Rotation-tunneling spectrum and astrochemical modeling of dimethylamine, CH$_3$NHCH$_3$, and searches for it in space [GA]

http://arxiv.org/abs/2305.11656


Methylamine has been the only simple alkylamine detected in the interstellar medium for a long time. With the recent secure and tentative detections of vinylamine and ethylamine, respectively, dimethylamine has become a promising target for searches in space. Its rotational spectrum, however, has been known only up to 45 GHz until now. Here we investigate the rotation-tunneling spectrum of dimethylamine in selected regions between 76 and 1091 GHz using three different spectrometers in order to facilitate its detection in space. The quantum number range is extended to $J = 61$ and $K_a = 21$, yielding an extensive set of accurate spectroscopic parameters. To search for dimethylamine, we refer to the spectral line survey ReMoCA carried out with the Atacama Large Millimeter/submillimeter Array toward the high-mass star-forming region Sagittarius B2(N) and a spectral line survey of the molecular cloud G+0.693$-$0.027 employing the IRAM 30 m and Yebes 40 m radio telescopes. We report nondetections of dimethylamine toward the hot molecular cores Sgr B2(N1S) and Sgr B2(N2b) as well as G+0.693$-$0.027 which imply that dimethylamine is at least 14, 4.5 and 39 times less abundant than methylamine toward these sources, respectively. The observational results are compared to computational results from a gas-grain astrochemical model. The modeled methylamine to dimethylamine ratios are compatible with the observational ratios. However, the model produces too much ethylamine compared with methylamine which could mean that the already fairly low levels of dimethylamine in the models may also be too high.

Read this paper on arXiv…

H. Müller, R. Garrod, A. Belloche, et. al.
Mon, 22 May 23
27/60

Comments: Mon. Not. R. Astron. Soc., accepted. 33 pages including tables, figures, and appendix

Rotational spectroscopy of oxirane-\textit{2,2}-$d_2$, $c$-CD$_2$CH$_2$O, and its tentative detection toward IRAS 16293$-$2422~B [GA]

http://arxiv.org/abs/2304.12045


We prepared a sample of oxirane doubly deuterated at one C atom and studied its rotational spectrum in the laboratory for the first time between 120~GHz and 1094~GHz. Accurate spectroscopic parameters up to eighth order were determined, and the calculated rest frequencies were used to identify $c$-CD$2$CH$_2$O tentatively in the interstellar medium in the Atacama Large Millimeter/submillimeter Array Protostellar Interferometric Line Survey (PILS) of the Class 0 protostellar system IRAS 16293$-$2422. The $c$-CD$_2$CH$_2$O to $c$-C$_2$H$_4$O ratio was estimated to be $\sim$0.054 with $T{\rm rot} = 125$ K. This value translates to a D-to-H ratio of $\sim$0.16 per H atom which is higher by a factor of 4.5 than the $\sim$0.036 per H atom obtained for $c$-C$_2$H$_3$DO. Such increase in the degree of deuteration referenced to one H atom in multiply deuterated isotopologs compared to their singly deuterated variants have been observed commonly in recent years.

Read this paper on arXiv…

H. Müller, J. Jørgensen, J. Guillemin, et. al.
Tue, 25 Apr 23
38/72

Comments: Journal of Molecular Spectroscopy, in press; Per Jensen special issue. 12 pages here

An efficient Monte Carlo model for the slowing down of photoelectrons. Application to H-$α$ in exoplanet atmospheres [EPA]

http://arxiv.org/abs/2211.14128


Photoelectrons, the fast electrons produced in the photoionization of planetary atmospheres, drive transformations in the atmospheric gas that are often inhibited by energy considerations for thermal electrons. The transformations include excitation and ionization of atoms and molecules, which affect the detectability of these gases and constrain the fraction of incident stellar radiation that transforms into heat. To gain insight into these important questions, we build a Monte Carlo model that solves the slowing down of photoelectrons in a gas with arbitrary amounts of H and He atoms and thermal electrons. Our novel multi-score scheme differs from similar tools in that it efficiently handles rare collisional channels, as in the case of low-abundance excited atoms that undergo superelastic and inelastic collisions. The model is validated and its performance demonstrated. Further, we investigate whether photoelectrons might affect the population of the excited hydrogen H(2) detected at some exoplanet atmospheres by transmission spectroscopy in the H-$\alpha$ line. For the ultra-hot Jupiter HAT-P-32b, we find that photoelectron-driven excitation of H(2) is inefficient at the pressures probed by the line core but becomes significant (yet sub-dominant) deeper in the atmosphere where the line wings form. The contribution of photoelectrons to the destruction of H(2) either by collisional deexcitation or ionization is entirely negligible, a conclusion likely to hold for exoplanet atmospheres at large. Importantly, photoelectrons dominate the gas ionization at the altitudes probed by the H-$\alpha$ line, a fact that will likely affect, even if indirectly, the population of H(2) and other tracers such as metastable helium. Future modeling of these excited levels should incorporate photoelectron-driven ionization.

Read this paper on arXiv…

A. Muñoz
Mon, 28 Nov 22
54/93

Comments: Accepted for publication in Icarus

An accurate set of H$_3$O$^+ -$ H$_2$ collisional rate coefficients for non-LTE modelling of warm interstellar clouds [GA]

http://arxiv.org/abs/2211.02309


Hydronium (H$_3$O$^+$) was first detected in 1986 in interstellar molecular clouds. It was reported in many galactic diffuse and dense regions, as well as in extragalactic sources. H$_3$O$^+$ plays a major role both in interstellar oxygen and water chemistry. However, despite the large number of H$_3$O$^+$ observations, its collisional excitation was investigated only partially. In the present work we study the state-to-state rotational de-excitation of $ortho$- and $para$-H$_3$O$^+$ in collisions both with $ortho$- and $para$-H$_2$. The cross sections are calculated within the close-coupling formalism using a highly accurate potential energy surface developed for this system. The rate coefficients are computed up to $300$ K kinetic temperature. Transitions between the lowest 21 rotation-inversion states were studied for $para$-H$_3$O$^+$, and the lowest 11 states for $ortho$-H$_3$O$^+$, i.e. all levels with rotational energies below 430 K ($\sim 300$ cm$^{-1}$) are considered (up to $j\leq5$). In order to estimate the impact of the new rate coefficients on the astrophysical models for H$_3$O$^+$, radiative transfer calculations were also carried out. We have examined how the new collisional data affect the line intensities with respect to older data previously used for the interpretation of observations. By analysing all detected transitions we find that our new, accurate rate coefficients have a significant impact (typically within a factor of 2) on radiation temperatures, allowing more accurate estimation of column densities and relative abundances of hydronium, especially in warm molecular clouds, paving the path towards better interpretation of interstellar water and oxygen chemistry.

Read this paper on arXiv…

S. Demes, F. Lique, A. Faure, et. al.
Mon, 7 Nov 22
43/67

Comments: 13 pages, 7 figures

Laboratory and astronomical discovery of magnesium dicarbide, MgC$_2$ [SSA]

http://arxiv.org/abs/2210.17348


We report the detection of magnesium dicarbide, MgC$_2$, in the laboratory at centimeter wavelengths and assign $^{24}$MgC$_2$, $^{25}$MgC$_2$, and $^{26}$MgC$_2$ to 14 unidentified lines in the radio spectrum of the circumstellar envelope of the evolved carbon star IRC+10216. The structure of MgC$_2$ is found to be T-shaped with a highly ionic bond between the metal atom and the C$_2$ unit, analogous to other dicarbides containing electropositive elements. A two-temperature excitation model of the MgC$_2$ emission lines observed in IRC+10216 yields a very low rotational temperature of $6\pm1$ K, a kinetic temperature of $22\pm13$ K, and a column density of $(1.0 \pm 0.3) \times 10^{12}$ cm$^{-2}$. The abundance of MgC$_2$ relative to the magnesium-carbon chains MgCCH, MgC$_4$H, and MgC$_6$H is $1{:}2{:}22{:}20$ and provides a new constraint on the sequential radiative association-dissociative recombination mechanisms implicated in the production of metal-bearing molecules in circumstellar environments.

