore) surfaces of H2O-rich ices is therefore a key process to constrain. This study aims to quantify the diffusion kinetics and barrier of the abundant ice constituent CO into H2O dominated ices at low temperatures (15-23 K), by measuring the mixing rate of initially layered H2O(:CO2)/CO ices. The mixed fraction of CO as a function of time is determined by monitoring the shape of the infrared CO stretching band. Mixing is observed at all investigated temperatures on minute time scales, and can be ascribed to CO diffusion in H2O ice pores. The diffusion coefficient and final mixed fraction depend on ice temperature, porosity, thickness and composition. The experiments are analyzed by applying Fick’s diffusion equation under the assumption that mixing is due to CO diffusion into an immobile H2O ice. The extracted energy barrier for CO diffusion into amorphous H2O ice is ~160 K. This is effectively a surface diffusion barrier. The derived barrier is low compared to current surface diffusion barriers in use in astrochemical models. Its adoption may significantly change the expected timescales for different ice processes in interstellar environments.

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T. Lauck, L. Karssemeijer, K. Shulenberger, et. al.
Mon, 2 Mar 15
24/39

Comments: Accepted for publication in ApJ; 12 pages, including 13 figures

Rotational spectroscopy as a tool to investigate interactions between vibrational polyads in symmetric top molecules: low-lying states v8 <= 2 of methyl cyanide, CH$_3$CN [GA]

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http://arxiv.org/abs/1502.06867


Spectra of methyl cyanide were recorded to analyze interactions in low-lying vibrational states and to construct line lists for radio astronomical observations as well as for infrared spectroscopic investigations of planetary atmospheres. The rotational spectra cover large portions of the 36-1627 GHz region. In the infrared (IR), a spectrum was recorded for this study in the region of 2nu8 around 717 cm-1 with assignments covering 684-765 cm-1. Additional spectra in the nu8 region were used to validate the analysis.
The large amount and the high accuracy of the rotational data extend to much higher J and K quantum numbers and allowed us to investigate for the first time in depth local interactions between these states which occur at high K values. In particular, we have detected several interactions between v8 = 1 and 2. Notably, there is a strong Delta(v8) = +- 1, Delta(K) = 0, Delta(l) = +-3 Fermi resonance between v8 = 1^-1 and v8 = 2^+2 at K = 14. Pronounced effects in the spectrum are also caused by resonant Delta(v8) = +- 1, Delta(K) = -+ 2, Delta(l) = +- 1 interactions between v8 = 1 and 2. An equivalent resonant interaction occurs between K = 14 of the ground vibrational state and K = 12, l = +1 of v8 = 1 for which we present the first detailed account. A preliminary account was given in an earlier study on the ground vibrational state. Similar resonances were found for CH3CCH and, more recently, for CH3NC, warranting comparison of the results. From data pertaining to v8 = 2, we also investigated rotational interactions with v4 = 1 as well as Delta(v8) = +- 1, Delta(K) = 0, Delta(l) = +-3 Fermi interactions between v8 = 2 and 3.
We have derived N2- and self-broadening coefficients for the nu8, 2nu8 – nu8, and 2nu8 bands from previously determined nu4 values. Subsequently, we determined transition moments and intensities for the three IR bands.

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H. Muller, L. Brown, B. Drouin, et. al.
Wed, 25 Feb 15
46/55

Comments: 20 pages, 12 figures, astract abbreviated; Journal of Molecular Spectroscopy, accepted

Hydrogenation at low temperatures does not always lead to saturation: the case of HNCO [SSA]

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http://arxiv.org/abs/1502.03282


Context. It is generally agreed that hydrogenation reactions dominate chemistry on grain surfaces in cold, dense molecular cores, saturating the molecules present in ice mantles. Aims. We present a study of the low temperature reactivity of solid phase isocyanic acid (HNCO) with hydrogen atoms, with the aim of elucidating its reaction network. Methods. Fourier transform infrared spectroscopy and mass spectrometry were employed to follow the evolution of pure HNCO ice during bombardment with H atoms. Both multilayer and monolayer regimes were investigated. Results. The hydrogenation of HNCO does not produce detectable amounts of formamide (NH2CHO) as the major product. Experiments using deuterium reveal that deuteration of solid HNCO occurs rapidly, probably via cyclic reaction paths regenerating HNCO. Chemical desorption during these reaction cycles leads to loss of HNCO from the surface. Conclusions. It is unlikely that significant quantities of NH2CHO form from HNCO. In dense regions, however, deuteration of HNCO will occur. HNCO and DNCO will be introduced into the gas phase, even at low temperatures, as a result of chemical desorption.

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J. Noble, P. Theule, E. Congiu, et. al.
Thu, 12 Feb 15
34/68

Comments: 9 pages, 8 figures, accepted for publication in A&A

Glycine's Radiolytic Destruction in Ices: First In-Situ Laboratory Measurements for Mars [IMA]

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http://arxiv.org/abs/1502.02688


We report new laboratory studies of the radiation-induced destruction of glycine-containing ices for a range of temperatures and compositions that allow extrapolation to Martian conditions. In-situ infrared spectroscopy was used to study glycine decay rates as a function of temperature (from 15 to 280 K) and initial glycine concentrations in six mixtures whose compositions ranged from dry glycine to H$_2$O + glycine (300:1). Results are presented in several systems of units, with cautions concerning their use. The half-life of glycine under the surface of Mars is estimated as an extrapolation of this data set to Martian conditions, and trends in decay rates are described as are applications to Mars’s near-surface chemistry.

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P. Gerakines and R. Hudson
Wed, 11 Feb 15
65/72

Comments: 17 pages, 4 tables, 9 figures

Quantum dynamics of CO-H$_2$ in full dimensionality [SSA]

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http://arxiv.org/abs/1502.02645


Accurate rate coefficients for molecular vibrational transitions due to collisions with H$_2$, critical for interpreting infrared astronomical observations, are lacking for most molecules. Quantum calculations are the primary source of such data, but reliable values that consider all internal degrees of freedom of the collision complex have only been reported for H$_2$-H$_2$ due to the difficulty of the computations. Here we present essentially exact full-dimensional dynamics computations for rovibrational quenching of CO due to H$_2$ impact. Using a high-level six-dimensional potential surface, time-independent scattering calculations, within a full angular-momentum-coupling formulation, were performed for the deexcitation of vibrationally excited CO. Agreement with experimentally-determined results confirms the accuracy of the potential and scattering computations, representing the largest of such calculations performed to date. This investigation advances computational quantum dynamics studies representing initial steps toward obtaining CO-H$_2$ rovibrational quenching data needed for astrophysical modeling.

