Tag Archives: Chemical Physics

The Case of H$_2$C$_3$O Isomers, Revisited: Solving the Mystery of the Missing Propadienone [GA]

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http://arxiv.org/abs/1904.11396


To date, two isomers of H$_2$C$_3$O have been detected, namely, propynal (HCCCHO) and cylclopropenone (c-H$_2$C$_3$O). A third, propadienone (CH$_2$CCO), has thus far eluded observers despite the fact that it is the lowest in energy of the three. This previously noted result is in contradiction of the minimum energy principle, which posits that the abundances of isomers in interstellar environments can be predicted based on their relative stabilities – and suggests, rather, the importance of kinetic over thermodynamic effects in explaining the role of such species.
Here, we report results of \textit{ab initio} quantum chemical calculations of the reaction between H and (a) HC$_3$O, (b) H$_2$C$_3$O (both propynal and propadienone), and (c) CH$_2$CHCO. We have found that, among all possible reactions between atomic hydrogen and either propadienone or propynal, only the destruction of propadienone is barrierless and exothermic. That this destruction pathway is indeed behind the non-detection of CH$_2$CCO is further suggested by our finding that the product of this process, the radical CH$_2$CHCO, can subsequently react barrierlessly with H to form propenal (CH$_2$CHCHO) which has, in fact, been detected in regions where the other two H$_2$C$_3$O isomers are observed. Thus, these results not only shed light on a previously unresolved astrochemical mystery, but also further highlight the importance of kinetics in understanding the abundances of interstellar molecules.

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C. Shingledecker, S. Álvarez-Barcia, V. Korn, et. al.
Fri, 26 Apr 19
24/69

Comments: ApJ, accepted: 14 pages, 2 figures

Graphene Molecule Compared With Fullerene C60 As Circumstellar Carbon Dust Of Planetary Nebula [GA]

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http://arxiv.org/abs/1904.10140


It had been understood that astronomically observed infrared spectrum of carbon rich planetary nebula as like Tc 1 and Lin 49 comes from fullerene (C60). Also, it is well known that graphene is a raw material for synthesizing fullerene. This study seeks some capability of graphene based on the quantum-chemical DFT calculation. It was demonstrated that graphene plays major role rather than fullerene. We applied two astrophysical conditions, which are void creation by high speed proton and photo-ionization by the central star. Model molecule was ionized void-graphene (C23) having one carbon pentagon combined with hexagons. By molecular vibrational analysis, we could reproduce six major bands from 6 to 9 micrometer, large peak at 12.8, and largest peak at 19.0. Also, many minor bands could be reproduced from 6 to 38 micrometer. Also, deeply void induced molecules (C22) and (C21) could support observed bands.

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N. Ota
Wed, 24 Apr 19
48/51

Comments: 9 pages, 7 figures

Dicarbon formation in collisions of two carbon atoms [CEA]

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http://arxiv.org/abs/1904.07831


Radiative association cross sections and rates are computed, using a quantum approach, for the formation of C$_2$ molecules (dicarbon) during the collision of two ground state C($^3$P) atoms. We find that transitions originating in the C$\;^1\Pi_g$, d$\;^3\Pi_g$, and 1$\;^5\Pi_u$ states are the main contributors to the process. The results are compared and contrasted with previous results obtained from a semi-classical approximation. New ab initio potential curves and transition dipole moment functions have been obtained for the present work using the multi-reference configuration interaction approach with the Davidson correction (MRCI+Q) and aug-cc-pCV5Z basis sets, substantially increasing the available molecular data on dicarbon. Applications of the current computations to various astrophysical environments and laboratory studies are briefly discussed focusing on these rates.

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J. Babb, R. Smyth and B. McLaughlin
Wed, 17 Apr 19
41/75

Comments: Accepted ApJ

Dispersion forces stabilise ice coatings at certain gas hydrate interfaces which prevent water wetting [EPA]

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http://arxiv.org/abs/1904.06557


Gas hydrates formed in oceans and permafrost occur in vast quantities on Earth representing both a massive potential fuel source and a large threat in climate forecasts. They have been predicted to be important on other bodies in our solar systems such as Enceladus, a moon of Saturn. CO$_2$-hydrates likely drive the massive gas-rich water plumes seen and sampled by the spacecraft Cassini, and the source of these hydrates is thought to be due to buoyant gas hydrate particles. Dispersion forces cause gas hydrates to be coated in a 3-4 nm thick film of ice, or to contact water directly, depending on which gas they contain. These films are shown to significantly alter the properties of the gas hydrate clusters, for example, whether they float or sink. It is also expected to influence gas hydrate growth and gas leakage.

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M. Boström, R. Corkery, E. Lima, et. al.
Tue, 16 Apr 19
64/88

Comments: N/A

Formation of Acetaldehyde on CO-rich Ices [GA]

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http://arxiv.org/abs/1904.06112


The radicals HCO and CH$_3$ on carbon monoxide ice surfaces were simulated using density functional theory. Their binding energy on amorphous CO ice shows broad distributions, with approximative average values of 500 K for HCO and 200 K for CH$_3$. If they are located on the surface close to each other (3 to 4 \AA), molecular dynamics calculations based on density functional theory show that they can form acetaldehyde (CH$_3$CHO) or CH$_4$ + CO in barrier-less reactions, depending on the initial orientation of the molecules with respect to each other. In some orientations, no spontaneous reactions were found, the products remained bound to the surface. Sufficient configurational sampling, inclusion of the vibrational zero point energy, and a thorough benchmark of the applied electronic structure method are important to predict reliable binding energies for such weakly interacting systems. From these results it is clear that complex organic molecules, like acetaldehyde, can be formed by recombination reactions of radicals on CO surfaces.

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T. Lamberts, M. Markmeyer, F. Kolb, et. al.
Mon, 15 Apr 19
25/61

Comments: Accepted in ACS Earth and Space Chemistry

On Simulating the Proton-Irradiation of O$_2$ and H$_2$O Ices Using Astrochemical-type Models, with Implications for Bulk Reactivity [GA]

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http://arxiv.org/abs/1904.04143


Many astrochemical models today explicitly consider the species that comprise the bulk of interstellar dust grain ice-mantles separately from those in the top few monolayers. Bombardment of these ices by ionizing radiation – whether in the form of cosmic rays, stellar winds, or radionuclide emission – represents an astrochemically viable means of driving a rich chemistry even in the bulk of the ice-mantle, now supported by a large body of work in laboratory astrophysics. In this study, using an existing rate equation-based astrochemical code modified to include a method of considering radiation chemistry recently developed by us, we attempted to simulate two such studies in which (a) pure O$_2$ ice at 5 K and, (b) pure H$_2$O ice at 16 K and 77 K, were bombarded by keV H$^+$ ions.
Our aims are twofold: (1) to test the capability of our newly developed method to replicate the results of ice-irradiation experiments, and (2) to determine in such a well-constrained system how bulk chemistry is best handled using the same gas-grain codes that are used to model the interstellar medium (ISM). We find that our modified astrochemical model is able to reproduce both the abundance of O$_3$ in the 5 K pure O$_2$ ice, as well as both the abundance of H$_2$O$_2$ in the 16 K water ice and the previously noted decrease of hydrogen peroxide at higher temperatures. However, these results require the assumption that radicals and other reactive species produced via radiolysis react quickly and non-diffusively with neighbors in the ice.

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C. Shingledecker, A. Vasyunin, E. Herbst, et. al.
Tue, 9 Apr 19
82/105

Comments: ApJ, accepted. 30 pages, 5 figures

Experimental and theoretical studies of D + H$_3^+$ $\rightarrow$ H$_2$D$^+$ + H [IMA]

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http://arxiv.org/abs/1904.02955


Deuterated molecules are important chemical tracers of prestellar and protostellar cores. Up to now, the titular reaction has been assumed to contribute to the generation of these deuterated molecules. We have measured the merged-beams rate coefficient for this reaction as function of the relative collision energy in the range of about 10 meV to 10 eV. By varying the internal temperature of the reacting H$_3^+$ molecules, we found indications for the existence of a reaction barrier. We have performed detailed theoretical calculations for the zero-point-corrected energy profile of the reaction and determined a new value for the barrier height of $\approx$ 68 meV. Furthermore, we have calculated the tunneling probability through the barrier. Our experimental and theoretical results show that the reaction is essentially closed at astrochemically relevant temperatures. We derive a thermal rate coefficient of $<1\times 10^{-12}$ cm$^3$ s$^{-1}$ for temperatures below 75 K with tunneling effects included and below 155 K without tunneling.

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P. Hillenbrand, K. Bowen, J. Liévin, et. al.
Mon, 8 Apr 19
25/45

Comments: Accepted for publication in ApJ; 21 pages, 7 figures, and 5 table

Vibrational dynamics of phenylphenanthrenes with phenyl group at different positions [CL]

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http://arxiv.org/abs/1903.10397


Polycyclic aromatic hydrocarbons (PAHs) appear to be ubiquitous in terrestrial atmosphere as well as in the interstellar medium (ISM). In the astrophysical context presence of PAHs is interpreted by the observations of mid-IR emission bands at 3.3, 6.2, 7.7, 8.6, 11.2 and 12.6 micron (3030, 1610, 1300, 1160, 890 and 790~cm$^{-1}$). The profile of these emission bands exhibit definite variations from source to source, which are explained due to sources having different types of PAHs in different ionization states. The model spectra of mixtures of PAHs show good fit for most bands except for the 6.2 micron feature. Thus a wider variety of PAHs need to be considered for this feature. PAH formation in circumstellar medium of planetary nebulae and dense clouds indicate formation of phenyl radical and phenyl substituted PAHs. To understand the modifications in infrared lineaments upon phenyl substitution and also due to substitution at different positions, study is made on five possible isomers of phenylphenanthrene. The features that may account for possible phenylphenanthrenes in protoplanetary nebulae atmospheres are also discussed. Possible contribution of phenylphenanthrenes towards the 6.2 micron feature and other astrophysical emission bands is assessed.

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R. Singh and S. Rastogi
Tue, 26 Mar 19
24/72

Comments: 8 figures and 8 tables

The CO2-broadened H2O continuum in the 100-1500 cm-1 region. Measurements, predictions and empirical model [EPA]

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http://arxiv.org/abs/1903.08972


Transmission spectra of H$_2$O+CO$_2$ mixtures have been recorded, at 296, 325 and 366 K, for various pressures and mixture compositions using two experimental setups. Their analysis enables to retrieve values of the ‘continuum’ absorption by the CO$_2$-broadened H$_2$O line wings between 100 and 1500 cm$^{-1}$. The results are in good agreement with those, around 1300 cm$^{-1}$, of the single previous experimental study available. Comparisons are also made with direct predictions based on line-shape correction factors $\chi$ calculated, almost thirty years ago, using a quasistatic approach and an input H$_2$O-CO$_2$ intermolecular potential. They show that this model quite nicely predicts, with slightly overestimated values, the continuum over a spectral range where it varies by more than three orders of magnitude. An empirical correction is proposed, based on the experimental data, which should be useful for radiative transfer and climate studies in CO$_2$ rich planetary atmospheres.

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H. Tran, M. Turbet, S. Hanoufa, et. al.
Fri, 22 Mar 19
3/70

Comments: Accepted for publication in the Journal of Quantitative Spectroscopy & Radiative Transfer (JQSRT); 11 pages and 3 figures

Emissivity of Ammonia Ice [EPA]

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http://arxiv.org/abs/1903.04960


Bodies in the outer Solar System may consist of, or be covered by, solid ammonia. Their temperature depends on its emissivity. We calculate that emissivity as a function of frequency, angle and polarization and convolve it with a Planck function to obtain its integrated emissivity.

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S. Wang and J. Katz
Wed, 13 Mar 19
41/125

Comments: 7 pp., 3 figs

New spectral characterization of dimethyl ether isotopologues CH$_3$OCH$_3$ and $^{13}$CH$_3$OCH$_3$ in the THz region [GA]

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http://arxiv.org/abs/1902.06444


The torsional Raman spectra of two astrophysically detected isotopologues of dimethyl-ether, ($^{12}$CH$3$O$^{12}$CH$_3$ and $^{13}$CH$_3$O$^{12}$CH$_3$), have been recorded at room temperature and cooled in supersonic jet, and interpreted with the help of highly correlated ab initio calculations. Dimethyl-ether displays excited torsional and vibrational levels at low energy that can be populated at the temperatures of the star forming regions, obliging to extend the analysis of the rotational spectrum over the ground state. Its spectrum in the THz region is rather complex due to the coupling of the torsional overtones $2\nu{11}$ and $2\nu_{15}$ with the COC bending mode, and the presence of many hot bands. The torsional overtones are set here at $2\nu_{11}=385.2$~cm$^{-1}$ and $2\nu_{15}=482.0$~cm$^{-1}$ for $^{12}$CH$3$O$^{12}$CH$_3$, and $2\nu{11}=385.0$~cm$^{-1}$ and $2\nu_{15}=481.1$~cm$^{-1}$ for $^{13}$CH$3$O$^{12}$CH$_3$. The new assignment of $2\nu{11}$ is downshifted around $\sim 10$~cm$^{-1}$ with respect to the literature. All the other (hot) bands have been re-assigned consistently. In addition, the infrared-forbidden torsional fundamental band $\nu_{11}$ is observed here at 197.8~cm$^{-1}$. The new spectral characterization in the THz region reported here provides improved values of the Hamiltonian parameters, to be used in the analysis of the rotational spectra of DME isotopologues for further astrophysical detections.

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J. Fernández, G. Tejeda, M. Carvajal, et. al.
Tue, 19 Feb 19
39/57

Comments: Accepted for publication in The Astrophysical Journal Supplement Series

A Consistent Reduced Network for HCN Chemistry in Early Earth and Titan Atmospheres: Quantum Calculations of Reaction Rate Coefficients [EPA]

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http://arxiv.org/abs/1902.05574


HCN is a key ingredient for synthesizing biomolecules such as nucleobases and amino acids. We calculate 42 reaction rate coefficients directly involved with or in competition with the production of HCN in the early Earth or Titan atmospheres. These reactions are driven by methane and nitrogen radicals produced via UV photodissociation or lightning. For every reaction in this network, we calculate rate coefficients at 298 K using canonical variational transition state theory (CVT) paired with computational quantum chemistry simulations at the BHandHLYP/augcc-pVDZ level of theory. We also calculate the temperature dependence of the rate coefficients for the reactions that have barriers from 50 to 400 K. We present 15 new reaction rate coefficients with no previously known value; 93% of our calculated coefficients are within an order of magnitude of the nearest experimental or recommended values. Above 320 K, the rate coefficient for the new reaction H2CN -> HCN + H dominates. Contrary to experiments, we find the HCN reaction pathway, N + CH3 -> HCN + H2, to be inefficient and suggest that the experimental rate coefficient actually corresponds to an indirect pathway, through the H2CN intermediate. We present CVT using energies computed with density functional theory as a feasible and accurate method for calculating a large network of rate coefficients of small-molecule reactions.

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B. Pearce, P. Ayers and R. Pudritz
Mon, 18 Feb 19
20/37

Comments: 34 pages, 8 figures, 14 tables, accepted for publication in J Phys Chem A

Searching for stable fullerenes in space with computational chemistry [GA]

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http://arxiv.org/abs/1902.03090


We report a computational study of the stability and infrared (IR) vibrational spectra of neutral and singly ionised fullerene cages containing between 44 and 70 carbon atoms. The stability is characterised in terms of the standard enthalpy of formation per CC bond, the HOMO-LUMO gap, and the energy required to eliminate a C$2$ fragment. We compare the simulated IR spectra of these fullerene species to the observed emission spectra of several planetary nebulae (Tc 1, SMP SMC 16, and SMP LMC 56) where strong C${60}$ emission has been detected. Although we could not conclusively identify fullerenes other than C${60}$ and C${70}$, our results point to the possible presence of smaller (44, 50, and 56-atom) cages in those astronomical objects. Observational confirmation of our prediction should become possible when the James Webb Space Telescope comes online.