Read this paper on arXiv…

P. Changala, H. Gupta, J. Cernicharo, et. al.
Tue, 1 Nov 22
92/100

Comments: 15 pages, 5 figures

C60 Cation as the Carrier of the 9577 and 9632 Angstrom Diffuse Interstellar Bands: Further Support from the VLT/X-Shooter Spectra [GA]

http://arxiv.org/abs/2111.05769


Ever since their first detection over 100 years ago, the mysterious diffuse interstellar bands (DIBs), a set of several hundred broad absorption features seen against distant stars in the optical and near infrared wavelength range, largely remain unidentified. The close match both in wavelengths and in relative strengths recently found between the experimental absorption spectra of gas-phase buckminsterfullerene ions (C60+) and four DIBs at 9632, 9577, 9428 and 9365 Angstrom (and, to a lesser degree, a weaker DIB at 9348 Angstrom) suggests C60+ as a promising carrier. However, arguments against the C60+ identification remain and are mostly concerned with the large variation in the intensity ratios of the 9632 and 9577 DIBs. In this work, we search for these DIBs in the ESO VLT/X-shooter archival data and identify the 9632, 9577, 9428 and 9365 Angstrom DIBs in a sample of 25 stars. While the 9428 and 9365 Angstrom DIBs are too noisy to allow any reliable analysis, the 9632 and 9577 Angstrom DIBs are unambiguously detected and, after correcting for telluric water vapor absorption, their correlation can be used to probe their origin. To this end, we select a sub-sample of nine hot, O- or B0-type stars of which the stellar Mg II contamination to the 9632 Angstrom DIB is negligibly small. We find their equivalent widths, after normalized by reddening to eliminate their common correlation with the density of interstellar clouds, exhibit a tight, positive correlation. This supports C60+ as the carrier of the 9632 and 9577 Angstrom DIBs.

Read this paper on arXiv…

T. Nie, F. Xiang and A. Li
Thu, 11 Nov 21
51/60

Comments: 8 pages, 4 figures, 2 tables; accepted for publication in the Monthly Notices of the Royal Astronomical Society

Where have all the interstellar silicon carbides gone? [GA]

http://arxiv.org/abs/2111.03920


The detection of the 11.3-micron emission feature characteristic of the Si–C stretch in carbon-rich evolved stars reveals that silicon carbide (SiC) dust grains are condensed in the outflows of carbon stars. SiC dust could be a significant constituent of interstellar dust since it is generally believed that carbon stars inject a considerable amount of dust into the interstellar medium (ISM). The presence of SiC dust in the ISM is also supported by the identification of presolar SiC grains of stellar origin in primitive meteorites. However, the 11.3-micron absorption feature of SiC has never been seen in the ISM and oxidative destruction of SiC is often invoked. In this work we quantitatively explore the destruction of interstellar SiC dust through oxidation based on molecular dynamics simulations and density functional theory calculations. We find that the reaction of an oxygen atom with SiC molecules and clusters is exothermic and could cause CO-loss. Nevertheless, even if this is extrapolable to bulk SiC dust, the destruction rate of SiC dust through oxidation could still be considerably smaller than the (currently believed) injection rate from carbon stars. Therefore, the lack of the 11.3-micron absorption feature of SiC dust in the ISM remains a mystery. A possible solution may lie in the currently believed stellar injection rate of SiC (which may have been overestimated) and/or the size of SiC dust (which may actually be considerably smaller than submicron in size).

Read this paper on arXiv…

T. Chen, C. Xiao, A. Li, et. al.
Tue, 9 Nov 21
41/102

Comments: 7 pages, 5 figures; accepted for publication in MNRAS

Trajectory-based simulation of far-infrared CIA profiles of CH$_4-$N$_2$ for modeling Titan's atmosphere [EPA]

http://arxiv.org/abs/2111.02312


We report the results of the trajectory-based simulation of far-infrared collision-induced absorption (CIA) due to CH$_4-$N$_2$ pairs at temperatures between 70 and 400 K. Our analysis utilizes recently calculated high-level potential energy (PES) and induced dipole surfaces (IDS) [Finenko, A. A., Chistikov, D. N., Kalugina, Y. N., Conway E. K., Gordon, I. E., Phys. Chem. Chem. Phys., 2021, doi: 10.1039/d1cp02161c]. Treating collision partners as rigid rotors, the time evolution of interaction-induced dipole is accumulated over a vast ensemble of classical trajectories and subsequently transformed into CIA spectrum via Fourier transform. In our calculations, both bound and unbound states are properly accounted for, and the rigorous theory of lower-order spectral moments is addressed to check the accuracy of simulated profiles. Classically derived trajectory-based profiles are subject to two approximate desymmetrization procedures so that resulting profiles conform to the quantum principle of detailed balance. The simulated profiles are compared to laboratory measurements and employed for modeling Titan’s spectra in the 50-500 cm$^{-1}$ range. Based on the desymmetrized simulated profiles, a new semi-empirical model for CH$_4-$N$_2$ CIA is proposed for modeling Titan’s infrared spectra. Synthetic spectra derived using this model yield an excellent agreement with the data recorded by the Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft at low and high emission angles.

Read this paper on arXiv…

A. Finenko, B. Bézard, I. Gordon, et. al.
Thu, 4 Nov 21
16/73

Comments: 18 pages, 11 figures, accepted for publication in ApJS . The semiempirical CH4-N2 profiles tabulated in the HITRAN CIA format are available on the ApJS website or upon reasonable request

Alternation of Singlet and Triplet States in Carbon-Based Chain Molecules and Its Astrochemical Implications. Results of an Extensive Theoretical Study [CL]

http://arxiv.org/abs/2110.11071


A variety of homologous carbon chains (HCnH, HCnN, CnS, CnO, and OCnO) are found to exhibit an appealing even-odd effect. Chains containing a number of carbon atoms of a certain parity possess singlet ground states, while members of opposite parity have triplet ground states. From a general perspective, it is important that this even-odd effect confounds straightforward chemical intuition. Whether the most stable form is a triplet or a singlet is neither simply related to the fact that the species in question is a normal (closed-shell, nonradical) molecule nor a (di)radical or to the (e.g., cumulene-type) C-C bond succession across the chain. From a computational perspective, the present results are important also because they demonstrate that electron correlations in carbon-based chains are extremely strong. Whether the gold-standard CCSD(T) (coupled-cluster expansions with single and double excitations and triple excitations corrections) framework suffices to describe such strongly correlated systems remains an open question that calls for further clarification. Most importantly for astrochemistry, the present results may explain why certain members are not astronomically observed although larger members of the same homologous series are detected; the missing species are exactly those for which the present calculations predict triplet ground states.