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B. Yang, P. Zhang, X. Wang, et. al.
Tue, 10 Feb 15
60/63

Comments: N/A

Infrared spectra of methyl-, and nitrogen-modified void coronene; modeling a carrier of interstellar polycyclic aromatic hydrocarbon [CL]

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http://arxiv.org/abs/1502.01766


Void induced coronene C23H12++ was suggested to be a possible carrier of the astronomically observed polycyclic aromatic hydrocarbon (PAH), which shows unique molecular structure with carbon two pentagons connected with five hexagons. Well observed astronomical infrared spectrum from 3-15 micron could be almost reproduced based on density functional theory. However, there remain several discrepancies with observed spectra, especially on 11-15 micron band weaker intensity. Observed 11.2 micron intensity is comparable to 7.6-7.8 micron one. Methyl-modified molecule C24H14++ revealed that calculated peak height of 11.4 micron show fairly large intensity up to 70-90% compared with that of 7.6-7.8 micron band. Also, nitrogen atom was substituted to peripheral C-H site of void coronene to be C22H11N1++. Pentagon site substituted case show 60% peak height. This molecule also reproduced well 12-15 micron peak position and relative intensity. Vibration mode analysis demonstrated that 11.3 micron mode comes from C-H out of plane bending. Heavy nitrogen plays as like an anchor role for molecule vibration.

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N. Ota
Mon, 9 Feb 15
37/47

Comments: 8 pages, 4 figures, 2 tables

The effect of selective desorption mechanisms during interstellar ice formation [GA]

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http://arxiv.org/abs/1502.00504


Major components of ices on interstellar grains in molecular clouds – water and carbon oxides – occur at various optical depths. This implies that selective desorption mechanisms are at work. An astrochemical model of a contracting low-mass molecular cloud core is presented. Ice was treated as consisting of the surface and three subsurface layers (sublayers). Photodesorption, reactive desorption, and indirect reactive desorption were investigated. The latter manifests itself through desorption from H+H reaction on grains. Desorption of shallow subsurface species was included. Modeling results suggest the existence of a “photon-dominated ice” during the early phases of core contraction. Subsurface ice is chemically processed by interstellar photons, which produces complex organic molecules. Desorption from the subsurface layer results in high COM gas-phase abundances at Av = 2.4…10mag. This may contribute towards an explanation for COM observations in dark cores. It was found that photodesorption mostly governs the onset of ice accumulation onto grains. Reaction-specific reactive desorption is efficient for small molecules that form via highly exothermic atom-addition reactions. Higher reactive desorption efficiency results in lower gas-phase abundances of COMs. Indirect reactive desorption allows to closely reproduce the observed H2O:CO:CO2 ratio towards a number of background stars. Presumably this can be done by any mechanism whose efficiency fits with the sequence CO > CO2 >> H2O. After the freeze-out has ended, the three sublayers represent chemically distinct parts of the mantle. 8…10.5mag is the likely AV threshold for the appearance of CO ice. The lower value is supported by observations.

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J. Kalvans
Tue, 3 Feb 15
24/80

Comments: Under review; recommended for publication in ApJ

Formation of hydroxylamine on dust grains via ammonia oxidation [IMA]

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http://arxiv.org/abs/1501.04701


The quest to detect prebiotic molecules in space, notably amino acids, requires an understanding of the chemistry involving nitrogen atoms. Hydroxylamine (NH$_2$OH) is considered a precursor to the amino acid glycine. Although not yet detected, NH$_2$OH is considered a likely target of detection with ALMA. We report on an experimental investigation of the formation of hydroxylamine on an amorphous silicate surface via the oxidation of ammonia. The experimental data are then fed into a simulation of the formation of NH$_2$OH in dense cloud conditions. On ices at 14 K and with a modest activation energy barrier, NH$_2$OH is found to be formed with an abundance that never falls below a factor 10 with respect to NH$_3$. Suggestions of conditions for future observations are provided.

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J. He, G. Vidali, J. Lemaire, et. al.
Wed, 21 Jan 15
29/52

Comments: 9 pages, 9 figures

A New Determination of the Binding Energy of Atomic Oxygen on Dust Grain Surfaces: Experimental Results and Simulations [GA]

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http://arxiv.org/abs/1501.03836


The energy to desorb atomic oxygen from an interstellar dust grain surface, $E_{\rm des}$, is an important controlling parameter in gas-grain models; its value impacts the temperature range over which oxygen resides on a dust grain. However, no prior measurement has been done of the desorption energy. We report the first direct measurement of $E_{\rm des}$ for atomic oxygen from dust grain analogs. The values of $E_{\rm des}$ are $1660\pm 60$~K and $1850\pm 90$~K for porous amorphous water ice and for a bare amorphous silicate film, respectively, or about twice the value previously adopted in simulations of the chemical evolution of a cloud. We use the new values to study oxygen chemistry as a function of depth in a molecular cloud. For $n=10^4$ cm$^{-3}$ and $G_0$=10$^2$ ($G_0$=1 is the average local interstellar radiation field), the main result of the adoption of the higher oxygen binding energy is that H$_2$O can form on grains at lower visual extinction $A_{\rm V}$, closer to the cloud surface. A higher binding energy of O results in more formation of OH and H$_2$O on grains, which are subsequently desorbed by FUV radiation, with consequences for gas-phase chemistry. For higher values of $n$ and $G_0$, the higher binding energy can lead to a large increase in the column of H$_2$O but a decrease in the column of O$_2$.