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A. Candian, M. Rachid, H. MacIsaac, et. al.
Mon, 11 Feb 19
17/51

Comments: 11 pages, 13 figures, 1 table. Accepted for publication on MNRAS

Quantum mechanical study of the high-temperature $\mathrm{H}^+ + \mathrm{HD} \to \mathrm{D}^+ + \mathrm{H}_2$ reaction for the primordial universe chemistry [CEA]

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http://arxiv.org/abs/1902.01759


We use the time-independent quantum-mechanical formulation of reactive collisions in order to investigate the state-to-state $\mathrm{H}^+ + \mathrm{HD} \to \mathrm{D}^+ + \mathrm{H}_2$ chemical reaction. We compute cross sections for collision energies up to 1.8 electron-volts and rate coefficients for temperatures up to 10000 kelvin. We consider HD in the lowest vibrational level $v=0$ and rotational levels $j=0$ to 4, and H$_2$ in vibrational levels $v’=0$ to 3 and rotational levels $j’=0$ to 9. For temperatures below 4000 kelvin, the rate coefficients strongly vary with the initial rotational level $j$, depending on whether the reaction is endothermic ($j\le 2$) or exothermic ($j\ge 3$). The reaction is also found less and less probable as the final vibrational quantum number $v’$ increases. Our results illustrate the importance of studying state-to-state reactions, in the context of the chemistry of the primordial universe.

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M. Lepers, G. Guillon and P. Honvault
Wed, 6 Feb 19
54/65

Comments: 7 pages, 9 figures

Photodetachment and Photoreactions of Substituted Naphthalene Anions in a Tandem Ion Mobility Spectrometer [IMA]

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http://arxiv.org/abs/1902.00890


Substituted naphthalene anions (deprotonated 2-naphthol and 6-hydroxy-2-naphthoic acid) are spectroscopically probed in a tandem drift tube ion mobility spectrometer (IMS). Target anions are selected according to their drift speed through nitrogen buffer gas in the first IMS stage before being exposed to a pulse of tunable light that induces either photodissociation or electron photodetachment, which is conveniently monitored by scavenging the detached electrons with trace \ce{SF6} in the buffer gas. The photodetachment action spectrum of the 2-naphtholate anion exhibits a band system spanning 380-460\,nm~with a prominent series of peaks spaced by 440\,\cme, commencing at 458.5\,nm, and a set of weaker peaks near the electron detachment threshold corresponding to transitions to dipole-bound states. The two deprotomers of 6-hydroxy-2-naphthoic acid are separated and spectroscopically probed independently. The molecular anion formed from deprotonation of the hydroxy group possesses a photodetachment action spectrum similar to that of the 2-naphtholate anion with an onset at 470\,nm and a maximum at 420\,nm. Near threshold, photoreaction with \ce{SF6} is observed with displacement of an OH group by an F atom. In contrast, the anion formed from deprotonation of the carboxylic acid group features a photodissociation action spectrum, recorded on the \ce{CO2} loss channel, lying to much shorter wavelength with an onset at 360\,nm and maximum photoresponse at 325\,nm.

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J. Bull, J. Buntine, M. Scholz, et. al.
Tue, 5 Feb 19
30/86

Comments: N/A

Laboratory photo-chemistry of covalently bonded fluorene clusters: observation of an interesting PAH bowl-forming mechanism [IMA]

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http://arxiv.org/abs/1901.03325


The fullerene C${60}$, one of the largest molecules identified in the interstellar medium (ISM), has been proposed to form top-down through the photo-chemical processing of large (more than 60 C-atoms) polycyclic aromatic hydrocarbon (PAH) molecules. In this article, we focus on the opposite process, investigating the possibility that fullerenes form from small PAHs, in which bowl-forming plays a central role. We combine laboratory experiments and quantum chemical calculations to study the formation of larger PAHs from charged fluorene clusters. The experiments show that with visible laser irradiation, the fluorene dimer cation – [C${13}$H${9}$$-$C${13}$H${9}$]$^+$ – and the fluorene trimer cation – [C${13}$H${9}$$-$C${13}$H${8}$$-$C${13}$H${9}$]$^+$ – undergo photo-dehydrogenation and photo-isomerization resulting in bowl structured aromatic cluster-ions, C${26}$H${12}$$^+$ and C${39}$H$_{20}$$^+$, respectively. To study the details of this chemical process, we employ quantum chemistry that allows us to determine the structures of the newly formed cluster-ions, to calculate the hydrogen loss dissociation energies, and to derive the underlying reaction pathways. These results demonstrate that smaller PAH clusters (with less than 60 C-atoms) can convert to larger bowled geometries that might act as building blocks for fullerenes, as the bowl-forming mechanism greatly facilitates the conversion from dehydrogenated PAHs to cages. Moreover, the bowl-forming induces a permanent dipole moment that – in principle – allows to search for such species using radio astronomy.

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W. Zhang, Y. Si, J. Zhen, et. al.
Mon, 14 Jan 19
12/43

Comments: 8 pages, 7 figures, accepted

Laboratory spectroscopic study of the $^{15}$N isotopomers of cyanamide, H$_2$NCN, and a search for them toward IRAS 16293$-$2422 B [SSA]

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http://arxiv.org/abs/1901.00421


Cyanamide is one of the few interstellar molecules containing two chemically different N atoms. It was detected recently toward the solar-type protostar IRAS 16293-2422 B together with H$_2$N$^{13}$CN and HDNCN in the course of the Atacama Large Millemeter/submillimeter Array (ALMA) Protostellar Interferometric Line Survey (PILS). The detection of the 15N isotopomers or the determination of upper limits to their column densities was hampered by the lack of accurate laboratory data at the frequencies of the survey. We wanted to determine spectroscopic parameters of the $^{15}$N isotopomers of cyanamide that are accurate enough for predictions well into the submillimeter region and to search for them in the PILS data. We investigated the laboratory rotational spectra of H$_2^{15}$NCN and H$_2$NC$^{15}$N in the selected region between 192 and 507~GHz employing a cyanamide sample in natural isotopic composition. Additionally, we recorded transitions of H$_2$N$^{13}$CN. We obtained new or improved spectroscopic parameters for the three isotopic species. Neither of the $^{15}$N isotopomers of cyanamide were detected unambiguously in the PILS data. Two relatively clean lines can be tentatively assigned to H$_2^{15}$NCN. If confirmed, their column densities would imply a low $^{14}$N/$^{15}$N ratio for cyanamide toward this source. The resulting line lists should be accurate enough for observations up to about 1 THz. More sensitive observations, potentially at different frequencies, may eventually lead to the astronomical detection of these isotopic species.

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A. Coutens, O. Zakharenko, F. Lewen, et. al.
Thu, 3 Jan 19
42/48

Comments: Astron. Astrophys., accepted; 8 pages

ExoMol line lists XXXI: Spectroscopy of lowest eights electronic states of C$_2$ [SSA]

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http://arxiv.org/abs/1812.07116


Accurate line lists for the carbon dimer, C$2$, are presented. These line lists cover rovibronic transitions between the eight lowest electronic states: $X\,{}^{1}\Sigma{g}^{+}$, $a\,{}^{3}\Pi_{u}$, $A\,{}^{1}\Pi_{u}$, $b\,^{3}\Sigma_{g}^{-}$, $b\,^{3}\Sigma_{g}^{-}$, $c\,^{3}\Sigma_{u}^{+}$, $d\,{}^{3}\Pi_{g}$, $B\,{}^{1}\Delta_{g}$, $B^\prime\,{}^{1}\Sigma_{g}^{+}$. Potential energy curves (PECs) and transition dipole moment curves are computed on a large grid of geometries using the aug-cc-pwCVQZ-DK/MRCI level of theory including core and core-valence correlations and scalar relativistic energy corrections. The same level of theory is used to compute spin-orbit and electronic angular momentum couplings. The PECs and couplings are refined by fitting to the empirical (MARVEL) energies of $^{12}$C$_2$ using the nuclear-motion program Duo. The transition dipole moment curves are represented as analytical functions to reduce the numerical noise when computing transition line strengths. Partition functions, full line lists, Land\'{e}-factors and lifetimes for three main isotopologues of C$_2$ ($^{12}$C$_2$,$^{13}$C$_2$ and $^{12}$C$^{13}$C) are made available in electronic form from the CDS (this http URL) and ExoMol (www.exomol.com) databases.

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S. Yurchenko, I. Szabó, E. Pyatenko, et. al.
Wed, 19 Dec 18
63/84

Comments: N/A

Deuterated methyl mercaptan (CH$_3$SD): Laboratory rotational spectroscopy and search toward IRAS 16293-2422 B [SSA]

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http://arxiv.org/abs/1812.02454


Methyl mercaptan (also known as methanethiol), CH3SH, has been found in the warm and dense parts of high — as well as low — mass star-forming regions. The aim of the present study is to obtain accurate spectroscopic parameters of the S-deuterated methyl mercaptan CH$_3$SD to facilitate astronomical observations by radio telescope arrays at (sub)millimeter wavelengths. We have measured the rotational spectrum associated with the large-amplitude internal rotation of the methyl group of methyl mercaptan using an isotopically enriched sample in the 150-510 GHz frequency range using the K\”oln millimeter wave spectrometer. The analysis of the spectra has been performed up to the second excited torsional state. We present modeling results of these data with the RAM36 program. CH$_3$SD was searched for, but not detected, in data from the Atacama Large Millimeter/submillimeter Array (ALMA) Protostellar Interferometric Line Survey (PILS) of the deeply embedded protostar IRAS 16293-2422. The derived upper limit corresponds to a degree of deuteration of at most ~18%.

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O. Zakharenko, F. Lewen, V. Ilyushin, et. al.
Fri, 7 Dec 18
12/66

Comments: Astron. Astrophys., in press; 8 pages

Gas phase detection and rotational spectroscopy of ethynethiol, HCCSH [GA]

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http://arxiv.org/abs/1811.12798


We report the gas-phase detection and spectroscopic characterization of ethynethiol ($\mathrm{HCCSH}$), a metastable isomer of thioketene ($\mathrm{H_2C_2S}$) using a combination of Fourier-transform microwave and submillimeter-wave spectroscopies. Several $a$-type transitions of the normal species were initially detected below 40 GHz using a supersonic expansion-electrical discharge source, and subsequent measurement of higher-frequency, $b$-type lines using double resonance provided accurate predictions in the submillimeter region. With these, searches using a millimeter-wave absorption spectrometer equipped with a radio frequency discharge source were conducted in the range 280 – 660 GHz, ultimately yielding nearly 100 transitions up to $^rR_0(36)$ and $^rQ_0(68)$. From the combined data set, all three rotational constants and centrifugal distortion terms up to the sextic order were determined to high accuracy, providing a reliable set of frequency predictions to the lower end of the THz band. Isotopic substitution has enabled both a determination of the molecular structure of $\mathrm{HCCSH}$ and, by inference, its formation pathway in our nozzle discharge source via the bimolecular radical-radical recombination reaction $\mathrm{SH + C_2H}$, which is calculated to be highly exothermic (-477 kJ/mol) using the HEAT345(Q) thermochemical scheme.

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K. Lee, M. Martin-Drumel, V. Lattanzi, et. al.
Mon, 3 Dec 18
37/63

Comments: Accepted for publication in Molecular Physics. 29 pages, 4 tables, 6 figures. Supporting information is available

Quantitative estimates of chemical disequilibrium in Titan's atmosphere [EPA]

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http://arxiv.org/abs/1811.10760


We apply previously introduced measures of chemical disequilibrium to Cassini mass spectroscopy data on the atmosphere of Titan. In the analysis presented here, we use an improved description, avoiding the meanfield approximation in previous work. The results of the analysis are nearly exactly the same as those found earlier and confirm that, with respect to the measures used, Titan’s atmosphere lies between living and many nonliving systems. Some details of the mathematical analysis, which appear to be new, are included.

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B. Intoy and J. Halley
Wed, 28 Nov 18
25/74

Comments: 16 pages, 5 figures

The Astrochemistry Implications of Quantum Chemical Normal Modes Vibrational Analysis [GA]

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http://arxiv.org/abs/1811.08547


Understanding the molecular vibrations underlying each of the unknown infrared emission (UIE) bands (such as those found at 3.3, 3.4, 3.5, 6.2, 6.9, 7.7, 11.3, 15.8, 16.4, 18.9 mm) observed in or towards astronomical objects is a vital link to uncover the molecular identity of their carriers. This is usually done by customary classifications of normal mode frequencies such as stretching, deformation, rocking, wagging, skeletal mode, etc. A large literature on this subject exists and since 1952 ambiguities in classifications of normal modes via this empirical approach were pointed out by Morino and Kuchitsu [1]. New ways of interpretation and analyzing vibrational spectra were sought within the theoretical framework of quantum chemistry [2,3]. Many of these methods cannot easily be applied [3] to the large, complex molecular systems which are one of the key research interests of astrochemistry. In considering this demand, a simple and new method of analyzing and classifying the normal mode vibrational motions of molecular systems was introduced [4]. This approach is a fully quantitative method of analysis of normal mode displacement vector matrices and classification of the characteristic frequencies (fundamentals) underlying the observed IR bands. Outcomes of applying such an approach show some overlap with customary empirical classifications, usually at short wavelengths. It provides a quantitative breakdown of a complex vibration (at longer wavelengths) into the contributed fragments like their aromatic or aliphatic components. In addition, in molecular systems outside the classical models of chemical bonds and structures where the empirical approach cannot be applied, this quantitative method enables an interpretation of vibrational motion(s) underlying the IR bands.

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S. Sadjadi and Q. Parker
Thu, 22 Nov 18
46/57

Comments: N/A

The Astrochemistry Implications of Quantum Chemical Normal Modes Vibrational Analysis [GA]

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http://arxiv.org/abs/1811.08547


Understanding the molecular vibrations underlying each of the unknown infrared emission (UIE) bands (such as those found at 3.3, 3.4, 3.5, 6.2, 6.9, 7.7, 11.3, 15.8, 16.4, 18.9 mm) observed in or towards astronomical objects is a vital link to uncover the molecular identity of their carriers. This is usually done by customary classifications of normal mode frequencies such as stretching, deformation, rocking, wagging, skeletal mode, etc. A large literature on this subject exists and since 1952 ambiguities in classifications of normal modes via this empirical approach were pointed out by Morino and Kuchitsu [1]. New ways of interpretation and analyzing vibrational spectra were sought within the theoretical framework of quantum chemistry [2,3]. Many of these methods cannot easily be applied [3] to the large, complex molecular systems which are one of the key research interests of astrochemistry. In considering this demand, a simple and new method of analyzing and classifying the normal mode vibrational motions of molecular systems was introduced [4]. This approach is a fully quantitative method of analysis of normal mode displacement vector matrices and classification of the characteristic frequencies (fundamentals) underlying the observed IR bands. Outcomes of applying such an approach show some overlap with customary empirical classifications, usually at short wavelengths. It provides a quantitative breakdown of a complex vibration (at longer wavelengths) into the contributed fragments like their aromatic or aliphatic components. In addition, in molecular systems outside the classical models of chemical bonds and structures where the empirical approach cannot be applied, this quantitative method enables an interpretation of vibrational motion(s) underlying the IR bands.

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S. Sadjadi and Q. Parker
Thu, 22 Nov 18
4/57

Comments: N/A

Perspectives on Astrophysics Based on Atomic, Molecular, and Optical (AMO) Techniques [CL]

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http://arxiv.org/abs/1811.06157


About two generations ago, a large part of AMO science was dominated by experimental high energy collision studies and perturbative theoretical methods. Since then, AMO science has undergone a transition and is now dominated by quantum, ultracold, and ultrafast studies. But in the process, the field has passed over the complexity that lies between these two extremes. Most of the Universe resides in this intermediate region. We put forward that the next frontier for AMO science is to explore the AMO complexity that describes most of the Cosmos.