Read this paper on arXiv…

I. Baldea
Fri, 22 Oct 21
22/133

Comments: The present posting includes important corrections of the values presented in the supporting information of the published paper

Structure, stability and optical absorption spectra of small Ti$_n$C$_x$ clusters: a first-principles approach [GA]

http://arxiv.org/abs/2109.10050


Titanium-carbide molecular clusters are thought to form in the circumstellar envelopes (CSEs) of carbon-rich Asymptotic Giant Branch stars (AGBs) but, to date, their detection has remained elusive. To facilitate the astrophysical identification of those clusters in AGBs and post-AGBs environments, the molecular structures and optical absorption spectra of small Ti$n$C$_x$ clusters, with n = 1-4 and x = 1-4, and some selected larger clusters, Ti$_3$C$_8$, Ti$_4$C$_8$, Ti$_6$C${13}$, Ti$7$C${13}$, Ti$8$C${12}$, Ti$9$C${15}$, and Ti${13}$C${22}$, have been calculated. The density functional formalism, within the B3LYP approximation for electronic exchange and correlation, was used to find the lowest energy structures. Except the clusters having a single Ti atom, the rest exhibit three-dimensional structures. Those are formed by a Ti fragment surrounded in general by carbon dimers. The optical spectra of Ti$_n$C$_x$, computed by time-dependent density functional theory, using the corrected CAM-B3LYP functional, show absorption features in the visible and near infrared regions which may help in the identification of these clusters in space. In addition, most of the clusters have sizable electric dipole moments, allowing their detection by radioastronomical observations.

Read this paper on arXiv…

S. Gámez-Valenzuela, J. Alonso, G. Santoro, et. al.
Wed, 22 Sep 21
36/57

Comments: N/A

Electric quadrupole transitions in carbon dioxide [CL]

http://arxiv.org/abs/2106.00585


Recent advances in the high sensitivity spectroscopy have made it possible, in combination with accurate theoretical predictions, to observe for the first time very weak electric quadrupole transitions in a polar polyatomic molecule of water. Here we present accurate theoretical predictions of the complete quadrupole ro-vibrational spectrum of a non-polar molecule CO$_2$, important in atmospheric and astrophysical applications. Our predictions are validated by recent cavity enhanced absorption spectroscopy measurements and are used to assign few weak features in the recent ExoMars ACS MIR spectroscopic observations of the martian atmosphere. Predicted quadrupole transitions appear in some of the mid-infrared CO$_2$ and water vapor transparency regions, making them important for detection and characterization of the minor absorbers in water- and CO$_2$-rich environments, such as present in the atmospheres of Earth, Venus and Mars.

Read this paper on arXiv…

A. Yachmenev, A. Campargue, S. Yurchenko, et. al.
Wed, 2 Jun 21
5/48

Comments: N/A

Void defect induced magnetism and structure change of carbon materials-1, Graphene nano ribbon [CL]

http://arxiv.org/abs/2104.09740


Void defect is a possible origin of ferromagnetic like feature of pure carbon material. Applying density functional theory to void defect induced graphene nano ribbon (GNR), a detailed relationship between multiple spin state and structure change was studied. An equitorial triangle of an initial initial void having six electrons is distorted to isosceles triangle by rebonding carbon atoms. Among possible spin states, the most stable state was Sz=2/2. The case of Sz=4/2 is remarkable that initial flat ribbon turned to three dimentional curled one having highly polarized spin configuration at ribbon edges. Total energy of Sz=4/2 was very close to that of Sz=2/2, which suggests coexistence of flat and curled ribbons. As a model of three dimensional graphite, bilayered AB stacked GNR was analyzed. Spin distribution was limited to the void created layer. Distributed void triangle show 60 degree clockwise rotation for differrent site void, which was consistent with experimental observation using the scanning tunneling microscope. (To be published on Journal of the Magnetic Society of Japan, 2021 )

Read this paper on arXiv…

N. Ota and L. Nemes
Wed, 21 Apr 2021
12/72

Comments: 7 pages, 7 figures, 2 tables

Modeling Anharmonic Infrared Spectra of Thermally Excited Pyrene(C$_{16}$H$_{10}$): the combined view of DFT AnharmonicCaOs and approximate DFT molecular dynamics [GA]

http://arxiv.org/abs/2102.06582


Aromatic Infrared Bands (AIBs) are a set of bright and ubiquitous emission bands, observed in regions illuminated by stellar ultraviolet photons, from our galaxy all the way out to cosmological distances. The forthcoming James Webb Space Telescope will unveil unprecedented spatial and spectral details in the AIB spectrum; significant advancement is thus necessary now to model the infrared emission of polycyclic aromatic hydrocarbons, their presumed carriers, with enough detail to exploit the information content of the AIBs. This requires including anharmonicity in such models, and to do so systematically for all species included, requiring a difficult compromise between accuracy and efficiency. We propose a new recipe using minimal assumptions on the general behaviour of band positions and widths with temperature, which can be defined by a small number of empirical parameters. We explore here the performances of a full quantum method, AnharmoniCaOs, relying on an ab initio potential, and Molecular Dynamics simulations using a Density Functional based Tight Binding potential to determine these parameters for the case of pyrene, for which high temperature gas-phase data are available. The first one is very accurate and detailed, but it becomes computationally very expensive for increasing T; the second trades some accuracy for speed, making it suitable to provide approximate, general trends at high temperatures. We propose to use, for each species and band, the best available empirical parameters for a fast, yet sufficiently accurate spectral model of PAH emission properly including anharmonicity. Modelling accuracy will depend critically on these empirical parameters, allowing for an incremental improvement in model results, as better estimates become gradually available.

Read this paper on arXiv…

S. Chakraborty, G. Mulas, M. Rapacioli, et. al.
Mon, 15 Feb 21
28/53

Comments: submitted to the Journal of Molecular Spectroscopy

Evolution and Mass Loss of Cool Ageing Stars: a Daedalean Story [SSA]

http://arxiv.org/abs/2011.13472


The chemical enrichment of the Universe; the mass spectrum of planetary nebulae, white dwarfs and gravitational wave progenitors; the frequency distribution of Type I and II supernovae; the fate of exoplanets … a multitude of phenomena which is highly regulated by the amounts of mass that stars expel through a powerful wind. For more than half a century, these winds of cool ageing stars have been interpreted within the common interpretive framework of 1-dimensional (1D) models. I here discuss how that framework now appears to be highly problematic.
* Current 1D mass-loss rate formulae differ by orders of magnitude, rendering contemporary stellar evolution predictions highly uncertain.
These stellar winds harbour 3D complexities which bridge 23 orders of magnitude in scale, ranging from the nanometer up to thousands of astronomical units. We need to embrace and understand these 3D spatial realities if we aim to quantify mass loss and assess its effect on stellar evolution. We therefore need to gauge
* the 3D life of molecules and solid-state aggregates: the gas-phase clusters that form the first dust seeds are not yet identified. This limits our ability to predict mass-loss rates using a self-consistent approach.
* the emergence of 3D clumps: they contribute in a non-negligible way to the mass loss, although they seem of limited importance for the wind-driving mechanism.
* the 3D lasting impact of a (hidden) companion: unrecognised binary interaction has biased previous mass-loss rate estimates towards values that are too large.
Only then will it be possible to drastically improve our predictive power of the evolutionary path in 4D (classical) spacetime of any star.