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J. He, J. Shi, T. Hopkins, et. al.
Mon, 19 Jan 15
33/50

Comments: 7 pages, 8 figures

Vibrational mode analysis of void induced coronene as a possible carrier of the astronomical aromatic infrared bands [CL]

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http://arxiv.org/abs/1501.01716


Void induced di-cation coronene C23H12++ is a possible carrier of the astronomically observed polycyclic aromatic hydrocarbon (PAH). Based on density functional theory, multiple spin state analysis was done for neutral void coronene C23H12. Singlet spin state was most stable (lowest total energy). By the Jahn-Teller effect, there occurs serious molecular deformation. Point group D6h of pure coronene transformed to C2 symmetry having carbon two pentagons. Advanced singlet stable molecules were di-cation C23H12++ and di-anion C23H12- -. Molecular configuration was almost similar with neutral C23H12. However, electric dipole moment of these two charged molecules show reversed direction with 1.19 and 2.63 Debey. Calculated infrared spectrum of C23H12++ show a very likeness to observed one of two astronomical sources of HD44179 and NGC7027. Harmonic vibrational mode analysis was done for C23H12++. At 3.2 micrometer, C-H stretching at pentagons was featured. From 6.4 to 8.7 micrometer, C-C stretching mode was observed. In-plane-bending of C-H was in a range of 7.6-9.2 micrometer. Both C-H out-of plane bending and C-C stretching were accompanied from 11.4 to 14.3 micrometer. Astronomically observed emission peaks of 3.3, 6.2, 7.6, 7.8, 8.6, 11.2, 12.7, 13.5 and 14.3 micrometer were compared well with calculated peaks of 3.2, 6.5, 7.6, 7.8, 8.6, 11.4, 12.9, 13.5, and 14.4 micrometer.

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N. Ota
Fri, 9 Jan 15
11/49

Comments: 8 pages, 4 figures, 4 tables

A Theoretical Study on the Vibrational Spectra of PAH Molecules with Aliphatic Sidegroups [GA]

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http://arxiv.org/abs/1501.00359


The role of aliphatic side groups on the formation of astronomical unidentified infrared emission (UIE) features is investigated by applying the density functional theory (DFT) to a series of molecules with mixed aliphatic-aromatic structures. The effects of introducing various aliphatic groups to a fixed polycyclic aromatic hydrocarbon (PAH) core (ovalene) are studied. Simulated spectra for each molecule are produced by applying a Drude profile at $T$=500 K while the molecule is kept at its electronic ground state. The vibrational normal modes are classified using a semi-quantitative method. This allows us to separate the aromatic and aliphatic vibrations and therefore provide clues to what types of vibrations are responsible for the emissions bands at different wavelengths. We find that many of the UIE bands are not pure aromatic vibrational bands but may represent coupled vibrational modes. The effects of aliphatic groups on the formation of the 8 $\mu$m plateau are qua ntitatively determined. The vibrational motions of methyl ($-$CH$_3$) and methyl ene ($-$CH$_2-$) groups can cause the merging of the vibrational bands of the pa rent PAH and the forming of broad features. These results suggest that aliphatic structures can play an important role in th e UIE phenomenon.

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S. Sadjadi, Y. Zhang and S. Kwok
Mon, 5 Jan 15
16/57

Comments: 29 pages, 13 figures, Accepted for publication in ApJ

Photoionization of the $3s^23p^4~ ^3P$ and the $3s^23p^4~^1D,~^1S$ states of sulfur: experiment and theory [EPA]

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http://arxiv.org/abs/1412.8715


Photoionization of neutral atomic sulfur in the ground and metastable states was studied experimentally at a photon energy resolution of 44 meV FWHM. Relative cross section measurements were recorded by using tunable vacuum ultraviolet (VUV) radiation in the energy range 9 — 30 eV obtained from a laser-produced plasma and the atomic species were generated by photolysis of molecular precursors. Photoionization of this atom is characterized by multiple Rydberg series of autoionizing resonances superimposed on a direct photoionization continuum. A wealth of resonance features observed in the experimental spectra are spectroscopically assigned and their energies and quantum defects tabulated. The cross-section measurements are compared with state-of-the-art theoretical cross-section calculations obtained from the Dirac Coulomb R-matrix method. Resonances series in the spectra are identified and compared indicating similar features in both the theoretical and experimental spectra.

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M. Barthel, R. Flesch, E. Ruhl, et. al.
Wed, 31 Dec 14
11/32

Comments: Accepted for publication in Physical Review A, 20 pages, 8 figures, 3 tables

Cosmic-ray-induced dissociation and reactions in warm interstellar ices [GA]

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http://arxiv.org/abs/1412.3940


Context. Cosmic ray particles that hit interstellar grains in dark molecular cores may induce whole-grain heating. The high temperature of a CR-heated grain allows energy barriers for bulk diffusion and reactions to be overcome. Additionally, ice molecules are destroyed by direct cosmic-ray induced dissociation. Aims. We provide a justified estimate of the significance of cosmic-ray induced surface-mantle diffusion, chemical reactions in ice, and dissociation of ice species in a star-forming interstellar cloud core. Methods. We considered a gas clump in a collapsing low-mass prestellar core and during the initial stages of protostellar envelope heating with a three-phase chemical kinetics model. The model includes a proper treatment of the stochastic aspect of whole-grain heating and new experimental data for dissociation. Results. We found that the cosmic-ray-induced effects are mostly limited to an increase in abundance for carbon-chain species. The effect on major species abundances is a few percentage points at most. The HNC to HCN ice abundance ratio in ice is increased. Conclusions. Among the processes considered in the model, dissociation is probably the most significant, while diffusion and reactions on warm grains are less important. All three processes facilitate the synthesis of complex molecules, including organic species.

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J. Kalvans
Mon, 15 Dec 14
1/53

Comments: N/A

Sulfur-bearing species in molecular clouds [SSA]

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http://arxiv.org/abs/1410.8293


We study several molecules that could help in the solution of the missing sulfur problem in dense clouds and circumstellar regions, as well as in the clarification of the sulfur chemistry in comets. These sulfur molecules are: the trimer (CH2S)3 and the tetramer (CH2S)4 of thioformaldehyde, pentathian S5CH2, hexathiepan S6CH2, thiirane C2H4S, trisulfane HSSSH, and thioacetone (CH3)2CS. Infrared spectra of these species are calculated using density functional theory methods. The majority of calculated bands belong to the mid-infrared, with some of them occurring in the near and far-infrared region. We suggest that some of unidentified spectral features measured by Infrared Space Observatory in several active galactic nuclei and starburst galaxies could be caused by 1,3,5-trithiane ((CH2S)3), 1,3,5,7-tetrathiocane ((CH2S)4), and thiirane (C2H4S). The objects whose unidentified infrared features we compare with calculated bands are: NGC 253, M82, NGC 1068, Circinus, Arp 220, 30 Doradus, Orion KL, and Sgr B2.