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D. Savin, J. Babb, P. Bellan, et. al.
Fri, 16 Nov 18
2/78

Comments: White paper submission to the Decadal Assessment and Outlook Report on Atomic, Molecular, and Optical (AMO) Science (AMO 2020)

Astronomical Infrared Spectrum of Planetary Nebula Lin49 and Tc1 Identified by Ionized Polycyclic-Pure-Carbon C23 and C60 [GA]

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http://arxiv.org/abs/1811.05043


Astronomical dust molecule of carbon-rich nebula-Lin49 and nebula-Tc1 could be identified to be polycyclic-pure-carbon C23 by the quantum-chemical calculation. Two driving forces were assumed. One is high speed proton attack on coronene-C24H12, which created void-induced C23H12. Another is high energy photon irradiation, which brought deep photo-ionization and finally caused dehydrogenation to be C23. Infrared spectrum calculation show that a set of ionized C23 (neutral, mono, and di-cation) could reproduce observed many peaks of 28 bands at wavelength from 6 to 38 micrometer. Previously predicted neutral fullerene-C60 could partially reproduce observed spectrum by 5 bands. Also, we tried calculation on ionized-C60, which show fairly good coincidence with observed 10 bands

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N. Ota
Wed, 14 Nov 18
2/75

Comments: 9 pages, 10 figures

A theoretical investigation of the reaction between the amidogen, NH, and the ethyl, C2H5, radicals: a possible gas-phase formation route of interstellar and planetary ethanimine [EPA]

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http://arxiv.org/abs/1810.10416


The reaction between the amidogen, NH, radical and the ethyl, C2H5, radical has been investigated by performing electronic structure calculations of the underlying doublet potential energy surface. Rate coefficients and product branching ratios have also been estimated by combining capture and RRKM calculations. According to our results, the reaction is very fast, close to the gas-kinetics limit. However, the main product channel, with a yield of ca. 86-88% in the range of temperatures investigated, is the one leading to methanimine and the methyl radical. The channels leading to the two E-, Z- stereoisomers of ethanimine account only for ca. 5-7% each. The resulting ratio [E-CH3CHNH]/[Z-CH3CHNH] is ca. 1.2, that is a value rather lower than that determined in the Green Bank Telescope PRIMOS radio astronomy survey spectra of Sagittarius B2 North (ca. 3). Considering that ice chemistry would produce essentially only the most stable isomer, a possible conclusion is that the observed [E-CH3CHNH]/[Z-CH3CHNH] ratio is compatible with a combination of gas-phase and grain chemistry. More observational and laboratory data are needed to definitely address this issue.

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N. Balucani, D. Skouteris, C. Ceccarelli, et. al.
Thu, 25 Oct 18
30/65

Comments: N/A

Synchrotron radiation interaction with cryosorbed layers for astrochemical investigations [GA]

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http://arxiv.org/abs/1810.10392


Photon-stimulated desorption (PSD) is a process of interest for the two seemingly unrelated topics of accelerator vacuum dynamics and astrochemistry. Here we present an approach to studying PSD of interstellar ice analogs, i.e. condensed films of molecules of astrophysical interest at cryogenic temperatures, using synchrotron radiation. We present results obtained in the VUV range on various pure and layered ices, focusing on elucidating the desorption mechanisms, and results in the X-ray range for H$_2$O.

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R. Dupuy, M. Bertin, G. Féraud, et. al.
Thu, 25 Oct 18
52/65

Comments: Proceedings of the ECLOUD’18 6th workshop on electron-cloud effects (3-7 June 2018 La Biodola (Isola d’Elba) Italy). To be published as a CERN yellow report

HSCO$^+$ and DSCO$^+$: a multi-technique approach in the laboratory for the spectroscopy of interstellar ions [GA]

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http://arxiv.org/abs/1810.09215


Protonated molecular species have been proven to be abundant in the interstellar gas. This class of molecules is also pivotal for the determination of important physical parameters for the ISM evolution (e.g. gas ionisation fraction) or as tracers of non-polar, hence not directly observable, species. The identification of these molecular species through radioastronomical observations is directly linked to a precise laboratory spectral characterisation. The goal of the present work is to extend the laboratory measurements of the pure rotational spectrum of the ground electronic state of protonated carbonyl sulfide (HSCO$^+$) and its deuterium substituted isotopomer (DSCO$^+$). At the same time, we show how implementing different laboratory techniques allows the determination of different spectroscopical properties of asymmetric-top protonated species. Three different high-resolution experiments were involved to detected for the first time the $b-$type rotational spectrum of HSCO$^+$, and to extend, well into the sub-millimeter region, the $a-$type spectrum of the same molecular species and DSCO$^+$. The electronic ground-state of both ions have been investigated in the 273-405 GHz frequency range, allowing the detection of 60 and 50 new rotational transitions for HSCO$^+$ and DSCO$^+$, respectively. The combination of our new measurements with the three rotational transitions previously observed in the microwave region permits the rest frequencies of the astronomically most relevant transitions to be predicted to better than 100 kHz for both HSCO$^+$ and DSCO$^+$ up to 500 GHz, equivalent to better than 60 m/s in terms of equivalent radial velocity. The present work illustrates the importance of using different laboratory techniques to spectroscopically characterise a protonated species at high frequency, and how a similar approach can be adopted when dealing with reactive species.

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V. Lattanzi, S. Spezzano, J. Laas, et. al.
Tue, 23 Oct 18
5/69

Comments: 7 pages, 4 figures. Accepted for publication in Astronomy and Astrophysics

Astronomical Creation of Cyclic-C3H2 and Chain-C3 Due to Interstellar Deep Photoionization [GA]

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http://arxiv.org/abs/1810.06006


Astronomical evolution mechanism of small size polycyclic aromatic hydrocarbon (PAH) was analyzed using the first principles quantum-chemical calculation. Starting model molecule was benzene (C6H6), which would be transformed to (C5H5) due to carbon void created by interstellar high speed proton attack. In a protoplanetary disk around a young star, molecules would be illuminated by high energy photon and ionized to be cationic-(C5H5). Calculation shows that from neutral to tri-cation, molecule keeps original configuration. At a step of sixth cation, there occurs surprising creation of cyclic-C3H2, which is the smallest PAH. Astronomical cyclic-C3H2 had been identified by radio astronomy. Deep photoionization of cyclic-C3H2 brings successive molecular change. Neutral and mono-cation keep cyclic configuration. At a step of di-cation, molecule was transformed to aliphatic chain-C3H2. Finally, chain-C3H2 was decomposed to pure carbon chain-C3 and two hydrogen atoms. Calculated infrared spectrum of those molecules was applied to observed spectrum of Herbig Ae young stars. Observed infrared spectrum could be partially explained by small molecules. Meanwhile, excellent coincidence was obtained by applying a larger molecules as like (C23H12)2+ or (C12H8)2+. Infrared observation is suitable for larger molecules and radio astronomy for smaller asymmetric molecules. It should be noted that these molecules could be identified in a natural way introducing two astronomical phenomena, that is, void-induced molecular deformation and deep photoionization.

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N. Ota
Tue, 16 Oct 18
43/83

Comments: 11 pages, 13 figures, 1 table

Can Formamide Be Formed on Interstellar Ice? An Atomistic Perspective [EPA]

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http://arxiv.org/abs/1810.02965


Interstellar formamide (NH2CHO) has recently attracted significant attention due to its potential role as a molecular building block in the formation of precursor biomolecules relevant for the origin of life. Its formation, whether on the surfaces of the interstellar grains or in the gas phase, is currently debated. The present article presents new theoretical quantum chemical computations on possible NH2CHO formation routes in water-rich amorphous ices, simulated by a 33-H2O-molecule cluster. We have considered three possible routes. The first one refers to a scenario used in several current astrochemical models, that is, the radical-radical association reaction between NH2 and HCO. Our calculations show that formamide can indeed be formed, but in competition with formation of NH3 and CO through a direct H transfer process. The final outcome of the NH2 + HCO reactivity depends on the relative orientation of the two radicals on the ice surface. We then analyzed two other possibilities, suggested here for the first time: reaction of either HCN or CN with water molecules of the ice mantle. The reaction with HCN has been found to be characterized by large energy barriers and, therefore, cannot occur under the interstellar ice conditions. On the contrary, the reaction with the CN radical can occur, possibly leading through multiple steps to the formation of NH2CHO. For this reaction, water molecules of the ice act as catalytic active sites since they help the H transfers involved in the process, thus reducing the energy barriers (compared to the gas-phase analogous reaction). Additionally, we apply a statistical model to estimate the reaction rate coefficient when considering the cluster of 33-H2O-molecules as an isolated moiety with respect to the surrounding environment, i.e., the rest of the ice.

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A. Rimola, D. Skouteris, N. Balucani, et. al.
Tue, 9 Oct 18
72/77

Comments: N/A

Fully anharmonic infrared cascade spectra of polycyclic aromatic hydrocarbons [GA]

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http://arxiv.org/abs/1810.01975


The infrared (IR) emission of polycyclic aromatic hydrocarbons (PAHs) permeates our universe; astronomers have detected the IR signatures of PAHs around many interstellar objects. The IR emission of interstellar PAHs differs from their emission as seen under conditions on Earth, as they emit through a collisionless cascade down through their excited vibrational states from high internal energies. The difficulty in reproducing interstellar conditions in the laboratory results in a reliance on theoretical techniques. However, the size and complexity of PAHs requires careful consideration when producing the theoretical spectra. In this work we outline the theoretical methods necessary to lead to a fully theoretical IR cascade spectra of PAHs including: an anharmonic second order vibrational perturbation theory (VPT2) treatment; the inclusion of Fermi resonances through polyads; and the calculation of anharmonic temperature band shifts and broadenings (including resonances) through a Wang–Landau approach. We also suggest a simplified scheme to calculate vibrational emission spectra that retains the essential characteristics of the full IR cascade treatment and can directly transform low temperature absorption spectra in IR cascade spectra. Additionally we show that past astronomical models were in error in assuming a 15 cm$^{-1}$ correction was needed to account for anharmonic emission effects.

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C. Mackie, T. Chen, A. Candian, et. al.
Fri, 5 Oct 18
23/53

Comments: N/A

Anharmonicity and the infrared emission spectrum of highly excited PAHs [IMA]

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http://arxiv.org/abs/1809.09868


Aims. Infrared (IR) spectroscopy is a powerful tool to study molecules in space. A key issue in such analyses is understanding the effect that temperature and anharmonicity have on different vibrational bands, and thus interpreting the IR spectra for molecules under various conditions.
Methods. We combined second order vibrational perturbation theory and the Wang-Landau random walk technique to produce accurate IR spectra of highly excited PAHs. We fully incorporated anharmonic effects, such as resonances, overtones, combination bands, and temperature effects.
Results. The results are validated against experimental results for the pyrene molecule (C16H10). In terms of positions, widths, and relative intensities of the vibrational bands, our calculated spectra are in excellent agreement with gas-phase experimental data.

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T. Chen, C. Mackie, A. Candian, et. al.
Thu, 27 Sep 18
60/68

Comments: N/A

Hyperfine-resolved rotation-vibration line list of ammonia (NH$_3$) [CL]

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http://arxiv.org/abs/1809.06915


A comprehensive, hyperfine-resolved rotation-vibration line list for the ammonia molecule ($^{14}$NH$_3$) is presented. The line list, which considers hyperfine nuclear quadrupole coupling effects, has been computed using robust, first principles methodologies based on a highly accurate empirically refined potential energy surface. Transitions between levels with energies below $8000$~cm$^{-1}$ and total angular momentum $F\leq14$ are considered. The line list shows excellent agreement with a range of experimental data and will significantly assist future high-resolution measurements of NH$_3$, both astronomically and in the laboratory.

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P. Coles, A. Owens, J. Küpper, et. al.
Thu, 20 Sep 18
44/55

Comments: N/A

Anharmonic vibrational spectroscopy of Polycyclic Aromatic Hydrocarbons (PAHs) [CL]

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http://arxiv.org/abs/1809.05669


While powerful techniques exist to accurately account for anharmonicity in vibrational molecular spectroscopy, they are computationally very expensive and cannot be routinely employed for large species and/or at non- zero vibrational temperatures. Motivated by the study of Polycyclic Aromatic Hydrocarbon (PAH) emission in space, we developed a new code, which takes into account all modes and can describe all IR transitions including bands becoming active due to resonances as well as overtones, combination and difference bands. In this article, we describe the methodology that was implemented and discuss how the main difficulties were overcome, so as to keep the problem tractable. Benchmarking with high-level calculations was performed on a small molecule. We carried out specific convergence tests on two prototypical PAHs, pyrene (C${16}$H${10}$) and coronene (C${24}$H${12}$), aiming at optimising tunable parameters to achieve both acceptable accuracy and computational costs for this class of molecules. We then report the results obtained at 0 K for pyrene and coronene, comparing the calculated spectra with available experimental data. The theoretical band positions were found to be significantly improved compared to harmonic Density Functional Theory (DFT) calculations. The band intensities are in reasonable agreement with experiments, the main limitation being the accuracy of the underlying calculations of the quartic force field. This is a first step towards calculating moderately high-temperature spectra of PAHs and other similarly rigid molecules using Monte Carlo sampling.

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G. Mulas, C. Falvo, P. Cassam-Chenaï, et. al.
Tue, 18 Sep 18
62/70

Comments: accepted for publication on Journal of Chemical Physics

Anomalous phosphine sensitivity coefficients as probes for a possible variation of the proton-to-electron mass ratio [CEA]

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http://arxiv.org/abs/1808.05427


A robust variational approach is used to investigate the sensitivity of the rotation-vibration spectrum of phosphine (PH$3$) to a possible cosmological variation of the proton-to-electron mass ratio, $\mu$. Whilst the majority of computed sensitivity coefficients, $T$, involving the low-lying vibrational states acquire the expected values of $T\approx-1$ and $T\approx-1/2$ for rotational and ro-vibrational transitions, respectively, anomalous sensitivities are uncovered for the $A_1!-!A_2$ splittings in the $\nu_2/\nu_4$, $\nu_1/\nu_3$ and $2\nu_4^{\ell=0}/2\nu_4^{\ell=2}$ manifolds of PH$_3$. A pronounced Coriolis interaction between these states in conjunction with accidentally degenerate $A_1$ and $A_2$ energy levels produces a series of enhanced sensitivity coefficients. Phosphine is expected to occur in a number of different astrophysical environments and has potential for investigating a drifting constant. Furthermore, the displayed behaviour hints at a wider trend in molecules of ${\bf C}{3\mathrm{v}}\mathrm{(M)}$ symmetry, thus demonstrating that the splittings induced by higher-order ro-vibrational interactions are well suited for probing $\mu$ in other symmetric top molecules in space, since these low-frequency transitions can be straightforwardly detected by radio telescopes.

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A. Owens, S. Yurchenko and V. Špirko
Fri, 17 Aug 18
26/53

Comments: N/A

Astronomical Quantum-chemical Origin of Ubiquitously Observed Interstellar Infrared Spectrum due to Polycyclic Aromatic Hydrocarbon [GA]

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http://arxiv.org/abs/1808.01070


Interstellar infrared observation shows featured spectrum due to polycyclic aromatic hydrocarbon (PAH)at wavelength 3.3,6.2,7.6,7.8,8.6,and 11.3 micrometer,which are ubiquitously observed in many astronomical dust clouds and galaxies. Our previous first principles calculation revieled that viod induced coronene (C23H12)2+ and circumcoronene (C53H18)1+ could reproduce such spectrum very well. In this study, quantum-mechanic origin was studied through atomic configuration change and atomic vibration mode analysis. By a high speed particle attack, carbon void would be introduced in PAH. Molecular configuration was deformed by the Jahn-Teller quantum effect. Carbon SP3 local bond was created among SP2 graphene like carbon network. Also, carbon tetrahedron local structure was created. Such peculiar structure is the quantum origin. Those metamorphosed molecules would be photo-ionized by the central star strong photon irradiation resulting cation molecules. Atomic vibration mode of cation molecule (C23H12)2+ was compared with that of neutral one (C23H12). At 3.3 micrometer, both molecules show show C-H stretching mode and give fairly large infrared intensity. At 6.2,7.6,7.8, and 8.6 micrometer bands, cation molecule show complex C-C stretching and shrinking mixing modes and remain large infrared emission. Whereas, neutral molecule gives harmonic motion, which cancelles each other resulting very small infrared intensity. At 11.3 micrometer, both neutral and cation molecules show C-H bending motion perpendicular to a molecular plane, which contributes to strong emission. Actual observed spectrum would be a sum of such quantum-mechanic origined molecules.

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N. Ota
Mon, 6 Aug 18
14/33

Comments: 13pages, 10 figures, 1 table, 6 appendix

Formation of anionic C,N-bearing chains in the Interstellar medium via reactions of H$^{-}$ with HC$_x$N for odd-valued x from 1 to 7 [CL]

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http://arxiv.org/abs/1807.11028


We investigate the relative efficiencies of low-temperature chemical reactions in the Interstellar medium (ISM) with H$^-$ anion reacting in the gas phase with cyanopolyyne neutral molecules, leading to the formation of anionic C$_x$N$^-$ linear chains of different length and of H$_2$. All the reactions turn out to be without barriers, highly exothermic reactions which provide a chemical route to the formation of anionic chains of the same length . Some of the anions have been observed in the dark molecular clouds and in the diffuse interstellar envelopes.Quantum calculations are carried for the corresponding reactive potential energy surfaces (RPESs) for all the odd-numbered members of the series (x=1, 3, 5, 7). We employ the Minimum Energy paths (MEPs) to obtain the relevant Transition State (TS) configurations and use the latter within the Variational Transition State ( VTS) model to obtain the chemical rates. The present results indicate that, at typical temperatures around 100 K, a set of significantly larger rate values exists for x=3 and x=5, while are smaller for CN$^-$ and C$_7$N$^-$. At those temperatures, however, all the rates turn out to be larger than the estimates in the current literature for the Radiative Electron Attachment (REA) rates, thus indicating the greater importance of the present chemical path with respect to REA processes at those temperatures. The physical reasons for our findings are discussed in detail and linked with the existing observational findings.