Read this paper on arXiv…

L. Decin
Mon, 30 Nov 20
85/117

Comments: preprint of invited review article to be published in the Annual Review of Astronomy and Astrophysics (2021) – 71 pages, Main Paper 58 pages, Supplemental Material 13 pages, 10 figures of which Figure 8 is also a movie. Movie does not display properly in all pdf file readers; therefore the movie has been uploaded separately or can be obtained via request to the author

Graphene Molecules Contributing to the Infrared Bands of Carbon Rich Planetary Nebulae [GA]

http://arxiv.org/abs/2007.03862


It is well known since 2010 that fullerene C60 is widespread through the interstellar space. Also, it is well known that graphene is a source material for synthesizing fullerene. Here, we simply assume the occurrence of graphene in space. Infrared spectra of graphene molecules are calculated to compare both to astronomical observational spectra and to laboratory experimental one. Model molecules for DFT calculation are selected by one simple astronomical assumption, that is, single void creation on charge neutral graphene of C13, C24 and C54, resulting C12, C23 and C53. They have a carbon pentagon ring within a hexagon network. Different void positions should be classified as different species. In every species, single void has 3 radical carbons holding 6 spins. These spins select stable state by recombination of carbon atoms to reduce molecular energy, and to give infrared spectrum due to molecular vibration. It was found that stable spin state was the triplet, not singlet. Most of species show prominent infrared spectral features closely resembling the astronomically observed infrared emission seen in the carbon rich planetary nebulae of Tc1 and Lin49. We could assign major bands at 18.9 micrometer, and sub-bands at 6.6, 7.0, 7.6, 8.1, 8.5, 9.0 and 17.4 micrometer. It is interesting that those graphene species were also assigned in the laboratory experiments on laser-induced carbon plasmas, which are analogies of carbon cluster creation in space. The conclusion is that graphene molecules could potentially contribute to the infrared emission bands of carbon-rich planetary nebulae.

Read this paper on arXiv…

N. Ota, A. Li, L. Nemes, et. al.
Thu, 9 Jul 20
21/70

Comments: 21 pages, 8 figures, 1 table, 1 appendix

Determination of accurate rest frequencies and hyperfine structure parameters of cyanobutadiyne, HC$_5$N [IMA]

http://arxiv.org/abs/2005.09312


Very accurate transition frequencies of HC$_5$N were determined between 5.3 and 21.4 GHz with a Fourier transform microwave spectrometer. The molecules were generated by passing a mixture of HC$_3$N and C$_2$H$_2$ highly diluted in neon through a discharge valve followed by supersonic expansion into the Fabry-Perot cavity of the spectrometer. The accuracies of the data permitted us to improve the experimental $^{14}$N nuclear quadrupole coupling parameter considerably and the first experimental determination of the $^{14}$N nuclear spin-rotation parameter. The transition frequencies are also well suited to determine in astronomical observations the local speed of rest velocities in molecular clouds with high fidelity. The same setup was used to study HC$_7$N, albeit with modest improvement of the experimental $^{14}$N nuclear quadrupole coupling parameter. Quantum chemical calculations were carried out to determine $^{14}$N nuclear quadrupole and spin-rotation coupling parameters of HC$_5$N, HC$_7$N, and related molecules. These calculations included evaluation of vibrational and relativistic corrections to the non-relativistic equilibrium quadrupole coupling parameters; their considerations improved the agreement between calculated and experimental values substantially.

Read this paper on arXiv…

T. Giesen, M. Harding, J. Gauss, et. al.
Wed, 20 May 20
61/66

Comments: 9 pages; J. Mol. Spectrosc., accepted

Rotational spectroscopy of singly $^{13}$C substituted isotopomers of propyne and determination of a semi-empirical equilibrium structure [IMA]

http://arxiv.org/abs/2001.11907


Submillimeter spectra of three isotopomers of propyne containing one $^{13}$C atom were recorded in natural isotopic composition in the region of 426 GHz to 785 GHz. Additional measurements were carried out near 110 GHz. Combining these with earlier data resulted in greatly improved spectroscopic parameters which permit reliable extrapolations up to about 1.5 THz. Coupled cluster quantum-chemical calculations were carried out in order to assess the differences between equilibrium and ground state rotational parameters of these and many other isotopic species to evaluate semi-empirical equilibrium structural parameters. In addition, we estimated the main spectroscopic parameters of the isotopomers of propyne with two $^{13}$C atoms, which have not yet been studied in the laboratory, but which may be detectable in astronomical sources with a large amount of $^{13}$C compared to the dominant $^{12}$C.

Read this paper on arXiv…

H. Müller, S. Thorwirth and F. Lewen
Mon, 3 Feb 20
7/46

Comments: 10 pages; J. Mol. Struct., in press

Superhydrogenated Polycyclic Aromatic Hydrocarbon Molecules: Vibrational Spectra in the Infrared [GA]

http://arxiv.org/abs/2001.00717


Superhydrogenated polycyclic aromatic hydrocarbons (PAHs) may be present in H-rich and ultraviolet-poor benign regions. The addition of excess H atoms to PAHs converts the aromatic bonds into aliphatic bonds, the strongest of which falls near 3.4 $\mu$m. Therefore, superhydrogenated PAHs are often hypothesized as a carrier of the 3.4 $\mu$m emission feature which typically accompanies the stronger 3.3 $\mu$m aromatic C–H stretching feature. To assess this hypothesis, we use density function theory to compute the IR vibrational spectra of superhydrogenated PAHs and their ions of various sizes (ranging from benzene, naphthalene to perylene and coronene) and of various degrees of hydrogenation (ranging from minimal hydrogenation to heavy hydrogenation). For each molecule, we derive the intrinsic oscillator strengths of the 3.3 $\mu$m aromatic C–H stretch ($A_{3.3}$) and the 3.4 $\mu$m aliphatic C–H stretch ($A_{3.4}$). By comparing the computationally-derived mean ratio of $\langle A_{3.4}/A_{3.3}\rangle\sim1.98$ with the mean ratio of the observed intensities $\langle I_{3.4}/I_{3.3}\rangle\sim0.12$, we find that the degree of superhydrogenation — the fraction of C atoms attached with excess H atoms — is only $\sim2.2\%$ for neutral PAHs which predominantly emit the 3.3 and 3.4 $\mu$m features. We also determine for each molecule the intrinsic band strengths of the 6.2 $\mu$m aromatic C–C stretch ($A_{6.2}$) and the 6.85 $\mu$m aliphatic C–H deformation ($A_{6.85}$). We derive the degree of superhydrogenation from the mean ratio of the observed intensities $\langle I_{6.85}/I_{6.2}\rangle\sim0.10$ and $\langle A_{6.85}/A_{6.2}\rangle\sim1.53$ for neutrals and $\langle A_{6.85}/A_{6.2}\rangle\sim1.23$ for cations to be $\lesssim 3.1\%$ for neutrals and $\lesssim 8.6\%$ for cations. We conclude that astrophysical PAHs are primarily aromatic and are only marginally superhydrogenated.

Read this paper on arXiv…

X. Yang, A. Li and R. Glaser
Mon, 6 Jan 20
32/49

Comments: 49 pages, 26 figures; accepted for publication in The Astrophysical Journal Supplement Series

Spectroscopy of corannulene cations in helium nanodroplets [CL]

http://arxiv.org/abs/1912.08692


Helium tagging in action spectroscopy is an efficient method for measuring the absorption spectrum of complex molecular ions with minimal perturbations to the gas phase spectrum. We have used superfluid helium nanodroplets doped with corannulene to prepare cations of these molecules complexed with different numbers of He atoms. In total we identify 13 different absorption bands from corannulene cations between 5500 {\AA} and 6000 {\AA}. The He atoms cause a small, chemically induced redshift to the band positions of the corannulene ion. By studying this effect as a function of the number of solvating atoms we are able to identify the formation of solvation structures that are not visible in the mass spectrum. The solvation features detected with the action spectroscopy agree very well with the results of atomistic modeling based on path-integral molecular dynamics simulations. By additionally doping our He droplets with D$_2$, we produce protonated corannulene ions. The absorption spectrum of these ions differs significantly from the case of the radical cations as the numerous narrow bands are replaced by a broad absorption feature that spans nearly 2000 {\AA} in width.