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G. Bilalbegovic and G. Baranovic
Fri, 31 Oct 14
41/69

Comments: accepted for publication in MNRAS

Hyperfine excitation of N$_2$H$^+$ by H$_2$: Toward a revision of N$_2$H$^+$ abundance in cold molecular clouds [GA]

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http://arxiv.org/abs/1410.5669


The modelling of emission spectra of molecules seen in interstellar clouds requires the knowledge of collisional rate coefficients. Among the commonly observed species, N$_2$H$^+$ is of particular interest since it was shown to be a good probe of the physical conditions of cold molecular clouds. Thus, we have calculated hyperfine-structure resolved excitation rate coefficients of N$_2$H$^+$(X$^1\Sigma^+$) by H$_2(j=0)$, the most abundant collisional partner in the cold interstellar medium. The calculations are based on a new potential energy surface, obtained from highly correlated {\it ab initio} calculations. State-to-state rate coefficients between the first hyperfine levels were calculated, for temperatures ranging from 5 K to 70 K. By comparison with previously published N$_2$H$^+$-He rate coefficients, we found significant differences which cannot be reproduced by a simple scaling relationship. As a first application, we also performed radiative transfer calculations, for physical conditions typical of cold molecular clouds. We found that the simulated line intensities significantly increase when using the new H$_2$ rate coefficients, by comparison with the predictions based on the He rate coefficients. In particular, we revisited the modelling of the N$_2$H$^+$ emission in the LDN 183 core, using the new collisional data, and found that all three of the density, gas kinetic temperature and N$_2$H$^+$ abundance had to be revised.

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F. Lique, F. Daniel, L. Pagani, et. al.
Wed, 22 Oct 14
23/64

Comments: N/A

On the empirical dipole polarizability of He from spectroscopy of HeH$^{+}$ [CL]

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http://arxiv.org/abs/1410.4895


Using a long-range polarization potential for HeH$^{+}$, we can obtain an empirical value for the Boltzmann constant $k_{B}$ with an order of magnitude greater precision than the previous best experimental value based on the dipole polarizability of $^{4}$He. We find that relativistic and QED effects of order $\alpha_{{\rm FS}}^{3}$ in the fine structure constant are crucial in the quadrupole polarizability in order to fit the dipole polarizbility with this precision using the polarization potential. By calculating finite-mass corrections for $^{3}$He, we also present theoretical values for the dipole and quadrupole polarizabilities of $^{3}$He with 9 and 7 digits of precision respectively.

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N. Dattani and M. Puchalski
Tue, 21 Oct 14
20/72

Comments: We thank Krzystof Pachucki, Z-C. Yan, Stephen Lundeen, Richard Drachman, Anand Bhatia, Robert Le Roy, Grzegorz Lach, and Jim Mitroy for helpful discussions

On the relevance of the H2 + O reaction pathway for the surface formation of interstellar water – A combined experimental and modeling study [GA]

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http://arxiv.org/abs/1409.3055


The formation of interstellar water has been commonly accepted to occur on the surfaces of icy dust grains in dark molecular clouds at low temperatures (10-20 K), involving hydrogenation reactions of oxygen allotropes. As a result of the large abundances of molecular hydrogen and atomic oxygen in these regions, the reaction H2 + O has been proposed to contribute significantly to the formation of water as well. However, gas phase experiments and calculations, as well as solid-phase experimental work contradict this hypothesis. Here, we use precisely executed temperature programmed desorption (TPD) experiments in an ultra-high vacuum setup combined with kinetic Monte Carlo simulations to establish an upper limit of the water production starting from H2 and O. These reactants are brought together in a matrix of CO2 in a series of (control) experiments at different temperatures and with different isotopological compositions. The amount of water detected with the quadrupole mass spectrometer upon TPD is found to originate mainly from contamination in the chamber itself. However, if water is produced in small quantities on the surface through H2 + O, this can only be explained by a combined classical and tunneled reaction mechanism. An absolutely conservative upper limit for the reaction rate is derived with a microscopic kinetic Monte Carlo model that converts the upper limit into a maximal possible reaction rate. Incorporating this rate into simulations run for astrochemically relevant parameters, shows that the upper limit to the contribution of the reaction H2 + O in OH, and hence water formation, is 11% in dense interstellar clouds. Our combined experimental and theoretical results indicate however, that this contribution is likely to be much lower.

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T. Lamberts, H. Cuppen, G. Fedoseev, et. al.
Thu, 11 Sep 14
6/67

Comments: 9 pages, 4 figures

Diffusion-desorption ratio of adsorbed CO and CO$_2$ on water ice [GA]

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http://arxiv.org/abs/1409.3038


Diffusion of atoms and molecules is a key process for the chemical evolution in the star forming regions of the interstellar medium. Accurate data on the mobility of many important interstellar species is however often not available and this provides a serious limitation for the reliability of models describing the physical and chemical processes in molecular clouds. Here we aim to provide the astrochemical modeling community with reliable data on the ratio between the energy barriers for diffusion and desorption for adsorbed CO and CO$_2$ on water ices. To this end, we use a fully atomistic, off-lattice kinetic Monte Carlo technique to generate dynamical trajectories of CO and CO$_2$ molecules on the surface of crystalline ice at temperatures relevant for the interstellar medium. The diffusion to desorption barrier ratios are determined to be 0.31 for CO and 0.39 for CO$_2$ . These ratios can be directly used to improve the accuracy of current gas-grain chemical models.

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L. Karssemeijer and H. Cuppen
Thu, 11 Sep 14
43/67

Comments: 4 pages, 3 figures, accepted for publication in Astronomy & Astrophysics

Laboratory Optical Spectroscopy of Thiophenoxy Radical and Its Profile Simulation as a Diffuse Interstellar Band Based on Rotational Distribution by Radiation and Collisions [SSA]

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http://arxiv.org/abs/1409.0182


The gas-phase optical absorption spectrum of a thiophenoxy radical (C6H5S), a diffuse interstellar band (DIB) candidate molecule, was observed in the discharge of thiophenol using a cavity ringdown spectrometer. The ground-state rotational constants of the thiophenoxy radical were theoretically calculated, and the excited-state rotational constants were determined from the observed rotational profile. The rotational profile of a near prolate molecule having a C2v symmetry was simulated on the basis of a rotational distribution model by radiation and collisions. Although the simulated profile did not agree with the observed DIBs, the upper limit of the column density for the thiophenoxy radical in the diffuse clouds toward HD 204827 was evaluated. The profile simulation indicates that rotational distribution by radiation and collisions is important to reproduce a rotational profile for a DIB candidate and that the near prolate C2v molecule is a possible candidate for DIB with a band width variation dependent on line-of-sight.