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F. Gianturco, M. Satta, E. Yurtsever, et. al.
Tue, 31 Jul 18
16/69

Comments: N/A

Exploring a dynamical path for C$_2$H$^-$ and NCO$^-$ formation in dark molecular clouds [CL]

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http://arxiv.org/abs/1807.11032


This paper deals with the possible formation of two molecular anions often considered likely components in the physical environments of the Interstellar Medium ( ISM) : $\mathrm{C_2H^-}$ and $\mathrm{NCO^-}$. They are both discussed here by computationally following the radiative association (RA) mechanism starting from $\mathrm{C_2^-}$, $\mathrm{H}$, $\mathrm{NC^-}$ and $\mathrm{O}$ as partners. The corresponding RA total cross sections produced by the calculations are in turn employed to generate the overall association rates over the relevant range of temperatures. The latter are found to be in line with other molecular ions formed by RA but not large enough to uniquivocally suggest this path as the main route to the anions formation in the ISM. Other possible paths of formation are also analysed and discussed. The presence of resonant structures during the association dynamics for both systems is found by the calculations and their consequences are discussed in some detail in the present study.

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I. Iskandarov, F. Carelli, E. Yurtsever, et. al.
Tue, 31 Jul 18
57/69

Comments: N/A

ExoMol molecular line lists XXX: a complete high-accuracy line list for water [EPA]

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http://arxiv.org/abs/1807.04529


A new line list for H$_2$$^{16}$O is presented. This line list, which is called POKAZATEL, includes transitions between rotation-vibrational energy levels up to 41000 cm$^{-1}$ in energy and is the most complete to date. The potential energy surface (PES) used for producing the line list was obtained by fitting a high-quality ab initio PES to experimental energy levels with energies of 41000 cm$^{-1}$ and for rotational excitations up to $J=5$. The final line list comprises all energy levels up to 41000 cm$^{-1}$ and rotational angular momentum $J$ up to 72. An accurate ab initio dipole moment surface (DMS) was used for the calculation of line intensities and reproduces high-precision experimental intensity data with an accuracy close to 1 %. The final line list uses empirical energy levels whenever they are available, to ensure that line positions are reproduced as accurately as possible. The POKAZATEL line list contains over 5 billion transitions and is available from the ExoMol website (www.exomol.com) and the CDS database.

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O. Polyansky, A. Kyuberis, N. Zobov, et. al.
Fri, 13 Jul 18
36/76

Comments: N/A

X-ray photodesorption from water ice in protoplanetary disks and X-ray dominated regions [GA]

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http://arxiv.org/abs/1807.03725


Water is the main constituent of interstellar ices, and it plays a key role in the evolution of many regions of the interstellar medium, from molecular clouds to planet-forming disks. In cold regions of the ISM, water is expected to be completely frozen out onto the dust grains. Nonetheless, observations indicate the presence of cold water vapor, implying that non-thermal desorption mechanisms are at play. Photodesorption by UV photons has been proposed to explain these observations, with the support of extensive experimental and theoretical work on ice analogues. In contrast, photodesorption by X-rays, another viable mechanism, has been little studied. The potential of this process to desorb key molecules, such as water, intact rather than fragmented or ionised, remains unexplored. We experimentally investigated X-ray photodesorption from water ice, monitoring all desorbing species. We find that desorption of neutral water is efficient, while ion desorption is minor. We derive for the first time yields that can be implemented in astrochemical models. These results open up the possibility of taking into account the X-ray photodesorption process in the modelling of protoplanetary disks or X-ray dominated regions.

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R. Dupuy, M. Bertin, G. Féraud, et. al.
Wed, 11 Jul 18
41/64

Comments: N/A

Tunnelling dominates the reactions of hydrogen atoms with unsaturated alcohols and aldehydes in the dense medium [GA]

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http://arxiv.org/abs/1806.05831


Hydrogen addition and abstraction reactions play an important role as surface reactions in the buildup of complex organic molecules in the dense interstellar medium. Addition reactions allow unsaturated bonds to be fully hydrogenated, while abstraction reactions recreate radicals that may undergo radical-radical recombination reactions. Previous experimental work has indicated that double and triple C–C bonds are easily hydrogenated, but aldehyde -C=O bonds are not. Here, we investigate a total of 29 reactions of the hydrogen atom with propynal, propargyl alcohol, propenal, allyl alcohol, and propanal by means of quantum chemical methods to quantify the reaction rate constants involved.
First of all, our results are in good agreement with and can explain the observed experimental findings. The hydrogen addition to the aldehyde group, either on the C or O side, is indeed slow for all molecules considered. Abstraction of the H atom of the aldehyde group, on the other hand, is among the faster reactions. Furthermore, hydrogen addition to C–C double bonds is generally faster than to triple bonds. In both cases, addition on the terminal carbon atom that is not connected to other functional groups is easiest. Finally, we wish to stress that it is not possible to predict rate constants based solely on the type of reaction: the specific functional groups attached to a backbone play a crucial role and can lead to a spread of several orders of magnitude in the rate constant.

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V. Zaverkin, T. Lamberts, M. Markmeyer, et. al.
Mon, 18 Jun 18
34/54

Comments: Accepted for publication in A&A

ExoMol line lists XXVIII: The rovibronic spectrum of AlH [SSA]

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http://arxiv.org/abs/1806.03279


A new line list for AlH is produced. The WYLLoT line list spans two electronic states $X\,{}^1\Sigma^+$ and $A\,{}^1\Pi$. A diabatic model is used to model the shallow potential energy curve of the $A\,{}^1\Pi$ state, which has a strong pre-dissociative character with only two bound vibrational states. Both potential energy curves are empirical and were obtained by fitting to experimentally derived energies of the $X\,{}^1\Sigma^+$ and $A\,{}^1\Pi$ electronic states using the diatomic nuclear motion codes Level and Duo. High temperature line lists plus partition functions and lifetimes for three isotopologues $^{27}$AlH, $^{27}$AlD and $^{26}$AlH were generated using ab initio dipole moments. The line lists cover both the $X$–$X$ and $A$–$X$ systems and are made available in electronic form at the CDS and ExoMol databases.

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S. Yurchenko, H. Williams, P. Leyland, et. al.
Mon, 11 Jun 18
37/50

Comments: N/A

Hydrogen transfer reactions of interstellar Complex Organic Molecules [GA]

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http://arxiv.org/abs/1806.02062


Radical recombination has been proposed to lead to the formation of complex organic molecules (COMs) in CO-rich ices in the early stages of star formation. These COMs can then undergo hydrogen addition and abstraction reactions leading to a higher or lower degree of saturation. Here, we have studied 14 hydrogen transfer reactions for the molecules glyoxal, glycoaldehyde, ethylene glycol, and methylformate and an additional three reactions where \ce{CH_nO} fragments are involved. Over-the-barrier reactions are possible only if tunneling is invoked in the description at low temperature. Therefore the rate constants for the studied reactions are calculated using instanton theory that takes quantum effects into account inherently. The reactions were characterized in the gas phase, but this is expected to yield meaningful results for CO-rich ices due to the minimal alteration of reaction landscapes by the CO molecules.
We found that rate constants should not be extrapolated based on the height of the barrier alone, since the shape of the barrier plays an increasingly larger role at decreasing temperature. It is neither possible to predict rate constants based only on considering the type of reaction, the specific reactants and functional groups play a crucial role. Within a single molecule, though, hydrogen abstraction from an aldehyde group seems to be always faster than hydrogen addition to the same carbon atom. Reactions that involve heavy-atom tunneling, e.g., breaking or forming a C–C or C–O bond, have rate constants that are much lower than those where H transfer is involved.

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S. Álvarez-Barcia, P. Russ, J. Kästner, et. al.
Thu, 7 Jun 18
8/51

Comments: Accepted for publication in MNRAS

Far infrared measurements of absorptions by CH4+CO2 and H2+CO2 mixtures and implications for greenhouse warming on early Mars [EPA]

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http://arxiv.org/abs/1805.02595


We present an experimental study of the absorption, between 40 and 640 cm$^{-1}$, by CO$_2$, CH$_4$ and H$_2$ gases as well as by H$_2$+CO$_2$ and CH$_4$+CO$_2$ mixtures at room temperature. A Fourier transform spectrometer associated to a multi-pass cell, whose optics were adjusted to obtain a 152 m pathlength, were used to record transmission spectra at total pressures up to about 0.98 bar. These measurements provide information concerning the collision-induced absorption (CIA) bands as well as about the wing of the CO$_2$ 15 $\mu$m band. Our results for the CIAs of pure gases are, within uncertainties, in agreement with previous determinations, validating our experimental and data analysis procedures. We then consider the CIAs by H$_2$+CO$_2$ and CH$_4$+CO$_2$ and the low frequency wing of the pure CO$_2$ 15 $\mu$m band, for which there are, to our knowledge, no previous measurements. We confirm experimentally the theoretical prediction of Wordsworth et al. 2017 that the H$_2$+CO$_2$ and CH$_4$+CO$_2$ CIAs are significantly stronger in the 50-550 cm$^{-1}$ region than those of H$_2$+N$_2$ and CH$_4$+N$_2$, respectively. However, we find that the shape and the strength of these recorded CIAs differ from the aforementioned predictions. For the pure CO$_2$ line-wings, we show that both the $\chi$-factor deduced from measurements near 4 $\mu$m and a line-mixing model very well describe the observed strongly sub-Lorentzian behavior in the 500-600 cm$^{-1}$ region. These experimental results open renewed perspectives for studies of the past climate of Mars and extrasolar analogues.

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M. Turbet, H. Tran, O. Pirali, et. al.
Tue, 8 May 18
48/69

Comments: Submitted to Icarus. 22 pages, 7 figures, 2 tables. Comments are welcome

Vibrational Satellites of C$_2$S, C$_3$S, and C$_4$S: Microwave Spectral Taxonomy as a Stepping Stone to the Millimeter-Wave Band [IMA]

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http://arxiv.org/abs/1804.04126


We present a microwave spectral taxonomy study of several hydrocarbon/CS$_2$ discharge mixtures in which more than 60 distinct chemical species, their more abundant isotopic species, and/or their vibrationally excited states were detected using chirped-pulse and cavity Fourier-transform microwave spectroscopies. Taken together, in excess of 85 unique variants were detected, including several new isotopic species and more than 25 new vibrationally excited states of C$_2$S, C$_3$S, and C$_4$S, which have been assigned on the basis of published vibration-rotation interaction constants for C$_3$S, or newly calculated ones for C$_2$S and C$_4$S. On the basis of these precise, low-frequency measurements, several vibrationally exited states of C$_2$S and C$_3$S were subsequently identified in archival millimeter-wave data in the 253–280 GHz frequency range, ultimately providing highly accurate catalogs for astronomical searches. As part of this work, formation pathways of the two smaller carbon-sulfur chains were investigated using $^{13}$C isotopic spectroscopy, as was their vibrational excitation. The present study illustrates the utility of microwave spectral taxonomy as a tool for complex mixture analysis, and as a powerful and convenient `stepping stone’ to higher frequency measurements in the millimeter and submillimeter bands.

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B. McGuire, M. Martin-Drumel, K. Lee, et. al.
Fri, 13 Apr 18
6/47

Comments: Accepted in PCCP

The threshold displacement energy of buckminsterfullerene C$_{60}$ and formation of the endohedral defect fullerene He@C$_{59}$ [CL]

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http://arxiv.org/abs/1804.04374


We have measured the threshold center-of-mass energy for knocking out a single carbon atom from C${60}^-$ in collisions with He. Combining this experimental result with classical molecular dynamics simulations, we determine a semi-empirical value of 24.1$\pm$0.5 eV for the threshold displacement energy, the minimum energy needed to remove a single carbon atom from the C${60}$ cage. We report the first observation of an endohedral complex with an odd number of carbon atoms, He@C$_{59}^-$, and discuss its formation and decay mechanisms.

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M. Stockett, M. Wolf, M. Gatchell, et. al.
Fri, 13 Apr 18
17/47

Comments: N/A

Hydrogenated pyrene: Statistical single-carbon loss below the knockout threshold [CL]

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http://arxiv.org/abs/1804.02862


An ongoing discussion revolves around the question of what effect hydrogenation has on carbon backbone fragmentation in Polycyclic Aromatic Hydrocarbons (PAHs). In order to shed more light on this issue, we have measured absolute single carbon loss cross sections in collisions between native or hydrogenated pyrene cations (C${16}$H${10+m}^{+}, m$ = 0, 6, 16) and He as functions of center-of-mass energies all the way down to 20 eV. Classical Molecular Dynamics (MD) simulations give further insight into energy transfer processes and also yield $m$-dependent threshold energies for prompt (femtoseconds) carbon knockout. Such fast, non-statistical fragmentation processes dominate CH$x$-loss for native pyrene ($m=0$), while much slower statistical fragmentation processes contribute significantly to single-carbon loss for the hydrogenated molecules ($m=6$ and $m=16$). The latter is shown by measurements of large CH$_x$-loss cross sections far below the MD knockout thresholds for C${16}$H${16}^{+}$ and C${16}$H$_{26}^{+}$.

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M. Wolf, L. Giacomozzi, M. Gatchell, et. al.
Tue, 10 Apr 18
35/79

Comments: 7 pages, 6 figures

ExoMol molecular line lists – XXVI: spectra of SH and NS [SSA]

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http://arxiv.org/abs/1803.09724


Line lists for the sulphur-containing molecules SH (the mercapto radical) and NS are computed as part of the ExoMol project. These line lists consider transitions within the $X$ ${}^2\Pi$ ground state for $^{32}$SH, $^{33}$SH, $^{34}$SH and $^{\text{32}}$SD, and $^{14}$N$^{32}$S, $^{14}$N$^{33}$S, $^{14}$N$^{34}$S, $^{14}$N$^{36}$S and $^{15}$N$^{32}$S. Ab initio potential energy (PEC) and spin-orbit coupling (SOC) curves are computed and then improved by fitting to experimentally observed transitions. Fully ab initio dipole moment curves (DMCs) computed at high level of theory are used to produce the final line lists. For SH, our fit gives a root-mean-square (rms) error of 0.03 cm$^{-1}$ between the observed ($v_{\rm max}=4$, $J_{\rm max} = 34.5$) and calculated transitions wavenumbers; this is extrapolated such that all $X$ $^2\Pi$ rotational-vibrational-electronic (rovibronic) bound states are considered. For $^{\text{32}}$SH the resulting line list contains about 81000 transitions and 2300 rovibronic states, considering levels up to $v_{\rm max} = 14$ and $J_{\rm max} = 60.5$. For NS the refinement used a combination of experimentally determined frequencies and energy levels and led to an rms fitting error of 0.002 cm$^{-1}$. Each NS calculated line list includes around 2.8 million transitions and 31000 rovibronic states with a vibrational range up to $v=53$ and rotational range to $J=235.5$, which covers up to 23000 cm$^{-1}$. Both line lists should be complete for temperatures up to 5000 K. Example spectra simulated using this line list are shown and comparisons made to the existing data in the CDMS database. The line lists are available from the CDS (this http URL) and ExoMol (www.exomol.com) data bases.

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S. Yurchenko, W. Bond, M. Gorman, et. al.
Wed, 28 Mar 18
9/148

Comments: N/A

Astrochemical Evolution Step From Acenaphthylene C12H8 To Pure Carbon C12 Around A Herbig Ae Young Star [SSA]

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http://arxiv.org/abs/1803.09035


Astrochemical evolution step of polycyclic aromatic hydrocarbon (PAH) around a Herbig Ae young star was analyzed using the first principles quantum chemical calculation. For simplicity, model molecule was selected to be acenaphthylene (C12H8) with hydrocarbon one pentagon combined with two hexagons. In a protoplanetary disk, molecules are illuminated by high energy photon from the central star and ionized to be cation (C12H8)n+ . Calculation shows that from n=0 to 6, molecule keeps its polycyclic hydrocarbon configuration. Whereas, at ionization step n=7, there occurs dehydrogenation of (C12H8) to pure carbon (C12). Such polycyclic pure carbon (PPC) would be attacked again by photons. At a stage of eighth ionization (C12)8+, there occur decomposition to aliphatic carbon chains, C9, C2, and mono carbon C1. Infrared spectra (IR) of those steps were calculated to identify observed spectra . Carrier molecules of Herbig Ae star WW Vul and HD145263 were identified by a combination of (C12H8)2+ and (C12H8)1+. Also, IR of HD37357 could be explained by (C12H8)2+, (C12H8)3+, and (C12H8)1+. Pure carbon molecules play an important role in many stars. IR of HD37258 was analyzed by a mixture of pure carbon (C12)2+, hydrocarbon (C12H8)2+ and neutral (C12H8)0+. Also, complex spectrum of HD38120 was analyzed by (C12)2+, (C12H8)2+ and (C12H8)3+. Acenaphthylene related molecules are just a typical example. We should apply various size molecules to understand total view around a new born star.