Read this paper on arXiv…

M. Gatchell, P. Martini, F. Laimer, et. al.
Thu, 19 Dec 19
37/82

Comments: 10 pages, 7 figures, published in Faraday Discuissions

Measurements and semi-empirical calculations of CO2+CH4 and CO2+H2 collision-induced absorptions across a wide range of wavenumbers and temperatures. Application for the prediction of early Mars surface temperature [EPA]

http://arxiv.org/abs/1912.05630


Reducing atmospheres have recently emerged as a promising scenario to warm the surface of early Mars enough to drive the formation of valley networks and other ancient aqueous features that have been detected so far on the surface of Mars. Here we present a series of experiments and calculations to better constrain CO2+CH4 and CO2+H2 collision-induced absorptions (CIAs) as well as their effect on the prediction of early Mars surface temperature. First, we carried out a new set of experimental measurements (using the AILES line of the SOLEIL synchrotron) of both CO2+CH4 and CO2+H2 CIAs. These measurements confirm the previous results of Turbet et al. 2019, Icarus vol. 321, while significantly reducing the experimental uncertainties. Secondly, we fitted a semi-empirical model to these CIAs measurements, allowing us to compute the CO2+CH4 and CO2+H2 CIAs across a broad spectral domain (0-1500cm-1) and for a wide range of temperatures (100-600K). Last, we performed 1-D numerical radiative-convective climate calculations (using the LMD Generic Model) to compute the surface temperature expected on the surface of early Mars for several CO2, CH4 and H2 atmospheric contents, taking into account the radiative effect of these revised CIAs. These calculations demonstrate that thick CO2+H2-dominated atmospheres remain a viable solution for warming the surface of Mars above the melting point of water, but not CO2+CH4-dominated atmospheres. Our calculated CO2+CH4 and CO2+H2 CIA spectra and predicted early Mars surface temperatures are provided to the community for future uses.

Read this paper on arXiv…

M. Turbet, C. Boulet and T. Karman
Fri, 13 Dec 19
73/75

Comments: Submitted for publication to Icarus. CO2+H2 and CO2+CH4 CIAs calculated tables (HITRAN format) – as well as predicted early Mars surface temperatures – are available on demand

Laboratory formation and photo-chemistry of fullerene/anthracene cluster cations [CL]

http://arxiv.org/abs/1910.11467


Besides buckminsterfullerene (C60), other fullerenes and their derivatives may also reside in space. In this work, we study the formation and photo-dissociation processes of astronomically relevant fullerene/anthracene (C14H10) cluster cations in the gas phase. Experiments are carried out using a quadrupole ion trap (QIT) in combination with time-of-flight (TOF) mass spectrometry. The results show that fullerene (C60, and C70)/anthracene (i.e., [(C14H10)nC60]+ and [(C14H10)nC70]+), fullerene (C56 and C58)/anthracene (i.e., [(C14H10)nC56]+ and [(C14H10)nC58]+) and fullerene (C66 and C68)/anthracene (i.e., [(C14H10)nC66]+ and [(C14H10)nC68]+) cluster cations, are formed in the gas phase through an ion-molecule reaction pathway. With irradiation, all the fullerene/anthracene cluster cations dissociate into mono$-$anthracene and fullerene species without dehydrogenation. The structure of newly formed fullerene/anthracene cluster cations and the bonding energy for these reaction pathways are investigated with quantum chemistry calculations.
Our results provide a growth route towards large fullerene derivatives in a bottom-up process and insight in their photo-evolution behavior in the ISM, and clearly, when conditions are favorable, fullerene/PAH clusters can form efficiently. In addition, these clusters (from 80 to 154 atoms or ~ 2 nm in size) offer a good model for understanding the physical-chemical processes involved in the formation and evolution of carbon dust grains in space, and provide candidates of interest for the DIBs that could motivate spectroscopic studies.

Read this paper on arXiv…

J. Zhen, W. Zhang, Y. Yang, et. al.
Mon, 28 Oct 19
9/40

Comments: 10 pages, 8 figures, accepted

The infrared spectrum of protonated buckminsterfullerene, C60H+ [CL]

http://arxiv.org/abs/1910.04533


Although fullerenes have long been hypothesized to occur in interstellar environments, their actual unambiguous spectroscopic identification is of more recent date. C60, C70 and C60+ now constitute the largest molecular species individually identified in the interstellar medium (ISM). Fullerenes have significant proton affinities and it was suggested that C60H+ is likely the most abundant interstellar analogue of C60. We present here the first laboratory infrared (IR) spectrum of gaseous C60H+. Symmetry breaking relative to C60 produces an IR spectrum that is much richer than that of C60. The experimental spectrum is used to benchmark theoretical spectra indicating that the B3LYP density functional with the 6-311+G(d,p) basis set accurately reproduces the spectrum. Comparison with IR emission spectra from two planetary nebulae, SMP LMC56 and SMC16, that have been associated with high C60 abundances, indicate that C60H+ is a plausible contributor to their IR emission.

Read this paper on arXiv…

J. Palotás, J. Martens, G. Berden, et. al.
Mon, 14 Oct 19
12/69

Comments: 9 pages, 4 figures

Investigations into the rotational spectrum of isotopic methyl mercaptan, (13)CH3SH, in the laboratory and towards Sagittarius B2(N2) [SSA]

http://arxiv.org/abs/1910.02259


Methyl mercaptan (CH3SH) is a known interstellar molecule with abundances high enough that the detection of some of its minor isotopologues is promising. The present study aims at providing accurate spectroscopic parameters for the (13)CH3SH isotopologue to facilitate searches in the interstellar medium at millimetre and submillimetre wavelengths. Through careful analysis of recent CH3SH spectra from 49-510 GHz and 1.1-1.5 THz recorded at natural isotopic composition, extensive assignments were possible not only for the ground torsional state of (13)CH3SH, but also in the first and second excited states. The torsion-rotation spectrum displays complex structure due to the large-amplitude internal rotation of the (13)CH3 group, similar to the main and other minor isotopic species of methyl mercaptan. The assigned transition frequencies have been fitted to within experimental error with a 52-parameter model employing the RAM36 programme. With predictions based on this fit, (13)CH3SH was searched for in spectra from the Atacama Large Millimetre/submillimetre Array (ALMA) towards the Galactic centre source Sgr B2(N2). Several transitions were expected to be observable, but all of them turned out to be severely blended with emission from other species, which prevents us from identifying (13)CH3SH in this source.

Read this paper on arXiv…

V. Ilyushin, O. Zakharenko, F. Lewen, et. al.
Tue, 8 Oct 19
44/88

Comments: 20 pages (with figures and tables); Can. J. Phys. (this https URL), accepted

Ultraslow radiative cooling of C$_n^-$ ($n=3-5$) [CL]

http://arxiv.org/abs/1909.07087


Ultraslow radiative cooling lifetimes and adiabatic detachment energies for three astrochemically relevant anions, C$_n^-$ ($n=3-5$), are measured using the Double ElectroStatic Ion Ring ExpEriment (DESIREE) infrastructure at Stockholm University. DESIREE maintains a background pressure of $\approx$10$^{-14}$\,mbar and temperature of $\approx$13\,K, allowing storage of mass-selected ions for hours and providing conditions coined a “molecular cloud in a box”. Here, we construct two-dimensional (2D) photodetachment spectra for the target anions by recording photodetachment signal as a function of irradiation wavelength and ion storage time (seconds to minute timescale). Ion cooling lifetimes, which are associated with infrared radiative emission, are extracted from the 2D photodetachment spectrum for each ion by tracking the disappearance of vibrational hot-band signal with ion storage time, giving $\frac{1}{e}$ cooling lifetimes of 3.1$\pm$0.1\,s (C$_3^-$), 6.8$\pm$0.5\,s (C$_4^-$) and 24$\pm$5\,s (C$_5^-$). Fits of the photodetachment spectra for cold ions, i.e. those stored for at least 30\,s, provides adiabatic detachment energies in good agreement with values from laser photoelectron spectroscopy. Ion cooling lifetimes are simulated using a Simple Harmonic Cascade model, finding good agreement with experiment and providing a mode-by-mode understanding of the radiative cooling properties. The 2D photodetachment strategy and radiative cooling modeling developed in this study could be applied to investigate the ultraslow cooling dynamics of wide range of molecular anions.