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M. Araki, K. Niwayama and K. Tsukiyama
Tue, 2 Sep 14
13/72

Comments: 15 pages, 5 figures

ArH2+ and NeH2+ as Global Minima in the Ar+/Ne+ + H2 Reactions: Energetic, Spectroscopic, and Structural Data [SSA]

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http://arxiv.org/abs/1408.7085


In light of the recent discovery of ArH+ in the Crab nebula, it is shown through high-level quantum chemical comptuations that the global minima on the Ar+/Ne+ + H2 potential energy surfaces are ArH2+ and NeH2+. Hence, ArH2+ may be a necessary intermediate in the Ar+ + H2 -> ArH+ + H formation reaction proposed in the same work where ArH+ is first reported in the Crab nebula. ArH2+ is also probably an intermediate in the alternative Ar + H+2 -> ArH+ + H reaction. Additionally, it is shown that Ne+ + H2 -> NeH+2 will subsequently most likely yield Ne + H+2 and not NeH+ + H offering a possible rationale as to the absence of NeH+ in spectra obtained from the interstellar medium (ISM). Following from this, spectroscopic data (both rotational and vibrational) are provided for NeH2+ and ArH2+ through the use of highly-accurate quantum chemical quartic and cubic force fields. All possible isotopologues are also included for 20Ne, 22Ne, 36Ar, 38Ar, 40Ar, 1H, and D. The dipole moments for these systems are quite large at 5.61 D for NeH2+ and 4.37 D for ArH2+. The spectroscopic constants provided will aid in the potential detection of these open-shell noble gas dihydride cations in the ISM.

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R. Theis, W. Morgan and R. Fortenberry
Mon, 1 Sep 14
7/36

Comments: 12 Pages, 4 Figures, 8 Tables, accepted for publication in MNRAS

Laboratory spectroscopy of 1,2-propanediol at millimeter and submillimeter wavelengths [CL]

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http://arxiv.org/abs/1408.5247


Ethanediol is one of the largest complex organic molecules detected in space thus far. It has been found in different types of molecular clouds. The two propanediol isomers are the next larger diols. Hence, they are viable candidates to be searched for in space. We wish to provide sufficiently large and accurate sets of spectroscopic parameters of 1,2-propanediol to facilitate searches for this molecule at millimeter and longer submillimeter wavelengths. We recorded rotational spectra of 1,2-propanediol in three wide frequency windows between 38 and 400~GHz. We made extensive assignments for the three lowest energy conformers to yield spectroscopic parameters up to eighth order of angular momentum. Our present data will be helpful for identifying 1,2-propanediol at moderate submillimeter or longer wavelengths with radio telescope arrays such as ALMA, NOEMA, or EVLA. In particular, its detection with ALMA in sources, in which ethanediol was detected, appears to be promising.

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J. Bossa, M. Ordu, H. Muller, et. al.
Mon, 25 Aug 14
8/43

Comments: 8 pages, 3 figures, Astron. Astrophys., accepted

Beryllium monohydride (BeH): Where we are now, after 86 years of spectroscopy [CL]

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http://arxiv.org/abs/1408.3301


BeH is one of the most important benchmark systems for ab initio methods and for studying Born-Oppenheimer breakdown. However the best empirical potential and best ab initio potential for the ground electronic state to date give drastically different predictions in the long-range region beyond which measurements have been made, which is about \sim1000 cm^{-1} for ^{9} BeH, \sim3000 cm^{-1} for ^{9} BeD, and \sim13000 cm^{-1} for ^{9} BeT. Improved empirical potentials and Born-Oppenheimer breakdown corrections have now been built for the ground electronic states X(1^{2}\Sigma^{+}) of all three isotopologues. The predicted dissociation energy for ^{9} BeH from the new empirical potential is now closer to the current best ab initio prediction by more than 66% of the discrepancy between the latter and the previous best empirical potential. The previous best empirical potential predicted the existence of unobserved vibrational levels for all three isotopologues, and the current best ab initio study also predicted the existence of all of these levels, and four more. The present empirical potential agrees with the ab initio prediction of all of these extra levels not predicted by the earlier empirical potential. With one exception, all energy spacings between vibrational energy levels for which measurements have been made, are predicted with an agreement of better than 1 cm^{-1} between the new empirical potential and the current best ab initio potential, but some predictions for unobserved levels are still in great disagreement, and the equilibrium bond lengths are different by orders of magnitude.

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N. Dattani
Fri, 15 Aug 14
30/45

Comments: Feedback encouraged. 9 Pages, 4 Figures, 4 Tables. The author thanks JSPS for financial support

The irradiation of water ice by C$^+$ ions in the cosmic environment [GA]

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http://arxiv.org/abs/1408.1316


We present a first principles molecular dynamics (FPMD) study of the interaction of low energy, positively charged, carbon (C+) projectiles with amorphous solid water clusters at 30 K. Reactions involving the carbon ion at an initial energy of 11 eV and 1.7 eV with 30-molecule clusters have been investigated. Simulations indicate that the neutral isoformyl radical, COH, and carbon monoxide, CO, are the dominant products of these reactions. All these reactions are accompanied by the transfer of a proton from the reacting water molecule to the ice, where it forms a hydronium ion. We find that COH is formed either via a direct, “knock-out”, mechanism following the impact of the C+ projectile upon a water molecule or by creation of a COH_2^+ intermediate. The direct mechanism is more prominent at higher energies. CO is generally produced following the dissociation of COH. More frequent production of the formyl radical, HCO, is observed here than in gas phase calculations. A less commonly occurring product is the dihydroxymethyl, CH(OH)_2, radical. Although a minor result, its existence gives an indication of the increasing chemical complexity which is possible in such heterogeneous environments.