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N. Ota
Wed, 28 Mar 18
61/148

Comments: 12pages, 10figures

Sites That Can Produce Left-Handed Amino Acids in the Supernova Neutrino Amino Acid Processing Model [CL]

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http://arxiv.org/abs/1802.08285


The Supernova Neutrino Amino Acid Processing model, which uses electron anti-neutrinos and the magnetic field from a source object such as a supernova to selectively destroy one amino acid chirality, is studied for possible sites that would produce meteoroids having partially left-handed amino acids. Several sites appear to provide the requisite magnetic field intensities and electron anti-neutrino fluxes. These results have obvious implications for the origin of life on Earth.

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R. Boyd, M. Famiano, T. Onaka, et. al.
Mon, 26 Feb 18
39/49

Comments: Accepted to ApJL

Full-dimensional Quantum Dynamics of SiO in Collision with H$_2$ [CL]

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http://arxiv.org/abs/1802.04702


We report the first full-dimensional potential energy surface (PES) and quantum mechanical close-coupling calculations for scattering of SiO due to H$_2$. The full-dimensional interaction potential surface was computed using the explicitly correlated coupled-cluster (CCSD(T)-F12b) method and fitted using an invariant polynomial approach. Pure rotational quenching cross sections from initial states $v_1=0$, $j_1$=1-5 of SiO in collision with H$_2$ are calculated for collision energies between 1.0 and 5000 cm$^{-1}$. State-to-state rotational rate coefficients are calculated at temperatures between 5 and 1000 K. The rotational rate coefficients of SiO with para-H$_2$ are compared with previous approximate results which were obtained using SiO-He PESs or scaled from SiO-He rate coefficients. Rovibrational state-to-state and total quenching cross sections and rate coefficients for initially excited SiO($v_1=1, j_1$=0 and 1) in collisions with para-H$_2$($v_2=0,j_2=0$) and ortho-H$_2$($v_2=0,j_2=1$) were also obtained. The application of the current collisional rate coefficients to astrophysics is briefly discussed.

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B. Yang, P. Zhang, C. Qu, et. al.
Wed, 14 Feb 18
22/68

Comments: N/A

Measurements and modeling of absorption by CO2+H2O mixtures in the spectral region beyond the CO2 nu3-band head [EPA]

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http://arxiv.org/abs/1802.01352


In this work, we measured the absorption by CO2+H2O mixtures from 2400 to 2600 cm-1 which corresponds to the spectral region beyond the nu3 band head of CO2. Transmission spectra of CO2 mixed with water vapor were recorded with a high-resolution Fourier-transform spectrometer for various pressure, temperature and concentration conditions. The continuum absorption by CO2 due to the presence of water vapor was determined by subtracting from measured spectra the contribution of local lines of both species, that of the continuum of pure CO2 as well as of the self- and CO2-continua of water vapor induced by the H2O-H2O and H2O-CO2 interactions. The obtained results are in very good agreement with the unique previous measurement (in a narrower spectral range). They confirm that the H2O-continuum of CO2 is significantly larger than that observed for pure CO2. This continuum thus must be taken into account in radiative transfer calculations for media involving CO2+H2O mixture. An empirical model, using sub-Lorentzian line shapes based on some temperature-dependent correction factors chi is proposed which enables an accurate description of the experimental results.

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H. Tran, M. Turbet, P. Chelin, et. al.
Tue, 6 Feb 18
34/62

Comments: Accepted in Icarus. 16 pages, 3 figures

Accurate millimetre and submillimetre rest frequencies for $cis$- and $trans$-dithioformic acid, HCSSH [GA]

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http://arxiv.org/abs/1801.07212


A better understanding of sulphur chemistry is needed to solve the interstellar sulphur depletion problem. A way to achieve this goal is to study new S-bearing molecules in the laboratory, obtaining accurate rest frequencies for an astronomical search. We focus on dithioformic acid, HCSSH, which is the sulphur analogue of formic acid. The aim of this study is to provide an accurate line list of the two HCSSH $trans$ and $cis$ isomers in their electronic ground state and a comprehensive centrifugal distortion analysis with an extension of measurements in the millimetre and submillimetre range. We studied the two isomers in the laboratory using an absorption spectrometer employing the frequency-modulation technique. The molecules were produced directly within a free-space cell by glow discharge of a gas mixture. We measured lines belonging to the electronic ground state up to 478 GHz, with a total number of 204 and 139 new rotational transitions, respectively, for $trans$ and $cis$ isomers. The final dataset also includes lines in the centimetre range available from literature. The extension of the measurements in the mm and submm range lead to an accurate set of rotational and centrifugal distortion parameters. This allows us to predict frequencies with estimated uncertainties as low as 5 kHz at 1 mm wavelength. Hence, the new dataset provided by this study can be used for astronomical search.

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D. Prudenzano, J. Laas, L. Bizzocchi, et. al.
Tue, 23 Jan 18
70/85

Comments: N/A

ExoMol line lists — XXII. The rotation-vibration spectrum of silane up to 1200 K [CL]

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http://arxiv.org/abs/1712.09354


A variationally computed $^{28}$SiH$_4$ rotation-vibration line list applicable for temperatures up to $T=1200\,$K is presented. The line list, called OY2T, considers transitions with rotational excitation up to $J=42$ in the wavenumber range $0$–$5000\,$cm$^{-1}$ (wavelengths $\lambda> 2\,\mu$m). Just under 62.7 billion transitions have been calculated between 6.1 million energy levels. Rovibrational calculations have utilized a new `spectroscopic’ potential energy surface determined by empirical refinement to 1452 experimentally derived energy levels up to $J=6$, and a previously reported \textit{ab initio} dipole moment surface. The temperature-dependent partition function of silane, the OY2T line list format, and the temperature dependence of the OY2T line list are discussed. Comparisons with the PNNL spectral library and other experimental sources indicate that the OY2T line list is robust and able to accurately reproduce weaker intensity features. The full line list is available from the ExoMol database and the CDS database.

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A. Owens, A. Yachmenev, W. Thiel, et. al.
Fri, 29 Dec 2017
19/65

Comments: 11 pages, 9 figures, 3 tables

Tunneling Reaction Kinetics for the Hydrogen Abstraction Reaction H + H$_2$S -> H$_2$ + HS in the Interstellar Medium [GA]

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http://arxiv.org/abs/1712.02088


The hydrogen abstraction reaction between H and H$_2$S, yielding HS and H$_2$ as products, has been studied within the framework of interstellar surface chemistry. High-temperature rate constants up to 2000 K are calculated in the gas phase and are in agreement with previously reported values. Subsequently low-temperature rate constants down to 55 K are presented for the first time that are of interest to astrochemistry, i.e., covering both bimolecular and unimolecular reaction mechanisms. For this, a so-called implicit surface model is used. Strictly speaking, this is a structural gas-phase model in which the restriction of the rotation in the solid state is taken into account. The calculated kinetic isotope effects are explained in terms of difference in activation and delocalization. All rate constants are calculated at the UCCSD(T)-F12/cc-VTZ-F12 level of theory. Finally, we show that the energetics of the reaction is only affected to a small extent by the presence of H$_2$O or H$_2$S molecular clusters that simulate an ice surface, calculated at the MPWB1K/def2-TZVP level of theory.

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T. Lamberts and J. Kastner
Thu, 7 Dec 17
56/72

Comments: Accepted for publication in Journal of Physical Chemistry A this http URL

Gas-Phase Spectra of MgO Molecules: A Possible Connection from Gas-Phase Molecules to Planet Formation [EPA]

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http://arxiv.org/abs/1711.03544


A more fine-tuned method for probing planet-forming regions, such as protoplanetary discs, could be rovibrational molecular spectroscopy observation of particular premineral molecules instead of more common but ultimately less related volatile organic compounds. Planets are created when grains aggregate, but how molecules form grains is an ongoing topic of discussion in astrophysics and planetary science. Using the spectroscopic data of molecules specifically involved in mineral formation could help to map regions where planet formation is believed to be occurring in order to examine the interplay between gas and dust. Four atoms are frequently associated with planetary formation: Fe, Si, Mg, and O. Magnesium, in particular, has been shown to be in higher relative abundance in planet-hosting stars. Magnesium oxide crystals comprise the mineral periclase making it the chemically simplest magnesium-bearing mineral and a natural choice for analysis. The monomer, dimer, and trimer forms of (MgO)_n with n = 1 – 3 are analyzed in this work using high-level quantum chemical computations known to produce accurate results. Strong vibrational transitions at 12.5 {\mu}m, 15.0 {\mu}m, and 16.5 {\mu}m are indicative of magnesium oxide monomer, dimer, and trimer making these wavelengths of particular interest for the observation of protoplanetary discs and even potentially planet-forming regions around stars. If such transitions are observed in emission from the accretion discs or absorptions from stellar spectra, the beginning stages of mineral and, subsequently, rocky body formation could be indicated.

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K. Kloska and R. Fortenberry
Mon, 13 Nov 17
16/46

Comments: 10 pages, 2 figures, 6 tables, Accepted in MNRAS

Gas-Phase Spectra of MgO Molecules: A Possible Connection from Gas-Phase Molecules to Planet Formation [EPA]

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http://arxiv.org/abs/1711.03544


A more fine-tuned method for probing planet-forming regions, such as protoplanetary discs, could be rovibrational molecular spectroscopy observation of particular premineral molecules instead of more common but ultimately less related volatile organic compounds. Planets are created when grains aggregate, but how molecules form grains is an ongoing topic of discussion in astrophysics and planetary science. Using the spectroscopic data of molecules specifically involved in mineral formation could help to map regions where planet formation is believed to be occurring in order to examine the interplay between gas and dust. Four atoms are frequently associated with planetary formation: Fe, Si, Mg, and O. Magnesium, in particular, has been shown to be in higher relative abundance in planet-hosting stars. Magnesium oxide crystals comprise the mineral periclase making it the chemically simplest magnesium-bearing mineral and a natural choice for analysis. The monomer, dimer, and trimer forms of (MgO)_n with n = 1 – 3 are analyzed in this work using high-level quantum chemical computations known to produce accurate results. Strong vibrational transitions at 12.5 {\mu}m, 15.0 {\mu}m, and 16.5 {\mu}m are indicative of magnesium oxide monomer, dimer, and trimer making these wavelengths of particular interest for the observation of protoplanetary discs and even potentially planet-forming regions around stars. If such transitions are observed in emission from the accretion discs or absorptions from stellar spectra, the beginning stages of mineral and, subsequently, rocky body formation could be indicated.

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K. Kloska and R. Fortenberry
Mon, 13 Nov 17
22/46

Comments: 10 pages, 2 figures, 6 tables, Accepted in MNRAS

The formation of urea in space I. Ion-molecule, neutral-neutral, and radical gas-phase reactions [GA]

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http://arxiv.org/abs/1711.03457


Many organic molecules have been observed in the interstellar medium thanks to advances in radioastronomy, and very recently the presence of urea was also suggested. While those molecules were observed, it is not clear what the mechanisms responsible to their formation are. In fact, if gas-phase reactions are responsible, they should occur through barrierless mechanisms (or with very low barriers). In the past, mechanisms for the formation of different organic molecules were studied, providing only in a few cases energetic conditions favorable to a synthesis at very low temperature. A particularly intriguing class of such molecules are those containing one N–C–O peptide bond, which could be a building block for the formation of biological molecules. Urea is a particular case because two nitrogen atoms are linked to the C–O moiety. Thus, motivated also by the recent tentative observation of urea, we have considered the synthetic pathways responsible to its formation.
We have studied the possibility of forming urea in the gas phase via different kinds of bi-molecular reactions: ion-molecule, neutral, and radical. In particular we have focused on the activation energy of these reactions in order to find possible reactants that could be responsible for to barrierless (or very low energy) pathways.
We have used very accurate, highly correlated quantum chemistry calculations to locate and characterize the reaction pathways in terms of minima and transition states connecting reactants to products.
Most of the reactions considered have an activation energy that is too high; but the ion-molecule reaction between NH$_2$OH$_2^+$ and formamide is not too high. These reactants could be responsible not only for the formation of urea but also of isocyanic acid, which is an organic molecule also observed in the interstellar medium.

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F. Brigiano, Y. Jeanvoine, A. Largo, et. al.
Fri, 10 Nov 17
16/55

Comments: 9 pages + 8 pages of appendix

ExoMol line lists XXIV: A new hot line list for silicon monohydride, SiH [SSA]

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http://arxiv.org/abs/1710.06964


SiH has long been observed in the spectrum of our Sun and other cool stars. Computed line lists for the main isotopologues of silicon monohydride, $^{28}$SiH, $^{29}$SiH, $^{30}$SiH and $^{28}$SiD are presented. These line lists consider rotation-vibration transitions within the ground $X$ $^{2}\Pi$ electronic state as well as transitions to the low-lying $A$ $^{2}\Delta$ and $a$ $^{4}\Sigma^-$ states. Ab initio potential energy (PECs) and dipole moment curves (DMCs) along with spin-orbit and electronic-angular-momentum couplings between them are calculated using the MRCI level of theory with the MOLPRO package. The PEC for the ground X $^2\Pi$ state is refined to available experimental data with a typical accuracy of around 0.01 cm$^{-1}$ or better. The $^{28}$SiH line list includes 11,785 rovibronic states and 1,724,841 transitions with associated Einstein-A coefficients for angular momentum $J$ up to $82.5$ and covering wavenumbers up to 31340 cm$^{-1}$ ($\lambda$ $<$ 0.319 $\mu$m). Spectra are simulated using the new line list and comparisons made with various experimental spectra. These line lists are applicable up to temperatures of ~5000 K, making them relevant to astrophysical objects such as exoplanetary atmospheres and cool stars and opening up the possibility of detection in the interstellar medium. These line lists are available at the ExoMol www.exomol.com and CDS database websites.

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S. Yurchenko, F. Sinden, L. Lodi, et. al.
Fri, 20 Oct 17
9/42

Comments: N/A

Associative detachment (AD) paths for H and CN- in the gas-phase: astrophysical implications [GA]

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http://arxiv.org/abs/1710.06343


The direct dynamical paths leading to Associative Detachment (AD) in the gas-phase, and specifically in the low-temperature regions of the Dark Molecular Clouds (DMC) in the ISM, or in cold trap laboratory experiments, are investigated with quantum chemical methods by using a high-level multi-reference Configuration Interaction (CI) approach that employs single and double excitations plus Davidson perturbative correction [MRSDCI(Q)] and the d-aug-cc-pV5Z basis set. The potential energy curves for H + CN- are constructed for different directions of the H partner approaching the CN- anion within the framework of the Born-Oppenheimer approximation. The present calculations found that the AD energetics at low temperature becomes favorable only along a selected range of approaching directions, thus showing that there is a preferred path of forming HCN at low temperatures, while that of forming its HNC isomer is found to be energetically forbidden. Given the existence in the ISM of different HCN/HNC ratios in different environments, we discuss the implications of our findings for selective formation of either isomer in the low-temperature conditions of the Molecular Cloud Cores.

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S. Jerosimic, F. Gianturco and R. Wester
Wed, 18 Oct 2017
42/62

Comments: N/A

The efficient photodesorption of nitric oxide (NO) ices A laboratory astrophysics study [GA]

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http://arxiv.org/abs/1710.01028


The study and quantification of UV photon-induced desorption of frozen molecules furthers our understanding of the chemical evolution of cold interstellar regions. Nitric oxide (NO) is an important intermediate species in both gas-phase and solid-phase chemical networks. In this work, we present quantitative measurements of the photodesorption of a pure NO ice.We used the tunable monochromatic synchrotron light of the DESIRS beamline of the SOLEIL facility near Paris to irradiate NO ices in the 6 – 13.6 eV range and measured desorption by quadrupole mass spectrometry.We find that NO photodesorption is very efficient, its yield being around 1e-2 molecule per incident photon for UV fields relevant to the diffuse and dense interstellar medium. We discuss the extrapolation of our results to an astrophysical context and we compare photodesorption of NO to previously studied molecules.