Read this paper on arXiv…

J. Bull, M. Scholz, E. Carrascosa, et. al.
Tue, 17 Sep 19
19/98

Comments: N/A

Rotational spectroscopy of methyl mercaptan CH$_3$$^{32}$SH at millimeter and submillimeter wavelengths [IMA]

http://arxiv.org/abs/1908.07790


We present a new global study of the millimeter (mm) wave, submillimeter (sub-mm) wave, and terahertz (THz) spectra of the lowest three torsional states of methyl mercaptan (CH$_3$SH). New measurements have been carried out between 50 and 510 GHz using the Kharkiv mm wave and the Cologne sub-mm wave spectrometers whereas THz spectra records were used from our previous study. The new data, involving torsion-rotation transitions with $J$ up to 61 and $K_a$ up to 18, were combined with previously published measurements and fit using the rho-axis-method torsion-rotation Hamiltonian. The final fit used 124 parameters to give an overall weighted root-mean-square deviation of 0.72 for the dataset consisting of 6965 microwave (MW) and 16345 far-infrared line frequencies sampling transitions within and between the ground, first, and second excited torsional states. This investigation presents a two-fold expansion in the $J$ quantum numbers and a significant improvement in the fit quality, especially for the MW part of the data, thus allowing us to provide more reliable predictions to support astronomical observations.

Read this paper on arXiv…

O. Zakharenko, V. Ilyushin, F. Lewen, et. al.
Thu, 22 Aug 19
58/70

Comments: 12 pages; Astron. Astrophys., in press

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

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.

Read this paper on arXiv…

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

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

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.

Read this paper on arXiv…

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

Inside a VAMDC data node – Putting standards into practical software [IMA]

http://arxiv.org/abs/1803.09217


Access to molecular and atomic data is critical for many forms of remote sensing analysis across different fields. Many atomic and molecular databases are however highly specialized for their intended application, complicating querying and combination data between sources. The Virtual Atomic and Molecular Data Centre, VAMDC, is an electronic infrastructure that allows each database to register as a “node”. Through services such as VAMDC’s portal website, users can then access and query all nodes in a homogenized way. Today all major Atomic and Molecular databases are attached to VAMDC.
This article describes the software tools we developed to help data providers create and manage a VAMDC node. It gives an overview of the VAMDC infrastructure and of the various standards it uses. The article then discusses the development choices made and how the standards are implemented in practice. It concludes with a full example of implementing a VAMDC node using a real-life case as well as future plans for the node software.

Read this paper on arXiv…

S. Regandell, T. Marquart and N. Piskunov
Wed, 28 Mar 18
142/148

Comments: 12 pages, 2 figures

Converged Close-Coupling R-Matrix calculations of Photoionization of Fe XVII in Astrophysical Plasmas: from Convergence to Completeness [SSA]

http://arxiv.org/abs/1801.02188


Extensive resonance structures are manifest in R-Matrix (RM) calculations. However, there exist a large number of highly excited electronic configurations that may contribute to background non-resonant bound-free opacity in high-temperature plasmas. Since RM calculations are very complex, and not essential for background contributions, the Relativistic Distorted Wave (RDW) method is utilized to complement (“top-up”) photoionization cross sections of Fe XVII obtained using Close-Coupling Breit-Pauli R-Matrix (CC-BPRM) method. There is good agreement between RDW and BPRM for background cross sections where resonances are not present, and individual fine structure levels can be correctly matched spectroscopically, though resonances are neglected in the RDW. To ensure completeness, a high energy range up to 500 Ry above the ionization threshold for each level is considered. Interestingly, the hydrogenic Kramer’s approximation also shows the same energy behavior as the RDW. Grouping separately, the BPRM configurations consist of 454 bound levels with resonances corresponding to configurations $1s^22s^22p^4nln’l’$ (n $\leq$ 3, n’ $\leq$ 10); including RDW configurations there are 51,558 levels in total. The topup contribution results in $\sim$20\% increment, in addition to the 35\% enhancement from BPRM calculations over the Opacity Project value for the Rosseland Mean Opacity at the Z-temperature of 2.11 $\times 10^6$K (Pradhan and Nahar 2017).

Read this paper on arXiv…

L. Zhao, W. Eissner, S. Nahar, et. al.
Tue, 9 Jan 18
59/94

Comments: 5 pages, 2 figures, Proc. Workshop on Astrophysical Opacities 2017, PASP

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

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.

Read this paper on arXiv…

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]

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.

Read this paper on arXiv…

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

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

The Interstellar Formation and Spectra of the Noble Gas, Proton-Bound HeHHe+, HeHNe+, & HeHAr+ Complexes [CL]

http://arxiv.org/abs/1711.03344


The sheer interstellar abundance of helium makes any bound molecules or complexes containing it of potential interest for astrophysical observation. This work utilizes high-level and trusted quantum chemical techniques to predict the rotational, vibrational, and rovibrational traits of HeHHe+, HeHNe+, and HeHAr+. The first two are shown to be strongly bound, while HeHAr+ is shown to be more of a van der Waals complex of argonium with a helium atom. In any case, the formation of HeHHe+ through reactions of HeH+ with HeH3+ is exothermic. HeHHe+ exhibits the quintessentially bright proton-shuttle motion present in all proton-bound complexes in the 7.4 micron range making it a possible target for telescopic observation at the mid-IR/far-IR crossover point and a possible tracer for the as-of-yet unobserved helium hydride cation. Furthermore, a similar mode in HeHNe+ can be observed to the blue of this close to 6.9 microns. The brightest mode of HeHAr+ is dimmed due the reduced interaction of the helium atom with the central proton, but this fundamental frequency can be found slightly to the red of the Ar-H stretch in the astrophysically detected argonium cation.

Read this paper on arXiv…

C. Stephan and R. Fortenberry
Fri, 10 Nov 17
9/55

Comments: 9 pages, 8 tables

Fragmentation of Fullerenes to Linear Carbon Chains [CL]

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.

Read this paper on arXiv…

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

Comments: 16 pages, 10 figures

Millimeter and Submillimeter Wave Spectroscopy of Propanal [SSA]

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.

Read this paper on arXiv…

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

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

Photon emission and atomic collision processes in two-phase argon doped with xenon and nitrogen [CL]

http://arxiv.org/abs/1702.03612


We present a comprehensive analysis of photon emission and atomic collision processes in two-phase argon doped with xenon and nitrogen. The dopants are aimed to convert the VUV emission of pure Ar to the UV emission of the Xe dopant in the liquid phase and to the near UV emission of the N2 dopant in the gas phase. Such a mixture is relevant to two-phase dark matter and low energy neutrino detectors, with enhanced photon collection efficiency for primary and secondary scintillation signals. Based on this analysis, we show that the recently proposed hypothesis of the enhancement of the excitation transfer from Ar to N2 species in the two-phase mode is either incorrect or needs assumption about a new extreme mechanism of excitation transfer coming into force at lower temperatures, in particular that of the resonant excitation transfer via ArN2 compound (van der Waals molecule).