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E. McBride, T. Millar and J. Kohanoff
Thu, 7 Aug 14
34/46

Comments: J Phys Chem A, in press, 24 pages, 6 figures

Coronene and pyrene (5, 7)-member ring defects: Infrared spectra, energetics and alternative formation pathways [SSA]

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http://arxiv.org/abs/1407.4662


PAHs are known to be one of the carriers of the ubiquitous aromatic IR bands. The IR spectra of many objects show IR emission features derived from PAH molecules of different size. Still some of the characteristics of the emitting population remain unclear. The emission bands show details which cannot be explained so far. These unidentified IR features require further laboratory and observational investigations. We present a theoretical study of the IR spectra of PAHs containing (5,7)-member ring defects, focusing on pyrene and coronene. Using density functional theory, we investigate the effects of such defects on the IR spectra of pyrene and coronene and their cations and anions. In addition, we explore parts of the potential energy surface of the neutral species and discuss alternative formation pathways. The addition of (5,7)-membered ring defects in pyrene and coronene results in a change of the IR spectra, both molecules lose their typical spectroscopic signature. We find shifts in the positions of the band as well as different intensities and a rise in the number of features. The boundaries in terms of the size of the PAHs exhibiting a (5,7)-membered ring defect are studied and shown. Investigation of the minimal energy pathway leads to a result of 8.21 eV for pyrene and 8.41 eV for coronene as minimum activation barriers for the transformation from ground state to defected state. Whereas pyrene retains some of its symmetry due to the symmetry exhibited by the Stone-Wales defect itself, coronene loses much more of its symmetry. The formation of these (5,7)-ring defects in PAHs may be well supported in AGB stars or PNe. Those environments strongly enable the transition from the ground state to the defect state. Therefore the knowledge of the IR spectra of these molecules will support future investigations aiming for a thorough understanding of the unidentified IR emission bands.

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S. Oettl, S. Huber, S. Kimeswenger, et. al.
Fri, 18 Jul 14
29/76

Comments: 8 pages, 9 figures, accepted for publication in A&A

Quantum reactive scattering of O($^3$P)+H$_2$ at collision energies up to 4.4 eV [CL]

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http://arxiv.org/abs/1407.3325


We report the results of quantum scattering calculations for the O($^3$P)+H$_2$ reaction for a range of collision energies from 0.4 to 4.4 eV, important for astrophysical and atmospheric processes. The total and state-to-state reactive cross sections are calculated using a fully quantum time-independent coupled-channel approach on recent potential energy surfaces of $^{3}A’$ and $^{3}A”$ symmetry. A larger basis set than in the previous studies was used to ensure convergence at higher energies. Our results agree well with the published data at lower energies and indicate the breakdown of reduced dimensionality approach at collision energies higher than 1.5 eV. Differential cross sections and momentum transfer cross sections are also reported.

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M. Gacesa and V. Kharchenko
Tue, 15 Jul 14
62/64

Comments: 11 pages, draft; submitted to JCP

PAHs with armchair edges and the 12.7 μm band [GA]

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http://arxiv.org/abs/1407.2010


In this Letter we report the results of Density Functional Theory (DFT) calculations on medium-sized neutral Polycyclic Aromatic Hydrocarbon (PAH) molecules with armchair edges. These PAH molecules possess strong C-H stretching and bending modes around 3 {\mu}m and in the fingerprint region (10-15 {\mu}m), and also strong ring deformation modes around 12.7 {\mu}m. Perusal of the entries in the NASA Ames PAHs Database (Bauschlicher et al. 2010) shows that ring deformation modes of PAHs are common – although generally weak. We then propose that armchair PAHs with NC >65 are responsible for the 12.7 {\mu}m Aromatic Infrared Band in HII regions and discuss astrophysical implications in the context of the PAH life-cycle.

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A. Candian, P. Sarre and A. Tielens
Wed, 9 Jul 14
33/74

Comments: 17 pages, 4 figures and 1 table, accepted for publication by ApJ Letters

Interactions of adsorbed CO$_2$ on water ice at low temperatures [GA]

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http://arxiv.org/abs/1406.6161


We present a computational study into the adsorption properties of CO$_2$ on amorphous and crystalline water surfaces under astrophysically relevant conditions. Water and carbon dioxide are two of the most dominant species in the icy mantles of interstellar dust grains and a thorough understanding of their solid phase interactions at low temperatures is crucial for understanding the structural evolution of the ices due to thermal segregation. In this paper, a new H$_2$O-CO$_2$ interaction potential is proposed and used to model the ballistic deposition of CO$_2$ layers on water ice surfaces, and to study the individual binding sites at low coverages. Contrary to recent experimental results, we do not observe CO$_2$ island formation on any type of water substrate. Additionally, density functional theory calculations are performed to assess the importance of induced electrostatic interactions.

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L. Karssemeijer, G. Wijs and H. Cuppen
Wed, 25 Jun 14
55/67

Comments: Accepted for publication in Physical Chemistry Chemical Physics

On the Validity of Collider-Mass Scaling for Molecular Rotational Excitation [GA]

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http://arxiv.org/abs/1406.5750


Rate coefficients for collisional processes such as rotational and vibrational excitation are essential inputs in many astrophysical models. When rate coefficients are unknown, they are often estimated using known values from other systems. The most common example is to use He-collider rate coefficients to estimate values for other colliders, typically H$_2$, using scaling arguments based on the reduced mass of the collision system. This procedure is often justified by the assumption that the inelastic cross section is independent of the collider. Here we explore the validity of this approach focusing on rotational inelastic transitions for collisions of H, para-H$_2$, $^3$He, and $^4$He with CO in its vibrational ground state. We compare rate coefficients obtained via explicit calculations to those deduced by standard reduced-mass scaling. Not surprisingly, inelastic cross sections and rate coefficients are found to depend sensitively on both the reduced mass and the interaction potential energy surface. We demonstrate that standard reduced-mass scaling is not valid on physical and mathematical grounds, and as a consequence, the common approach of multiplying a rate coefficient for a molecule-He collision system by the constant factor of ~1.4 to estimate the rate coefficient for para-H$_2$ collisions is deemed unreliable. Furthermore, we test an alternative analytic scaling approach based on the strength of the interaction potential and the reduced mass of the collision systems. Any scaling approach, however, may be problematic when low-energy resonances are present; explicit calculations or measurements of rate coefficients are to be preferred.