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R. Dupuy, G. Feraud, M. Bertin, et. al.
Wed, 4 Oct 17
17/73

Comments: A&A, Accepted

Nitrile versus isonitrile adsorption at interstellar grain surfaces II. Carbonaceous aromatic surfaces [GA]

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http://arxiv.org/abs/1709.03721


Almost 20 % of the ~ 200 different species detected in the interstellar and circumstellar media present a carbon atom linked to nitrogen by a triple bond. Of these 37 molecules, 30 are nitrile R-CN compounds, the remaining 7 belonging to the isonitrile R-NC family. How these species behave in their interactions with the grain surfaces is still an open question. In a previous work, we have investigated whether the difference between nitrile and isonitrile functional groups may induce differences in the adsorption energies of the related isomers at the surfaces of interstellar grains of various nature and morphologies. This study is a follow up of this work, where we focus on the adsorption on carbonaceous aromatic surfaces. The question is addressed by means of a concerted experimental and theoretical approach of the adsorption energies of CH$_3$CN and CH$_3$NC on the surface of graphite (with and without surface defects). The experimental determination of the molecule and surface interaction energies is carried out using temperature-programmed desorption (TPD) in an ultra-high vacuum (UHV) between 70 and 160 K. Theoretically, the question is addressed using first-principle periodic density functional theory (DFT) to represent the organised solid support. The adsorption energy of each compound is found to be very sensitive to the structural defects of the aromatic carbonaceous surface: these defects, expected to be present in a large numbers and great diversity on a realistic surface, significantly increase the average adsorption energies to more than 50\% as compared to adsorption on perfect graphene planes. The most stable isomer (CH$_3$CN) interacts more efficiently with the carbonaceous solid support than the higher energy isomer (CH$_3$NC), however.

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M. Bertin, M. Doronin, X. Michaut, et. al.
Wed, 13 Sep 17
53/72

Comments: N/A

High-resolution one-photon absorption spectroscopy of the $D\,{}^2Σ^- \leftarrow X\,{}^2Π$ system of radical OH and OD [CL]

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http://arxiv.org/abs/1709.02509


Vacuum-ultraviolet (VUV) photoabsorption spectra were recorded of the $A\,{}^2\Sigma^+(v’=0)\leftarrow{}X\,{}^2\Pi(v”=0)$, $D\,{}^2\Sigma^-(v’=0)\leftarrow{}X\,{}^2\Pi(v”=0)$ and $D\,{}^2\Sigma^-(v’=1)\leftarrow{}X\,{}^2\Pi(v”=0)$ bands of the OH and OD radicals generated in a plasma-discharge source using synchrotron radiation as a background continuum coupled with the VUV Fourier-transform spectrometer on the DESIRS beamline of synchrotron SOLEIL. High-resolution spectra permitted the quantification of transition frequencies, relative $f$-values, and natural line broadening. The $f$-values were absolutely calibrated with respect to a previous measurement of $A\,{}^2\Sigma^+(v’=0)\leftarrow{}X\,{}^2\Pi(v”=0)$ ([wang1979]). Lifetime broadening of the excited $D\,{}^2\Sigma^-(v=0)$ and $D\,{}^2\Sigma^-(v=1)$ levels is measured for the first time and compared with previous experimental limits, and implies a lifetime 5 times shorter than a theoretical prediction ([van_der_loo2005]). A local perturbation of the $D\,{}^2\Sigma^-(v=0)$ level in OH was found.

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A. Heays, N. Oliveira, B. Gans, et. al.
Mon, 11 Sep 17
19/49

Comments: N/A

Fragmentation of Fullerenes to Linear Carbon Chains [CL]

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http://arxiv.org/abs/1709.00317


Small cationic fullerene fragments, produced by electron impact ionization of C60, were mass-selected and accumulated in cryogenic Ne matrixes. Optical absorption spectroscopy of these fragments with up to 18 carbon atoms revealed linear structures. Considering the recent discovery of fullerenes in Space and the very strong absorptions of long linear carbon clusters both in the UV-Vis and IR spectral regions, these systems are good candidates to be observed in Space. We present laboratory data, supported by quantum-chemical calculations and discuss the relevance of long carbon chains for astronomy.

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D. Strelnikov, M. Link and M. Kappes
Mon, 4 Sep 17
55/61

Comments: 16 pages, 10 figures

Categorize Interstellar Infrared Spectrum by Polycyclic Pure-Carbon-Molecule and Hydrocarbon-Molecule [GA]

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http://arxiv.org/abs/1708.08043


By applying quantum chemical calculation, interstellar infrared spectrum was categorized to three classes. Type-A show unusual feature of strong peaks at 11.3,12.9, and 14.0 micrometer. Usually observed 6.2, 7.7, and 8.6 micrometer bands are weak or not recognized. Typical examples are NGC1316 and NGC4589. Such spectrum could be identified for the first time by pure carbon molecule (C23)2+ (dication) having two carbon pentagons combined with five hexagons. Calculated spectrum coincided well at 11.3, 13.0, and 14.0 micrometer. Also we could find more coincidence at 5.2, 5.6, 7.6, 8.8,10.6,15.7 and 17.2 micrometer. Type-B is ubiquitously observed IR, but show medium strength at 11.3 micrometer. Examples are NGC6946 and the red triangle nebula. Coronene modified PAH (C23H12)2+ show best coincidence for both wavelength and strength. Type-C is usualy observed one featuring very strong peak at 11.3 micrometer. Examples are NGC7023, NGC2023 and M17SW. One capable expanation of large 11.3 micrometer band is a mixture of Type-A and Type-B. Combination of polycyclic pure-carbon-molecule and hydrocarbon-molecule may give a variety of IR spectrum.

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N. Ota
Tue, 29 Aug 17
44/65

Comments: 9 pages, 7 figures

Path integral Monte Carlo simulations of dense carbon-hydrogen plasmas [CL]

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http://arxiv.org/abs/1708.07246


Carbon-hydrogen plasmas and hydrocarbon materials are of broad interest to laser shock experimentalists, high energy density physicists, and astrophysicists. Accurate equations of state (EOS) of hydrocarbons are valuable for various studies from inertial confinement fusion (ICF) to planetary science. By combining path integral Monte Carlo (PIMC) results at high temperatures and density functional theory molecular dynamics (DFT-MD) results at lower temperatures, we compute the EOS for hydrocarbons at 1184 separate ($\rho,T$)-points distributed over a range of compositions. These methods accurately treat electronic excitation and many-body interaction effects and thus provide a benchmark-quality EOS that surpasses that of semi-empirical and Thomas-Fermi-based methods in the warm dense matter regime. By comparing our first-principles EOS to the LEOS 5112 model for CH, we validate the specific heat assumptions in this model but suggest that the Grueneisen parameter is too large at low temperature. Based on our first-principles EOS, we predict the Hugoniot curve of polystyrene to be ~5% stiffer at maximum compression than that predicted by orbital-free DFT and other Thomas-Fermi-based approaches. By investigating the atomic structure and chemical bonding, we show a drastic decrease in the lifetime of chemical bonds in the pressure interval of 0.4-4 megabar. We find the assumption of linear mixing to be valid for describing the EOS and the shock Hugoniot curve of the dense, partially ionized hydrocarbons under consideration. We make predictions of the shock compression of glow-discharge polymers and investigate the effects of oxygen content and C:H ratio on their Hugoniot curve. Our full suite of first-principles simulation results may be used to benchmark future theoretical investigations pertaining to hydrocarbon EOS, and should be helpful in guiding the design of future gigabar experiments.

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S. Zhang, B. Militzer, L. Benedict, et. al.
Fri, 25 Aug 17
30/59

Comments: 13 pages, 12 figures, 1 table

Atom Tunneling in the Water Formation Reaction H$_2$ + OH $\rightarrow$ H$_2$O + H on an Ice Surface [GA]

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http://arxiv.org/abs/1708.05559


OH radicals play a key role as an intermediate in the water formation chemistry of the interstellar medium. For example the reaction of OH radicals with H$_2$ molecules is among the final steps in the astrochemical reaction network starting from O, O$_2$, and O$_3$. Experimentally it was shown that even at 10 K this reaction occurs on ice surfaces. As the reaction has a high activation energy only atom tunneling can explain such experimental findings.
In this study we calculated reaction rate constants for the title reaction on a water-ice I$_h$ surface. To our knowledge, low-temperature rate constants on a surface are not available in the literature. All surface calculations were done using a QM/MM framework (BHLYP/TIP3P) after a thorough benchmark of different density functionals and basis sets to highly accurate correlation methods. Reaction rate constants are obtained using instanton theory which takes atom tunneling into account inherently, with constants down to 110 K for the Eley-Rideal mechanism and down to 60 K for the Langmuir-Hinshelwood mechanism. We found that the reaction is nearly temperature independent below 80 K. We give kinetic isotope effects for all possible deuteration patterns for both reaction mechanisms. For the implementation in astrochemical networks, we also give fit parameters to a modified Arrhenius equation. Finally, several different binding sites and binding energies of OH radicals on the I$_h$ surface are discussed and the corresponding rate constants are compared to the gas-phase case.

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J. Meisner, T. Lamberts and J. Kastner
Mon, 21 Aug 17
27/44

Comments: Published online. Supporting information on this http URL

Influence of surface and bulk water ice on the reactivity of a water-forming reaction [GA]

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http://arxiv.org/abs/1708.05555


On the surface of icy dust grains in the dense regions of the interstellar medium a rich chemistry can take place. Due to the low temperature, reactions that proceed via a barrier can only take place through tunneling. The reaction H + H$_2$O$_2$ $\rightarrow$ H$_2$O + OH is such a case with a gas-phase barrier of $\sim$26.5 kJ/mol. Still the reaction is known to be involved in water formation on interstellar grains. Here, we investigate the influence of a water ice surface and of bulk ice on the reaction rate constant. Rate constants are calculated using instanton theory down to 74 K. The ice is taken into account via multiscale modeling, describing the reactants and the direct surrounding at the quantum mechanical level with density functional theory (DFT), while the rest of the ice is modeled on the molecular mechanical level with a force field. We find that H$_2$O$_2$ binding energies cannot be captured by a single value, but rather depend on the number of hydrogen bonds with surface molecules. In highly amorphous surroundings the binding site can block the routes of attack and impede the reaction. Furthermore, the activation energies do not correlate with the binding energies of the same sites. The unimolecular rate constants related to the Langmuir-Hinshelwood mechanism increase as the activation energy decreases. Thus, we provide a lower limit for the rate constant and argue that rate constants can have values up to two order of magnitude larger than this limit.

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T. Lamberts and J. Kastner
Mon, 21 Aug 17
43/44

Comments: Accepted for publication in ApJ

Editorial: Special Issue on the Atomic and Molecular Processes in the Ultracold Regime, the Chemical Regime, and Astrophysics [CL]

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http://arxiv.org/abs/1708.00943


This editorial introduces the J. Phys. B: Atomic, Molecular and Optical Physics Special Issue “Atomic and Molecular Processes in the Ultracold Regime, the Chemical Regime and Astrophysics” dedicated to Professor Alexander Dalgarno (1928-2015). After a brief biographical review, short summaries of the contributed papers and their relations to some of Prof. Dalgarno’s work are given.

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J. Babb, R. Cote, H. Sadeghpour, et. al.
Fri, 4 Aug 17
27/47

Comments: 7 pp

A calculation of the Deuterium Hugoniot using the classical-map hypernetted-chain (CHNC) approach [CL]

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http://arxiv.org/abs/1707.08880


The Hugoniot for Deuterium is calculated using the classical-map hyper-netted-chain (CHNC) approach using several models of the effective temperature that may be assigned to the electron-ion interaction. This effective temperature embodies the exchange-correlation and kinetic energy functional that is assigned to the electron-ion interaction. Deuterium pair distribution functions (calculated using the neutral-pseudo atom method) showing the formation of molecular pre-peaks are displayed to clarify the soft-turning over of the hugoniot in the pressure range of 0.2-0.6 Megabars. This contribution updates a previous CHNC calculation of the deuterium hugoniot given in Phys. Rev. B, 66, 014110 (2002).

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M. Dharma-wardana
Fri, 28 Jul 17
18/48

Comments: This paper contains some of the material submitted to the DOE/NNSA EOS workshop, May-31, Rochester, USA, 2017

Demonstration of radon removal from SF$_6$ using molecular sieves for dark matter searches [CL]

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http://arxiv.org/abs/1707.07772


The gas SF$_6$ has become of interest as a negative ion drift gas for use in directional dark matter searches. However, as for other targets in such searches, it is important that radon contamination can be removed as this provides a source of unwanted background events. In this work we demonstrate for the first time filtration of radon from SF$_6$ gas by using a molecular sieve. Four types of sieve from Sigma-Aldrich were investigated, namely 3A, 4A, 5A and 13X. A manufactured radon source was used for the tests. This was attached to a closed loop system in which gas was flowed through the filters and a specially adapted Durridge RAD7 radon detector. In these measurements, it was found that only the 5A type was able to significantly reduce the radon concentration without absorbing the SF$_6$ gas. The sieve was able to reduce the initial radon concentration of 3875 $\pm$ 13 Bqm$^{-3}$ in SF$_6$ gas by 87% when cooled with dry ice. The ability of the cooled 5A molecular sieve filter to significantly reduce radon concentration from SF$_6$ provides a promising foundation for the construction of a radon filtration setup for future ultra-sensitive SF$_6$ gas rare-event physics experiments.

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A. Ezeribe, W. Lynch, R. Mckeand, et. al.
Wed, 26 Jul 17
6/68

Comments: 12 pages, 10 figures

Deuteration of ammonia with D atoms on oxidized partly ASW covered graphite surface [IMA]

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http://arxiv.org/abs/1707.04063


The deuteration of ammonia by D atoms has been investigated experimentally in the sub-monolayer regime on realistic analogues of interstellar dust grain surfaces. About 0.8 monolayer of solid NH3 was deposited on top of an oxidized graphite surface held at 10 K, partly covered with ASW ice. Ammonia ice is subsequently exposed to D atoms for different exposure times using a differentially pumped beam-line. The deuteration experiments of ammonia were carried out by mass spectroscopy and temperature programmed desorption (TPD) technique. The experimental results showed the formation of three isotopologue ammonia species by direct exothermic H-D substitution surface reactions: NH3+D->NH2D+H, NH2D+D->NHD2+H, and NHD2+D->ND3+H. The formation of the deuterated isotopologues NH2D, NHD2, and ND3 at low surface temperature (10 K) is likely to occur through quantum tunneling process on the oxidized graphite surface. A kinetic model taking into account the diffusion of D atoms on the surface is developed to estimate the width and the hight of the activation energy barriers for the successive deuteration reactions of ammonia species by D atoms. Identical control experiments were performed using CH3OH and D atoms. The deuteration process of solid methanol is ruled by H abstraction and D addition mechanism, and is almost five orders of magnitude faster than ammonia deuteration process.

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H. Chaabouni, M. Minissale, S. Baouche, et. al.
Fri, 14 Jul 17
39/55

Comments: N/A

Millimeter and Submillimeter Wave Spectroscopy of Propanal [SSA]

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http://arxiv.org/abs/1707.02148


The rotational spectra of the two stable conformers syn- and gauche-propanal (CH3CH2CHO) were studied in the millimeter and submillimeter wave regions from 75 to 500 GHz with the Cologne (Sub-)Millimeter wave Spectrometer. Furthermore, the first excited states associated with the aldehyde torsion and with the methyl torsion, respectively, of the syn-conformer were analyzed. The newly obtained spectroscopic parameters yield better predictions, thus fulfill sensitivity and resolution requirements in new astronomical observations in order to unambiguously assign pure rotational transitions of propanal. This is demonstrated on a radio astronomical spectrum from the Atacama Large Millimeter/submillimeter Array Protostellar Interferometric Line Survey (ALMA-PILS). In particular, an accurate description of observed splittings, caused by internal rotation of the methyl group in the syn-conformer and by tunneling rotation interaction from two stable degenerate gauche-conformers, is reported. The rotational spectrum of propanal is of additional interest because of its two large amplitude motions pertaining to the methyl and the aldehyde group, respectively.