Read this paper on arXiv…

A. Buzulutskov
Tue, 14 Feb 17
19/71

Comments: 6 pages, 1 figure, 1 table

Photo-stability of super-hydrogenated PAHs determined by action spectroscopy experiments [GA]

http://arxiv.org/abs/1609.04264


We have investigated the photo-stability of pristine and super-hydrogenated pyrene cations C$_{16}$H$_{10+m}^+, m = 0,6, \mathrm{\ or\ } 16$) by means of gas-phase action spectroscopy. Optical absorption spectra and photo-induced dissociation mass spectra are presented. By measuring the yield of mass-selected photo-fragment ions as a function of laser pulse intensity, the number of photons (and hence the energy) needed for fragmentation of the carbon backbone was determined. Backbone fragmentation of pristine pyrene ions (C$_{16}$H$_{10}^+$) requires absorption of three photons of energy just below 3 eV, whereas super-hydrogenated hexahydropyrene (C$_{16}$H$_{16}^+$) must absorb two such photons and fully hydrogenated hexadecahydropyrene (C$_{16}$H$_{26}^+$) only a single photon. These results are consistent with previously reported dissociation energies for these ions. Our experiments clearly demonstrate that the increased heat capacity from the additional hydrogen atoms does not compensate for the weakening of the carbon backbone when pyrene is hydrogenated. In photodissociation regions, super-hydrogenated Polycyclic Aromatic Hydrocarbons (PAHs) have been proposed to serve as catalysts for H$_2$-formation. Our results indicate that carbon backbone fragmentation may be a serious competitor to H$_2$-formation at least for small hydrogenated PAHs like pyrene.

Read this paper on arXiv…

M. Wolf, H. Kiefer, J. Langeland, et. al.
Thu, 15 Sep 16
14/56

Comments: 6 pages, 4 figures, accepted in The Astrophysical Journal

Multipole Traps as Tools in Environmental Studies [CL]

http://arxiv.org/abs/1512.05522


Trapping of microparticles, nanoparticles and aerosols is an issue of major interest for physics and chemistry. We present a setup intended for microparticle trapping in multipole linear Paul trap geometries, operating under Standard Ambient Temperature and Pressure (SATP) conditions. A 16-electrode linear trap geometry has been designed and tested, with an aim to confine a larger number of particles with respect to quadrupole traps and thus enhance the signal to noise ratio, as well as to study microparticle dynamical stability in electrodynamic fields. Experimental tests and numerical simulations suggest that multipole traps are very suited for high precision mass spectrometry measurements in case of different microparticle species or to identify the presence of certain aerosols and polluting agents in the atmosphere. Particle traps represent versatile tools for environment monitoring or for the study of many-body Coulomb systems and dusty plasmas.

Read this paper on arXiv…

B. Mihalcea, C. Stan, L. Giurgiu, et. al.
Fri, 18 Dec 15
13/70

Comments: 17 pages, 8 figures

Multipole Electrodynamic Ion Trap Geometries for Microparticle Confinement under Standard Ambient Temperature and Pressure Conditions [CL]

http://arxiv.org/abs/1512.05503


Trapping of microparticles and aerosols is of great interest for physics and chemistry. We report microparticle trapping in multipole linear Paul trap geometries, operating under Standard Ambient Temperature and Pressure (SATP) conditions. An 8-electrode and a 12-electrode linear trap geometries have been designed and tested with an aim to achieve trapping for larger number of particles and to study microparticle dynamical stability in electrodynamic fields. We report emergence of planar and volume ordered structures of the microparticles, depending on the a.c. trapping frequency and particle specific charge ratio. The electric potential within the trap was mapped using the electrolytic tank method. Particle dynamics was simulated using a stochastic Langevin equation. We emphasize extended regions of stable trapping with respect to quadrupole traps, as well as good agreement between experiment and numerical simulations.

Read this paper on arXiv…

B. Mihalcea, L. Giurgiu, C. Stan, et. al.
Fri, 18 Dec 15
21/70

Comments: 24 pages, 11 figures

An optical spectrum of a large isolated gas-phase PAH cation: C78H26+ [IMA]

http://arxiv.org/abs/1512.00940


A gas-phase optical spectrum of a large polycyclic aromatic hydrocarbon (PAH) cation – C78H26 +- in the 410-610 nm range is presented. This large all-benzenoid PAH should be large enough to be stable with respect to photodissociation in the harsh conditions prevailing in the interstellar medium (ISM). The spectrum is obtained via multi-photon dissociation (MPD) spectroscopy of cationic C78H26 stored in the Fourier Transform Ion Cyclotron Resonance (FT-ICR) cell using the radiation from a mid-band optical parametric oscillator (OPO) laser. The experimental spectrum shows two main absorption peaks at 431 nm and 516 nm, in good agreement with a theoretical spectrum computed via time-dependent density functional theory (TD-DFT). DFT calculations indicate that the equilibrium geometry, with the absolute minimum energy, is of lowered, nonplanar C2 symmetry instead of the more symmetric planar D2h symmetry that is usually the minimum for similar PAHs of smaller size. This kind of slightly broken symmetry could produce some of the fine structure observed in some diffuse interstellar bands (DIBs). It can also favor the folding of C78H26 + fragments and ultimately theformation of fullerenes. This study opens up the possibility to identify the most promising candidates for DIBs amongst large cationic PAHs.

Read this paper on arXiv…

J. Zhen, G. Mulas, A. Bonnamy, et. al.
Fri, 4 Dec 15
2/64

Comments: 7 pages, 4 figures, 1 table, accepted in Molecular Astrophysics, 2015

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

http://arxiv.org/abs/1510.07820


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

Read this paper on arXiv…

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

Comments: 10 pages, 5 figures

Simulating the Formation of Carbon-rich Molecules on an idealised Graphitic Surface [EPA]

http://arxiv.org/abs/1510.05783


There is accumulating evidence for the presence of complex molecules, including carbon-bearing and organic molecules, in the interstellar medium. Much of this evidence comes to us from studies of chemical composition, photo- and mass-spectroscopy in cometary, meteoritic and asteroid samples, indicating a need to better understand the surface chemistry of astrophysical objects. There is also considerable interest in the origins of life-forming and life-sustaining molecules on Earth. Here, we perform reactive molecular dynamics simulations to probe the formation of carbon-rich molecules and clusters on carbonaceous surfaces resembling dust grains and meteoroids. Our results show that large chains form on graphitic surfaces at low temperatures (100K – 500K) and smaller fullerene-like molecules form at higher temperatures (2000K – 3000K). The formation is faster on the surface than in the gas at low temperatures but slower at high temperatures as surface interactions prevent small clusters from coagulation. We find that for efficient formation of molecular complexity, mobility about the surface is important and helps to build larger carbon chains on the surface than in the gas phase at low temperatures. Finally, we show that the temperature of the surface strongly determines what kind of structures forms and that low turbulent environments are needed for efficient formation.

Read this paper on arXiv…

D. Marshall and H. Sadeghpour
Wed, 21 Oct 15
54/66

Comments: 13 pages, 14 figures; the manuscript has been submitted to MNRAS for publication

Electronic transitions of jet-cooled SiC2, Si2Cn (n=1-3), Si3Cn (n=1,2), and SiC6H4 between 250 and 710 nm [CL]

http://arxiv.org/abs/1503.05138


Electronic transitions of the title molecules were measured between 250 and 710 nm using a mass-resolved 1+1′ resonant two-photon ionization technique at a resolution of 0.1 nm. Calculations at the B3LYP/aug-cc-pVQZ level of theory support the analyses. Because of their spectral properties, SiC$_2$, linear Si$_2$C$_2$, Si$_3$C, and SiC$_6$H$_4$ are interesting target species for astronomical searches in the visible spectral region. Of special relevance is the Si–C$_2$–Si chain, which features a prominent band at 516.4 nm of a strong transition ($f=0.25$). This band and one from SiC$_6$H$_4$ at 445.3 nm were also investigated at higher resolution (0.002 nm).