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K. Walker, B. Yang, P. Stancil, et. al.
Tue, 24 Jun 14
59/82

Comments: 23 pages, 8 figures. Accepted for publication in ApJ

Influence of surface coverage on the chemical desorption process [CL]

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http://arxiv.org/abs/1406.5594


In cold astrophysical environments, some molecules are observed in the gas phase whereas they should have been depleted, frozen on dust grains. In order to solve this problem, astrochemists have proposed that a fraction of molecules synthesized on the surface of dust grains could desorb just after their formation. Recently the chemical desorption process has been demonstrated experimentally, but the key parameters at play have not yet been fully understood. In this article we propose a new procedure to analyze the ratio of di-oxygen and ozone synthesized after O atoms adsorption on oxidized graphite. We demonstrate that the chemical desorption efficiency of the two reaction paths (O+O and O+O$_2$) is different by one order of magnitude. We show the importance of the surface coverage: for the O+O reaction, the chemical desorption efficiency is close to 80 $\%$ at zero coverage and tends to zero at one monolayer coverage. The coverage dependence of O+O chemical desorption is proved by varying the amount of pre-adsorbed N$_2$ on the substrate from 0 to 1.5 ML. Finally, we discuss the relevance of the different physical parameters that could play a role in the chemical desorption process: binding energy, enthalpy of formation, and energy transfer from the new molecule to the surface or to other adsorbates.

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M. Marco and D. Francois
Tue, 24 Jun 14
73/82

Comments: 20 pages, 8 figure, in press in JCP July 2014, Volume 141, Issue 2

The fast C(3P) + CH3OH reaction as an efficient loss process for gas-phase interstellar methanol [GA]

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http://arxiv.org/abs/1406.1363


Rate constants for the C(3P) + CH3OH reaction have been measured in a continuous supersonic flow reactor over the range 50 K to 296 K. C(3P) was created by the in-situ pulsed laser photolysis of CBr4, a multiphoton process which also produced some C(1D), allowing us to investigate simultaneously the low temperature kinetics of the C(1D) + CH3OH reaction. C(1D) atoms were followed by an indirect chemiluminescent tracer method in the presence of excess CH3OH. C(3P) atoms were detected by the same chemiluminescence technique and also by direct vacuum ultra-violet laser induced fluorescence (VUV LIF). Secondary measurements of product H(2S) atom formation have been undertaken allowing absolute H atom yields to be obtained by comparison with a suitable reference reaction. In parallel, statistical calculations have been performed based on ab-initio calculations of the complexes, adducts and transition states (TSs) relevant to the title reaction. By comparison with the experimental H atom yields, the preferred reaction pathways could be determined, placing important constraints on the statistical calculations. The experimental and theoretical work are in excellent agreement, predicting a negative temperature dependence of the rate constant increasing from 2.2 x 10-11 cm3 molecule-1 s-1 at 296 K to 20.0 x 10-11 cm3 molecule-1 s-1 at 50 K. CH3 and HCO are found to be the major products under our experimental conditions. As this reaction is not considered in current astrochemical networks, its influence on interstellar methanol abundances is tested using a model of dense interstellar clouds.

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R. Shannon, C. Cossou, J. Loison, et. al.
Fri, 6 Jun 14
8/69

Comments: Accepted to RSC Advances

An experimental and theoretical investigation of the N + C2 reaction at low temperature [GA]

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http://arxiv.org/abs/1406.1374


Rate constants for the N + C2 reaction have been measured in a continuous supersonic flow reactor over the range 57 K to 296 K by the relative rate technique employing the N + OH – H + NO reaction as a reference. Excess concentrations of atomic nitrogen were produced by the microwave discharge method and C2 and OH radicals were created by the in-situ pulsed laser photolysis of precursor molecules C2Br4 and H2O2 respectively. In parallel, quantum dynamics calculations were performed based on an accurate global potential energy surfaces for the three lowest lying quartet states of the C2N molecule. The 14A” potential energy surface is barrierless, having two deep potential wells corresponding to the NCC and CNC intermediates. Both the experimental and theoretical work show that the rate constant decreases to low temperature, although the experimentally measured values fall more rapidly than the theoretical ones except at the lowest temperatures. Astrochemical simulations indicate that this reaction could be the dominant source of CN in dense interstellar clouds.

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J. Loison, X. Hu, S. Han, et. al.
Fri, 6 Jun 14
65/69

Comments: Accepted to Physical Chemistry Chemical Physics

Vacuum-UV spectroscopy of interstellar ice analogs. II. Absorption cross-sections of nonpolar ice molecules [SSA]

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http://arxiv.org/abs/1405.7802


Dust grains in cold circumstellar regions and dark-cloud interiors at 10-20 K are covered by ice mantles. A nonthermal desorption mechanism is invoked to explain the presence of gas-phase molecules in these environments, such as the photodesorption induced by irradiation of ice due to secondary ultraviolet photons. To quantify the effects of ice photoprocessing, an estimate of the photon absorption in ice mantles is required. In a recent work, we reported the vacuum-ultraviolet (VUV) absorption cross sections of nonpolar molecules in the solid phase. The aim was to estimate the VUV-absorption cross sections of nonpolar molecular ice components, including CH4, CO2, N2, and O2. The column densities of the ice samples deposited at 8 K were measured in situ by infrared spectroscopy in transmittance. VUV spectra of the ice samples were collected in the 120-160 nm (10.33-7.74 eV) range using a commercial microwave-discharged hydrogen flow lamp. We found that, as expected, solid N2 has the lowest VUV-absorption cross section, which about three orders of magnitude lower than that of other species such as O2, which is also homonuclear. Methane (CH4) ice presents a high absorption near Ly-alpha (121.6 nm) and does not absorb below 148 nm. Estimating the ice absorption cross sections is essential for models of ice photoprocessing and allows estimating the ice photodesorption rates as the number of photodesorbed molecules per absorbed photon in the ice.

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G. Cruz-Diaz, G. Caro, Y. Chen, et. al.
Mon, 2 Jun 14
11/56

Comments: 9 pages, 6 figures, 7 tables

Vacuum-UV absorption spectroscopy of interstellar ice analogues. III. Isotopic effects [SSA]

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http://arxiv.org/abs/1405.7839


This paper reports the first measurements of solid-phase vacuum-ultraviolet (VUV) absorption cross-sections of heavy isotopologues present in icy dust grain mantles of dense interstellar clouds and cold circumstellar environments. Pure ices composed of D2O, CD3OD, 13CO2, and 15N15N were deposited at 8 K, a value similar to the coldest dust temperatures in space. The column density of the ice samples was measured in situ by infrared spectroscopy in transmittance. VUV spectra of the ice samples were collected in the 120-160 nm (10.33-7.74 eV) range using a commercial microwave discharged hydrogen flow lamp as the VUV source. Prior to this work, we have recently submitted a similar study of the light isotopologues (Cruz-Diaz, Mu\~noz Caro and Chen). The VUV spectra are compared to those of the light isotopologues in the solid phase, and to the gas phase spectra of the same molecules. Our study is expected to improve very significantly the models that estimate the VUV absorption of ice mantles in space, which have often used the available gas phase data as an approximation of the absorption cross sections of the molecular ice components. We will show that this work has also important implications for the estimation of the photodesorption rates per absorbed photon in the ice.