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O. Zingsheim, H. Muller, F. Lewen, et. al.
Mon, 10 Jul 17
33/64

Comments: 8 pages, J. Mol. Spectrosc., accepted

On the Energetics of the HCO$^+$ + C $\to$ CH$^+$ + CO Reaction and Some Astrochemical Implications [GA]

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http://arxiv.org/abs/1707.01456


We explore the energetics of the titular reaction, which current astrochemical databases consider open at typical dense molecular (i.e., dark) cloud conditions. As is common for reactions involving the transfer of light particles, we assume that there are no intersystem crossings of the potential energy surfaces involved. In the absence of any such crossings, we find that this reaction is endoergic and will be suppressed at dark cloud temperatures. Updating accordingly a generic astrochemical model for dark clouds changes the predicted gas-phase abundances of 224 species by greater than a factor of 2. Of these species, 43 have been observed in the interstellar medium. Our findings demonstrate the astrochemical importance of determining the role of intersystem crossings, if any, in the titular reaction.

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D. Savin, R. Bhaskar, S. Vissapragada, et. al.
Thu, 6 Jul 17
28/52

Comments: Accepted for publication in ApJ; 14 pages, 2 figures, and 1 table

C5H9N Isomers: Pointers to Possible Branched Chain Interstellar Molecules [GA]

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http://arxiv.org/abs/1707.00645


The astronomical observation of isopropyl cyanide further stresses the link between the chemical composition of the ISM and molecular composition of the meteorites in which there is a dominance of branched chain amino acids as compared to the straight. However, observations of more branched chain molecules in ISM will firmly establish this link. In the light of this, we have considered C5H9N isomeric group in which the next higher member of the alkyl cyanide and other branched chain isomers belong. High-level quantum chemical calculations have been employed in estimating accurate energies of these isomers. From the results, the only isomer of the group that has been astronomically searched, n-butyl cyanide is not the most stable isomer and therefore, which might explain why its search could only yield upper limits of its column density without a successful detection. Rather, the two most stable isomers of the group are the branched chain isomers, tert-butylnitrile and isobutyl cyanide. Based on the rotational constants of these isomers, it is found that the expected intensity of tert-butylnitrile is the maximum among this isomeric group. Thus, this is proposed as the most probable candidate for astronomical observation. A simple LTE (Local thermodynamic equilibrium) modelling has also been carried out to check the possibility of detecting tert-butyl cyanide in the millimetre-wave region.

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E. Etim, P. Gorai, A. Das, et. al.
Tue, 4 Jul 17
66/74

Comments: 16 pages, 1 figure

A new model of the chemistry of ionizing radiation in solids: CIRIS [CL]

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http://arxiv.org/abs/1706.05430


The collisions between high-energy ions and solids can result in significant physical and chemical changes to the material. These effects are potentially important for better understanding the chemistry of interstellar and planetary bodies, which are exposed to cosmic radiation and the solar wind, respectively, however, modeling such collisions on a detailed microscopic basis has thus far been largely unsuccessful. To that end, a new model, entitled CIRIS: the Chemistry of Ionizing Radiation in Solids, was created to calculate the physical and chemical effects of the irradiation of solid materials. With the new code, we simulate O2 ice irradiated with 100 keV protons. Our models are able to reproduce the measured ozone abundances of a previous experimental study, as well as independently predict the approximate thickness of the ice used in that work.

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C. Shingledecker, R. Gal and E. Herbst
Tue, 20 Jun 17
53/72

Comments: N/A

Diffuse Interstellar Bands: A Comprehensive Laboratory Study [CL]

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http://arxiv.org/abs/1706.04273


As a result of the search for the identity of the chromophores responsible for producing the diffuse interstellar bands, a comprehensive exposition of experimental data is presented, which implicates the following molecules- 1. The extremely stable organic molecules, magnesium tetrabenzoporphyrin (MgTBP) and H2TBP. 2. A paraffin matrix (referred to as grains) containing TBPs. 3. A low concentration of pyridine (also within the grains), whose transmission window at 2175 Angstroms, accounts for the ubiquitous UV bump. The blue emission spectra associated with the central star, HD44179, of the Red Rectangle displays the fluorescence excitation spectra of bare MgTBP. This unique spectrum matches the low temperature lab data of MgTBP in the vapor phase. An effective grain temperature of 2.728 K (plus or minus 0.008) was deduced, based on MgTBPs lowest measured vibrational state of 341 GHz.

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F. Johnson
Thu, 15 Jun 17
52/68

Comments: N/A

Collisional excitation of NH3 by atomic and molecular hydrogen [CL]

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http://arxiv.org/abs/1706.00732


We report extensive theoretical calculations on the rotation-inversion excitation of interstellar ammonia (NH3) due to collisions with atomic and molecular hydrogen (both para- and ortho-H2). Close-coupling calculations are performed for total energies in the range 1-2000 cm-1 and rotational cross sections are obtained for all transitions among the lowest 17 and 34 rotation-inversion levels of ortho- and para-NH3, respec- tively. Rate coeffcients are deduced for kinetic temperatures up to 200 K. Propensity rules for the three colliding partners are discussed and we also compare the new results to previous calculations for the spherically symmetrical He and para-H2 projectiles. Significant differences are found between the different sets of calculations. Finally, we test the impact of the new rate coeffcients on the calibration of the ammonia ther- mometer. We find that the calibration curve is only weakly sensitive to the colliding partner and we confirm that the ammonia thermometer is robust.

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N. Bouhafs, C. Rist, F. Daniel, et. al.
Mon, 5 Jun 17
27/43

Comments: N/A

ExoMol Line List XXI: Nitric Oxide (NO) [EPA]

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http://arxiv.org/abs/1705.05955


Line lists for the ground electronic ground state for six major isotopologues of nitric oxide are presented. The line lists are constructed using empirical energy levels (and line positions) and high-level {\it ab inito} intensities. The energy levels were obtained using a combination of two approaches, from an effective Hamiltonian and from solving the rovibronic Schr\”{o}dinger equation variationally. The effective hamiltonian model was obtained through a fit to the experimental line positions of NO available in the literature for all six isotopologues using the programs SPFIT and SPCAT. The variational model was built through a least squares fit of the \textit{ab inito} potential and spin-orbit curves to the experimentally derived energies and experimental line positions of the main isotopologue only using the Duo program. The \textit{ab inito} potential energy, spin-orbit and dipole moment curves (PEC, SOC and DMC) are computed using high-level {\it ab inito} methods and the MARVEL method is used to obtain energies of NO from experimental transition frequencies. Each line list covers a wavenumber range from 0 – 40,000 \cm with approximately 22,000 rovibronic states and 2.3-2.6 million transitions extending to $J_{max} = 184.5$ and $v_{max} = 51$. Partition functions are also calculated up to a temperature of 5000 K. The calculated absorption line intensities at 296 K using these line lists show excellent agreement with those included in the HITRAN and HITEMP databases. The computed NO line lists are the most comprehensive to date, covering a wider wavenumber and temperature range compared to both the HITRAN and HITEMP databases. These line lists are also more accurate than those used in HITEMP. The full line lists are available from the CDS and ExoMol databases; data will also be available from CDMS.

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A. Wong, S. Yurchenko, P. Bernath, et. al.
Thu, 18 May 17
4/60

Comments: N/A

Magnetic Bubble Chambers and Sub-GeV Dark Matter Direct Detection [CL]

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http://arxiv.org/abs/1701.06566


We propose a new application of single molecule magnet crystals: their use as “magnetic bubble chambers” for the direct detection of sub-GeV dark matter. The spins in these macroscopic crystals effectively act as independent nano-scale magnets. When anti-aligned with an external magnetic field they form meta-stable states with a relaxation time that can be very long at sufficiently low temperatures. The Zeeman energy stored in this system can be released through localized heating, caused for example by the scattering or absorption of dark matter, resulting in a spin avalanche (or “magnetic deflagration”) that amplifies the effects of the initial heat deposit, enabling detection. Much like the temperature and pressure in a conventional bubble chamber, the temperature and external magnetic field set the detection threshold for a single molecule magnet crystal. We discuss this detector concept for dark matter detection and propose ways to ameliorate backgrounds. If successfully developed, this detector concept can search for hidden photon dark matter in the meV – eV mass range with sensitivities exceeding current bounds by several orders of magnitude.

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P. Bunting, G. Gratta, T. Melia, et. al.
Mon, 8 May 17
11/54

Comments: 21 pages, 8 figures; v2 journal version

Molecular Isomer Identification of Titan Tholins Organic Aerosols by Photoelectron/Photoion Coincidence Spectroscopy Coupled to VUV Synchrotron Radiation [EPA]

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http://arxiv.org/abs/1705.01918


The chemical composition of Titan organic haze is poorly known. To address this issue, laboratory analogs named tholins are synthesized, and analyzed by methods requiring often an extraction process in a carrier solvent. These methods exclude the analysis of the insoluble tholins fraction and assume a hypothetical chemical equivalence between soluble and insoluble fractions. In this work, we present a powerful complementary analysis method recently developed on the DESIRS VUV synchrotron beamline at SOLEIL. It involves a soft pyrolysis of tholins at ~230 deg C and an electron ion coincidence analysis of the emitted volatiles compounds photoionized by the tunable synchrotron radiation. By comparison with reference photoelectron spectra (PES), the spectral information collected on the detected molecules yields their isomeric structure. The method is more readily applied to light species, while for heavier ones the number of possibilities and the lack of PES reference spectra in the literature limit its analysis. A notable pattern in the analyzed tholins is the presence of species containing adjacent doubly-bonded N atoms, which might be a signature of heterogeneous incorporation of N2 in tholins.

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B. Cunha-de-Miranda, G. Garcia, F. Gaie-Levrel, et. al.
Fri, 5 May 17
34/60

Comments: N/A

Spectrally-resolved UV photodesorption of CH4 in pure and layered ices [GA]

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http://arxiv.org/abs/1704.05398


Context. Methane is among the main components of the ice mantles of insterstellar dust grains, where it is at the start of a rich solid-phase chemical network. Quantification of the photon-induced desorption yield of these frozen molecules and understanding of the underlying processes is necessary to accurately model the observations and the chemical evolution of various regions of the interstellar medium. Aims. This study aims at experimentally determining absolute photodesorption yields for the CH4 molecule as a function of photon energy. The influence of the ice composition is also investigated. By studying the methane desorption from layered CH4:CO ice, indirect desorption processes triggered by the excitation of the CO molecules is monitored and quantified. Methods. Tunable monochromatic VUV light from the DESIRS beamline of the SOLEIL synchrotron is used in the 7 – 13.6 eV (177 – 91 nm) range to irradiate pure CH4 or layers of CH4 deposited on top of CO ice samples. The release of species in the gas phase is monitored by quadrupole mass spectrometry and absolute photodesorption yields of intact CH4 are deduced. Results. CH4 photodesorbs for photon energies higher than ~9.1 eV (~136 nm). The photodesorption spectrum follows the absorption spectrum of CH4, which confirms a desorption mechanism mediated by electronic transitions in the ice. When it is deposited on top of CO, CH4 desorbs between 8 and 9 eV with a pattern characteristic of CO absorption, indicating desorption induced by energy transfer from CO molecules. Conclusions. The photodesorption of CH4 from the pure ice in various interstellar environments is around 2.0 x 10^-3 molecules per incident photon. Results on CO-induced indirect desorption of CH4 provide useful insights for the generalization of this process to other molecules co-existing with CO in ice mantles.

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R. Dupuy, M. Bertin, G. Feraud, et. al.
Wed, 19 Apr 17
3/62

Comments: N/A

ExoMol molecular line lists XX: a comprehensive line list for H$_3^+$ [GA]

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http://arxiv.org/abs/1704.04096


H$_3^+$ is a ubiquitous and important astronomical species whose spectrum has been observed in the interstellar medium, planets and tentatively in the remnants of supernova SN1897a. Its role as a cooler is important for gas giant planets and exoplanets, and possibly the early Universe. All this makes the spectral properties, cooling function and partition function of H$_3^+$ key parameters for astronomical models and analysis. A new high-accuracy, very extensive line list for H$_3^+$ called MiZATeP was computed as part of the ExoMol project alongside a temperature-dependent cooling function and partition function as well as lifetimes for %individual excited states. These data are made available in electronic form as supplementary data to this article and at this http URL

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I. Mizus, A. Alijah, N. Zobov, et. al.
Fri, 14 Apr 17
24/38

Comments: 10 pages, 5 figures, 7 tables

Near-IR Photoluminescence of C60+ and Implications for Astronomy [CL]

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http://arxiv.org/abs/1704.01516


We have observed that C${60}^+$ ions isolated in cryogenic matrixes show pronounced near-IR photoluminescence upon excitation in the near-IR range. By contrast UV photoexcitation does not lead to measurable luminescence. After the recent unequivocal assignment of five Diffuse Interstellar Bands to near-IR absorption bands of C${60}^+$, we propose to search also for C$_{60}^+$ near-IR emission in those astronomical objects, where fullerenes have been already detected or may be potentially present.

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D. Strelnikov, B. Kern and M. Kappes
Thu, 6 Apr 17
36/43

Comments: N/A

Radiative association of C(${}^3P$) and H${}^+$: Triplet states [CL]

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http://arxiv.org/abs/1702.07777


The radiative association of C(${}^3P$) and H${}^+$ is investigated by calculating cross sections for photon emission into bound ro-vibrational states of CH${}^+$ from the vibrational continua of initial triplet d$\,{}^3\Pi$ or b$\,{}^3\Sigma^-$ states. Potential energy curves and transition dipole moments are calculated using multi-reference configuration interaction (MRCI) methods with AV6Z basis sets. The cross sections are evaluated using quantum-mechanical methods and rate coefficients are calculated. The rate coefficients are about 100 times larger than those for radiative association of C${}^+({}^2{P^o})$ and H from the A$\,{}^1\Pi$ state. We also confirm that the formation of CH${}^+$ by radiative association of C${}^+({}^2{P^o})$ and H via the triplet c$\,{}^3\Sigma^+$ state is a minor process.

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J. Babb and B. McLaughlin
Tue, 28 Feb 17
45/69

Comments: 7 pages

Equation of state and shock compression of warm dense sodium – a first-principles study [SSA]

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http://arxiv.org/abs/1702.06572


As one of the simple alkali metals, sodium has been of fundamental interest for shock physics experiments, but knowledge of its equation of state (EOS) in hot, dense regimes is not well known. By combining path integral Monte Carlo (PIMC) results for partially-ionized states [B. Militzer and K. P. Driver, Phys. Rev. Lett. 115, 176403 (2015)] at high temperatures and density functional theory molecular dynamics (DFT-MD) results at lower temperatures, we have constructed a coherent equation of state for sodium over a wide density-temperature range of $1.93-11.60$ g/cm$^{3}$ and $10^3-1.29\times10^8$ K. We find that a localized, Hartree-Fock nodal structure in PIMC yields pressures and internal energies that are consistent with DFT-MD at intermediate temperatures of $2\times10^6$ K. Since PIMC and DFT-MD provide a first-principles treatment of electron shell and excitation effects, we are able to identify two compression maxima in the shock Hugoniot curve corresponding to $K$-shell and $L$-shell ionization. Our Hugoniot curves provide a benchmark for widely-used EOS models, SESAME, LEOS, and Purgatorio. Due to the low ambient density, sodium has an unusually high first compression maximum along the shock Hugoniot curve. At beyond 10$^7$ K, we show that the radiation effect leads to very high compression along the Hugoniot curve, surpassing relativistic corrections, and observe an increasing deviation of the shock and particle velocities from a linear relation. We also compute the temperature-density dependence of thermal and pressure ionization processes.

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S. Zhang, K. Driver, F. Soubiran, et. al.
Thu, 23 Feb 17
3/48

Comments: 10 pages, 10 figures

ENDOR study of nitrogen hyperfine and quadrupole tensors in vanadyl porphyrins of heavy crude oil [CL]

Posted on by

http://arxiv.org/abs/1702.06066


We report the observation of pulsed electron-nuclear double resonance (ENDOR) spectrum caused by interactions of the nitrogen nuclei 14N with the unpaired electron of the paramagnetic vanadyl complexes VO2+ of vanadyl porphyrins in natural crude oil. We provide detailed experimental and theoretical characterization of the nitrogen hyperfine and quadrupole tensors.