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G. Cruz-Diaz, G. Caro and Y. Chen
Mon, 2 Jun 14
14/56

Comments: 7 pages, 4 figures, 3 tables

Vacuum-UV spectroscopy of interstellar ice analogs. I. Absorption cross-sections of polar-ice molecules [SSA]

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http://arxiv.org/abs/1405.7797


The VUV absorption cross sections of most molecular solids present in interstellar ice mantles with the exception of H2O, NH3, and CO2 have not been reported yet. Models of ice photoprocessing depend on the VUV absorption cross section of the ice to estimate the penetration depth and radiation dose, and in the past, gas phase cross section values were used as an approximation. We aim to estimate the VUV absorption cross section of molecular ice components. Pure ices composed of CO, H2O, CH3OH, NH3, or H2S were deposited at 8 K. The column density of the ice samples was measured in situ by infrared spectroscopy in transmittance. VUV spectra of the ice samples were collected in the 120-160 nm (10.33-7.74 eV) range using a commercial microwave-discharged hydrogen flow lamp. We provide VUV absorption cross sections of the reported molecular ices. Our results agree with those previously reported for H2O and NH3 ices. Vacuum-UV absorption cross section of CH3OH, CO, and H2S in solid phase are reported for the first time. H2S presents the highest absorption in the 120-160 nm range. Our method allows fast and readily available VUV spectroscopy of ices without the need to use a synchrotron beamline. We found that the ice absorption cross sections can be very different from the gas-phase values, and therefore, our data will significantly improve models that simulate the VUV photoprocessing and photodesorption of ice mantles. Photodesorption rates of pure ices, expressed in molecules per absorbed photon, can be derived from our data.

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G. Cruz-Diaz, G. Caro, Y. Chen, et. al.
Mon, 2 Jun 14
53/56

Comments: 10 pages

OH+ in astrophysical media: state-to-state formation rates, Einstein coefficients and inelastic collision rates with He [SSA]

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http://arxiv.org/abs/1405.4173


The rate constants required to model the OH$^+$ observations in different regions of the interstellar medium have been determined using state of the art quantum methods.
First, state-to-state rate constants for the H$_2(v=0,J=0,1)$+ O$^+$($^4S$) $\rightarrow$ H + OH$^+(X ^3\Sigma^-, v’, N)$ reaction have been obtained using a quantum wave packet method. The calculations have been compared with time-independent results to asses the accuracy of reaction probabilities at collision energies of about 1 meV. The good agreement between the simulations and the existing experimental cross sections in the $0.01-$1 eV energy range shows the quality of the results.
The calculated state-to-state rate constants have been fitted to an analytical form. Second, the Einstein coefficients of OH$^+$ have been obtained for all astronomically significant ro-vibrational bands involving the $X^3\Sigma^-$ and/or $A^3\Pi$ electronic states.
For this purpose the potential energy curves and electric dipole transition moments for seven electronic states of OH$^+$ are calculated with {\it ab initio} methods at the highest level and including spin-orbit terms, and the rovibrational levels have been calculated including the empirical spin-rotation and spin-spin terms. Third, the state-to-state rate constants for inelastic collisions between He and OH$^+(X ^3\Sigma^-)$ have been calculated using a time-independent close coupling method on a new potential energy surface. All these rates have been implemented in detailed chemical and radiative transfer models. Applications of these models to various astronomical sources show that inelastic collisions dominate the excitation of the rotational levels of OH$^+$. In the models considered the excitation resulting from the chemical formation of OH$^+$ increases the line fluxes by about 10 % or less depending on the density of the gas.

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S. Gomez-Carrasco, B. Godard, F. Lique, et. al.
Mon, 19 May 14
40/45

Comments: N/A

Ion-molecule reactions involving HCO$^+$ and N$_2$H$^+$: Isotopologue equilibria from new theoretical calculations and consequences for interstellar isotope fractionation [GA]

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http://arxiv.org/abs/1404.5487


$Aims$: We revisit with new augmented accuracy the theoretical dynamics of basic isotope exchange reactions involved in the $^{12}$C/$^{13}$C, $^{16}$O/$^{18}$O, and $^{14}$N/$^{15}$N balance because these reactions have already been studied experimentally in great detail. $Methods$: Electronic structure methods were employed to explore potential energy surfaces, full-dimensional rovibrational calculations to compute rovibrational energy levels that are numerically exact, and chemical network models to estimate the abundance ratios under interstellar conditions. $Results$: New exothermicities, derived for HCO$^+$ reacting with CO, provide rate coefficients markedly different from previous theoretical values in particular at low temperatures, resulting in new abundance ratios relevant for carbon chemistry networks. In concrete terms, we obtain a reduction in the abundance of H$^{12}$C$^{18}$O$^+$ and an increase in the abundance of H$^{13}$C$^{16}$O$^+$ and D$^{13}$C$^{16}$O$^+$. In all studied cases, the reaction of the ion with a neutral polarizable molecule proceeds through the intermediate proton-bound complex found to be very stable. For the complexes OCH$^+$…CO, OCH$^+$…OC, COHOC$^+$, N$_2$…HCO$^+$, N$_2$H$^+$…OC, and N$_2$HN$_2^+$, we also calculated vibrational frequencies and dissociation energies. $Conclusions$: The linear proton-bound complexes possess sizeable dipole moments, which may facilitate their detection.

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M. Mladenovic and E. Roueff
Wed, 23 Apr 14
19/46

Classical scattering in strongly attractive potentials [CL]

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http://arxiv.org/abs/1403.6000


Scattering in central attractive potentials is investigated systematically, in the limit of strong interaction, when large-angles scattering dominates. In particular, three important model interactions (Lennard-Jones, Yukawa, and exponential), which are qualitatively different from each other, are studied in detail. It is shown that for each of these interactions the dependence of the scattering angle on the properly normalized impact parameter exhibits a quasi-universal behavior. This implies simple scaling of the transport cross sections with energy in the considered limit. Accurate fits for the momentum transfer cross section are suggested. Applications of the obtained results are discussed.

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S. Khrapak
Tue, 25 Mar 14
66/79