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I. Gracheva, M. Gafurov, G. Mamin, et. al.
Tue, 21 Feb 17
29/70

Comments: 6 pages, 2 Figures

New quantum chemical computations of formamide deuteration support a gas-phase formation of this prebiotic molecule [SSA]

Posted on by

http://arxiv.org/abs/1701.06138


Based on recent work, formamide might be a potentially very important molecule in the emergence of terrestrial life. Although detected in the interstellar medium for decades, its formation route is still debated, whether in the gas phase or on the dust grain surfaces. Molecular deuteration has proven to be, in other cases, an efficient way to identify how a molecule is synthesised. For formamide, new published observations towards the IRAS16293-2422 B hot corino show that its three deuterated forms have all the same deuteration ratio, 2–5%, and that this is a factor 3–8 smaller than that measured for H2CO towards the IRAS16293-2422 protostar. Following a previous work on the gas-phase formamide formation via the reaction NH2 + H2CO -> HCONH2 + H, we present here new calculations of the rate coefficients for the production of monodeuterated formamide through the same reaction, starting from monodeuterated NH2 or H2CO. Some misconceptions regarding our previous treatment of the reaction are also cleared up. The results of the new computations show that, at the 100 K temperature of the hot corino, the rate of deuteration of the three forms is the same, within 20%. On the contrary, the reaction between non-deuterated species proceeds three times faster than that with deuterated ones. These results confirm that a gas-phase route for the formation of formamide is perfectly in agreement with the available observations.

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D. Skouteris, F. Vazart, C. Ceccarelli, et. al.
Tue, 24 Jan 17
4/63

Comments: MNRAS in press

Inelastic cross sections and rate coefficients for collisions between CO and H2 [IMA]

Posted on by

http://arxiv.org/abs/1701.05213


A five-dimensional coupled states (5D-CS) approximation is used to compute cross sections and rate coefficients for CO+H2 collisions. The 5D-CS calculations are benchmarked against accurate six-dimensional close-coupling (6D-CC) calculations for transitions between low-lying rovibrational states. Good agreement between the two formulations is found for collision energies greater than 10 cm-1. The 5D-CS approximation is then used to compute two separate databases which include highly excited states of CO that are beyond the practical limitations of the 6D-CC method. The first database assumes an internally frozen H2 molecule and allows rovibrational transitions for v < 5 and j < 30, where v and j are the vibrational and rotational quantum numbers of the initial state of the CO molecule. The second database allows H2 rotational transitions for initial CO states with v < 5 and j < 10. The two databases are in good agreement with each other for transitions that are common to both basis sets. Together they provide data for astrophysical models which were previously unavailable.

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C. Castro, K. Doan, M. Klemka, et. al.
Fri, 20 Jan 17
27/51

Comments: N/A

Rate Constants for Fine-Structure Excitations in O-H Collisions with Error Bars Obtained by Machine Learning [GA]

Posted on by

http://arxiv.org/abs/1701.01897


We present an approach using a combination of coupled channel scattering calculations with a machine- learning technique based on Gaussian Process regression to determine the sensitivity of the rate constants for non-adiabatic transitions in inelastic atomic collisions to variations of the underlying adiabatic interaction potentials. Using this approach, we improve the previous computations of the rate constants for the fine-structure transitions in collisions of O(3Pj) with atomic H. We compute the error bars of the rate constants corresponding to 20 % variations of the ab initio potentials and show that this method can be used to determine which of the individual adiabatic potentials are more or less important for the outcome of different fine-structure changing collisions.

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D. Vieira and R. Krems
Tue, 10 Jan 17
42/75

Comments: 10 pages, 5 figures, accepted for publication in ApJ

Importance of tunneling in H-abstraction reactions by OH radicals: The case of CH4 + OH studied through isotope-substituted analogs [GA]

Posted on by

http://arxiv.org/abs/1612.07027


We present a combined experimental and theoretical study focussing on the quantum tunneling of atoms in the reaction between CH4 and OH. The importance of this reaction pathway is derived by investigating isotope substituted analogs. Quantitative reaction rates needed for astrochemical models at low temperature are currently unavailable both in the solid state and in the gas phase. Here, we study tunneling effects upon hydrogen abstraction in CH4 + OH by focusing on two reactions: CH4 + OD -> CH3 + HDO and CD4 + OH -> CD3 + HDO. The experimental study shows that the solid-state reaction rate R(CH4 + OD) is higher than R(CD4 + OH) at 15 K. Experimental results are accompanied by calculations of the corresponding unimolecular and bimolecular reaction rate constants using instanton theory taking into account surface effects. From the work presented here, the unimolecular reactions are particularly interesting as these provide insight into reactions following a Langmuir-Hinshelwood process. The resulting ratio of the rate constants shows that the H abstraction (k(CH4 + OD)) is approximately ten times faster than D-abstraction (k(CD4 + OH)) at 65 K. We conclude that tunneling is involved at low temperatures in the abstraction reactions studied here. The unimolecular rate constants can be used by the modeling community as a first approach to describe OH-mediated abstraction reactions in the solid phase. For this reason we provide fits of our calculated rate constants that allow the inclusion of these reactions in models in a straightforward fashion.

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T. Lamberts, G. Fedoseev, J. Kastner, et. al.
Thu, 22 Dec 16
42/65

Comments: Accepted by A&A

Search for Interstellar monohydric Thiols [GA]

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http://arxiv.org/abs/1612.02688


It has been pointed out by various astronomers that very interesting relationship exists between interstellar alcohols and the corresponding thiols (sulfur analogue of alcohols) as far as the spectroscopic properties and chemical abundances are concerned. Monohydric alcohols such as methanol and ethanol are widely observed and 1-propanol is recently claimed to have been seen in Orion KL. Among the monohydric thiols, methanethiol (chemical analogue of methanol), has been firmly detected in Orion KL and Sgr B2(N2) and ethanethiol (chemical analogue of ethanol) has been claimed to be observed in Sgr B2(N2) though the confirmation of this detection is yet to come. It is very likely that higher order thiols could be observed in these regions. In this paper, we study the formation of monohydric alcohols and their thiol analogues. Based on our quantum chemical calculation and chemical modeling, we find that `Tg’ conformer of 1-propanethiol is a good candidate of astronomical interest. We present various spectroscopically relevant parameters of this molecule to assist its future detection in the Interstellar medium (ISM).

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P. Gorai, A. Das, A. Das, et. al.
Fri, 9 Dec 16
30/62

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

Quantum tunneling during interstellar surface-catalyzed formation of water: the reaction H + H$_2$O$_2$ $\rightarrow$ H$_2$O + OH [CL]

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http://arxiv.org/abs/1611.09188


The final step of the water formation network on interstellar grain surfaces starting from the H + O$_2$ route is the reaction between H and H$_2$O$_2$. This reaction is known to have a high activation energy and therefore at low temperatures it can only proceed via tunneling. To date, however, no rate constants are available at temperatures below 200 K. In this work, we use instanton theory to compute rate constants for the title reaction with and without isotopic substitutions down to temperatures of 50 K. The calculations are based on density functional theory, with additional benchmarks for the activation energy using unrestricted single-reference and multireference coupled-cluster single-point energies. Gas-phase bimolecular rate constants are calculated and compared with available experimental data not only for H + H$_2$O$_2$ $\rightarrow$ H$_2$O + OH, but also for H + H$_2$O$_2$ $\rightarrow$ H$_2$ + HO$_2$. We find a branching ratio where the title reaction is favored by at least two orders of magnitude at 114 K. In the interstellar medium this reaction predominantly occurs on water surfaces, which increases the probability that the two reactants meet. To mimic this one, two, or three spectator H2O molecules are added to the system. Eley-Rideal bimolecular and Langmuir-Hinshelwood unimolecular rate constants are presented here. The kinetic isotope effects for the various cases are compared to experimental data as well as to expressions commonly used in astrochemical models. Both the rectangular barrier and the Eckart approximations lead to errors of about an order of magnitude. Finally, fits of the rate constants are provided as input for astrochemical models.

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T. Lamberts, P. Samanta, A. Kohn, et. al.
Thu, 1 Dec 16
45/75

Comments: Accepted for publication by Phys. Chem. Chem. Phys. 2016. Supplementary information available: this http URL

Quantum tunneling during interstellar surface-catalyzed formation of water: the reaction H + H$_2$O$_2$ $\rightarrow$ H$_2$O + OH [CL]

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http://arxiv.org/abs/1611.09188


The final step of the water formation network on interstellar grain surfaces starting from the H + O$_2$ route is the reaction between H and H$_2$O$_2$. This reaction is known to have a high activation energy and therefore at low temperatures it can only proceed via tunneling. To date, however, no rate constants are available at temperatures below 200 K. In this work, we use instanton theory to compute rate constants for the title reaction with and without isotopic substitutions down to temperatures of 50 K. The calculations are based on density functional theory, with additional benchmarks for the activation energy using unrestricted single-reference and multireference coupled-cluster single-point energies. Gas-phase bimolecular rate constants are calculated and compared with available experimental data not only for H + H$_2$O$_2$ $\rightarrow$ H$_2$O + OH, but also for H + H$_2$O$_2$ $\rightarrow$ H$_2$ + HO$_2$. We find a branching ratio where the title reaction is favored by at least two orders of magnitude at 114 K. In the interstellar medium this reaction predominantly occurs on water surfaces, which increases the probability that the two reactants meet. To mimic this one, two, or three spectator H2O molecules are added to the system. Eley-Rideal bimolecular and Langmuir-Hinshelwood unimolecular rate constants are presented here. The kinetic isotope effects for the various cases are compared to experimental data as well as to expressions commonly used in astrochemical models. Both the rectangular barrier and the Eckart approximations lead to errors of about an order of magnitude. Finally, fits of the rate constants are provided as input for astrochemical models.

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T. Lamberts, P. Samanta, A. Kohn, et. al.
Tue, 29 Nov 16
63/77

Comments: Accepted for publication by Phys. Chem. Chem. Phys. 2016. Supplementary information available: this http URL

Predicted reentrant melting of dense hydrogen at ultra-high pressures [CL]

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http://arxiv.org/abs/1611.01418


The phase diagram of hydrogen is one of the most important challenges in high-pressure physics and astrophysics. Especially, the melting of dense hydrogen is complicated by dimer dissociation, metallization and nuclear quantum effect of protons, which together lead to a cold melting of dense hydrogen when above 500 GPa. Nonetheless, the variation of the melting curve at higher pressures is virtually uncharted. Here we report that using ab initio molecular dynamics and path integral simulations based on density functional theory, a new atomic phase is discovered, which gives an uplifting melting curve of dense hydrogen when beyond 2 TPa, and results in a reentrant solid-liquid transition before entering the Wigner crystalline phase of protons. The findings greatly extend the phase diagram of dense hydrogen, and put metallic hydrogen into the group of alkali metals, with its melting curve closely resembling those of lithium and sodium.

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H. Geng and Q. Wu
Mon, 7 Nov 16
43/48

Comments: 27 pages, 10 figures

Missing Fe: hydrogenated iron nanoparticles [SSA]

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http://arxiv.org/abs/1611.00309


Although it was found that the FeH lines exist in the spectra of some stars, none of the spectral features in the ISM have been assigned to this molecule. We suggest that iron atoms interact with hydrogen and produce Fe-H nanoparticles which sometimes contain many H atoms. We calculate infrared spectra of hydrogenated iron nanoparticles using density functional theory methods and find broad, overlapping bands. Desorption of H2 could induce spinning of these small Fe-H dust grains. Some of hydrogenated iron nanoparticles posses magnetic and electric moments and should interact with electromagnetic fields in the ISM. Fe_nH_m nanoparticles could contribute to the polarization of the ISM and the anomalous microwave emission. We discuss the conditions required to form FeH and Fe_nH_m in the ISM.

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G. Bilalbegovic, A. Maksimovic and V. Mohacek-Grosev
Wed, 2 Nov 16
53/55

Comments: accepted for publications in MNRAS Letters

Visible absorptions of potential diffuse ISM hydrocarbons: C$_9$H$_9$ and C$_9$H$_5$ radicals [CL]

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http://arxiv.org/abs/1610.07318


The laboratory detection of previously unobserved resonance-stabilized C$_9$H$_5$ and C$_9$H$_9$ radicals in the supersonic expansion of a hydrocarbon discharge source is reported. The radicals are tentatively assigned as acetylenic-substituted cyclopentadienyl C$_9$H$_5$ and vinyl-substituted benzyl C$_9$H$_9$ species. They are found to feature visible absorption bands that coincide with a few very weak diffuse interstellar bands toward HD183143 and HD204827.

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M. Steglich, S. Maity and J. Maier
Tue, 25 Oct 16
48/69

Comments: 5 figures, 3 tables

Nitrile versus isonitrile adsorption at interstellar grains surfaces: I – Hydroxylated surfaces [GA]

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http://arxiv.org/abs/1610.05010


Almost 20% of the ~ 200 different species detected in the interstellar and circumstellar media present a carbon atom linked to nitrogen by a triple bond. Among these 37 molecules, 30 are nitrile R-CN compounds, the remaining seven belonging to the isonitrile R-NC family. How these species behave in presence of the grain surfaces is still an open question. In this contribution we investigate whether the difference between nitrile and isonitrile functional groups may induce differences in the adsorption energies of the related isomers at the surfaces of interstellar grains of different nature and morphologies. The question was addressed by means of a concerted experimental and theoretical study of the adsorption energies of CH3CN and CH3NC on the surface water ice and silica. The experimental determination of the molecule – surface interaction energies was carried out using temperature programmed desorption (TPD) under an ultra-high vacuum (UHV) between 70 and 160 K. Theoretically, the question was addressed using first principle periodic density functional theory (DFT) to represent the organized solid support. The most stable isomer (CH3CN) interacts more efficiently with the solid support than the higher energy isomer (CH3NC) for water ice and silica. Comparing with the HCN and HNC pair of isomers, the simulations show an opposite behaviour, in which isonitrile HNC are more strongly adsorbed than nitrile HCN provided that hydrogen bonds are compatible with the nature of the model surface. The present study confirms that the strength of the molecule surface interaction between isomers is not related to their intrinsic stability but instead to their respective ability to generate different types of hydrogen bonds.

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M. Bertin, M. Doronin, J. Fillion, et. al.
Tue, 18 Oct 16
28/70

Comments: Astronomy and Astrophysics 2016

Submillimeter spectroscopy of H$_2$C$^{17}$O and a revisit of the rotational spectra of H$_2$C$^{18}$O and H$_2$C$^{16}$O [GA]

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http://arxiv.org/abs/1610.02174


The rotational spectrum of the formaldehyde isotopologue H2C(17)O was investigated between 0.56 and 1.50 THz using a sample of natural isotopic composition. In addition, transition frequencies were determined for H2C(18)O and H2C(16)O between 1.37 and 1.50 THz. The data were combined with critically evaluated literature data to derive improved sets of spectroscopic parameters which include (17)O or H nuclear hyperfine structure parameters.

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H. Muller and F. Lewen
Mon, 10 Oct 16
39/51

Comments: 7 pages, 1 figure, 1 table; J. Mol. Spectrosc., accepted

Formation of the prebiotic molecule NH$_2$CHO on astronomical amorphous solid water surfaces: accurate tunneling rate calculations [CL]

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http://arxiv.org/abs/1610.01007


Investigating how formamide forms in the interstellar medium is a hot topic in astrochemistry, which can contribute to our understanding of the origin of life on Earth. We have constructed a QM/MM model to simulate the hydrogenation of isocyanic acid on amorphous solid water surfaces to form formamide. The binding energy of HNCO on the ASW surface varies significantly between different binding sites, we found values between $\sim$0 and 100 kJ mol$^{-1}$. The barrier for the hydrogenation reaction is almost independent of the binding energy, though. We calculated tunneling rate constants of H + HNCO $\rightarrow$ NH$_2$CO at temperatures down to 103 K combining QM/MM with instanton theory. Tunneling dominates the reaction at such low temperatures. The tunneling reaction is hardly accelerated by the amorphous solid water surface compared to the gas phase for this system, even though the activation energy of the surface reaction is lower than the one of the gas-phase reaction. Both the height and width of the barrier affect the tunneling rate in practice. Strong kinetic isotope effects were observed by comparing to rate constants of D + HNCO $\rightarrow$ NHDCO. At 103 K we found a KIE of 231 on the surface and 146 in the gas phase. Furthermore, we investigated the gas-phase reaction NH$_2$ + H$_2$CO $\rightarrow$ NH$_2$CHO + H and found it unlikely to occur at cryogenic temperatures. The data of our tunneling rate constants are expected to significantly influence astrochemical models.

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L. Song and J. Kastner
Wed, 5 Oct 16
30/66

Comments: N/A