Read this paper on arXiv…

M. Steglich and J. Maier
Wed, 18 Mar 15
15/59

Comments: 15 pages, 9 figures, 5 Tables

The electronic spectra of protonated PANH molecules [SSA]

http://arxiv.org/abs/1503.03329


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

Read this paper on arXiv…

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

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

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

http://arxiv.org/abs/1502.06867


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

Read this paper on arXiv…

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

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

Terahertz spectroscopy of N$^{18}$O and isotopic invariant fit of several nitric oxide isotopologs [GA]

http://arxiv.org/abs/1412.4974


A tunable far-infrared laser sideband spectrometer was used to investigate a nitric oxide sample enriched in 18O between 0.99 and 4.75 THz. Regular, electric dipole transitions were recorded between 0.99 and 2.52 THz, while magnetic dipole transitions between the 2Pi(1/2) and 2Pi(3/2) spin-ladders were recorded between 3.71 and 4.75 THz. These data were combined with lower frequency data of N(18)$O (unlabeled atoms refer to (14)N and (16)O, respectively), with rotational data of NO, (15)NO, N(17)O, and (15)N(18)O, and with heterodyne infrared data of NO to be subjected to one isotopic invariant fit. Rotational, fine and hyperfine structure parameters were determined along with vibrational, rotational, and Born-Oppenheimer breakdown corrections. The resulting spectroscopic parameters permit prediction of rotational spectra suitable for the identification of various nitric oxide isotopologs especially in the interstellar medium by means of rotational spectroscopy.

Read this paper on arXiv…

H. Muller, K. Kobayashi, K. Takahashi, et. al.
Wed, 17 Dec 14
58/67

Comments: 8 pages, 1 figure; part of the Marilyn Jacox special issue of the Journal of Molecular Spectroscopy, in press

Sulfur-bearing species in molecular clouds [SSA]

http://arxiv.org/abs/1410.8293


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

Read this paper on arXiv…

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

Comments: accepted for publication in MNRAS

Dust in brown dwarfs and extra-solar planets IV. Assessing TiO2 and SiO nucleation for cloud formation modeling [SSA]

http://arxiv.org/abs/1410.6610


Clouds form in atmospheres of brown dwarfs and planets. The cloud particle formation processes are similar to the dust formation process studied in circumstellar shells of AGB stars and in Supernovae. Cloud formation modelling in substellar objects requires gravitational settling and element replenishment in addition to element depletion. All processes depend on the local conditions, and a simultaneous treatment is required. We apply new material data in order to assess our cloud formation model results regarding the treatment of the formation of condensation seeds. We re-address the question of the primary nucleation species in view of new (TiO2)_N-cluster data and new SiO vapour pressure data. We apply the density functional theory using the computational chemistry package Gaussian 09 to derive updated thermodynamical data for (TiO2)_N-clusters as input for our TiO2 seed formation model. We test different nucleation treatments and their effect on the overall cloud structure by solving a system of dust moment equations and element conservation or a pre-scribed Drift-Phoenix atmosphere structure. Updated Gibbs free energies for the (TiO2)_N-clusters are presented, and a slightly temperature dependent surface tension for T=500 … 2000K with an average value of sigma_infty = 480.6 erg 1/cm2. The TiO2-seed formation rate changes only slightly with the updated cluster data. A considerably larger effect on the rate of seed formation, and hence on grain size and dust number density, results from a switch to SiO-nucleation. Despite the higher abundance of SiO over TiO2 in the gas phase, TiO2 remains considerably more efficient in forming condensation seeds by homogeneous nucleation followed by heterogeneous grain growth. The paper discussed the effect on the cloud structure in more detail.

Read this paper on arXiv…

G. Lee, C. Helling, H. Giles, et. al.
Mon, 27 Oct 14
16/58

Comments: accepted for publication in A&A (abstract abridged)

Revised spectroscopic parameters of SH$^+$ from ALMA and IRAM 30 m observations [GA]

http://arxiv.org/abs/1409.1807


Hydrides represent the first steps of interstellar chemistry. Sulfanylium (SH$^+$), in particular, is a key tracer of energetic processes. We used ALMA and the IRAM 30 m telescope to search for the lowest frequency rotational lines of SH$^+$ toward the Orion Bar, the prototypical photo-dissociation region illuminated by a strong UV radiation field. On the basis of previous $Herschel$/HIFI observations of SH$^+$, we expected to detect emission of the two SH$^+$ hyperfine structure (HFS) components of the $N_J = 1_0 – 0_1$ fine structure (FS) component near 346 GHz. While we did not observe any lines at the frequencies predicted from laboratory data, we detected two emission lines, each $\sim$15 MHz above the SH$^+$ predictions and with relative intensities and HFS splitting expected for SH$^+$. The rest frequencies of the two newly detected lines are more compatible with the remainder of the SH$^+$ laboratory data than the single line measured in the laboratory near 346 GHz and previously attributed to SH$^+$. Therefore, we assign these new features to the two SH$^+$ HFS components of the $N_J = 1_0 – 0_1$ FS component and re-determine its spectroscopic parameters, which will be useful for future observations of SH$^+$, in particular if its lowest frequency FS components are studied. Our observations demonstrate the suitability of these lines for SH$^+$ searches at frequencies easily accessible from the ground.

Read this paper on arXiv…

H. Muller, J. Goicoechea, J. Cernicharo, et. al.
Mon, 8 Sep 14
31/69

Comments: 5 pages, 1 figure, Astron. Astrophys., accepted

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

http://arxiv.org/abs/1408.5247


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

Read this paper on arXiv…

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

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

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

http://arxiv.org/abs/1408.3301


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

Read this paper on arXiv…

N. Dattani
Fri, 15 Aug 14
30/45

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

Do cement nanoparticles exist in space ? [SSA]

http://arxiv.org/abs/1404.7392


The calcium-silicate-hydrate is used to model properties of cement on Earth. We study cementitious nanoparticles and propose these structures as components of cosmic dust grains. Quantum density functional theory methods are applied for the calculation of infrared spectra of Ca4Si4O14H4, Ca6Si3O13H2, and Ca12Si6O26H4 clusters. We find bands distributed over the near, mid and far-infrared region. A specific calcium-silicate-hydrate spectral feature at 14 microns, together with the bands at 10 and 18 microns which exist for other silicates as well, could be used for a detection of cosmic cement. We compare calculated bands with the 14 microns features in the spectra of HD 45677, HD 44179, and IRC+10420 which were observed by Infrared Space Observatory and classified as remaining. High abundance of oxygen atoms in cementitious nanoparticles could partially explain observed depletion of this element from the interstellar medium into dust grains.

Read this paper on arXiv…

G. Bilalbegovic, A. Maksimovic and V. Mohacek-Grosev
Wed, 30 Apr 14
54/68

Comments: accepted for publication in MNRAS

Absolute evaporation rates of non-rotating neutral PAH clusters [CL]

http://arxiv.org/abs/1404.2107


Clusters of polycyclic aromatic hydrocarbons (PAHs) have been proposed as candidates for evaporating very small grains, which are thought to be precursors of free-flying PAHs. Evaporation rates have been calculated so far only for species containing up to a few 100 C atoms, whereas interstellar PAH clusters could contain up to ~1000 C atoms. We present a method that generalises the calculation of the statistical evaporation rate of large PAH clusters and provides rates for species containing up to ~1000 C-atoms. The evaporation of non-rotating neutral homo-molecular PAH clusters containing up to 12 molecules from a family of highly symmetric compact PAHs is studied. Statistical calculations were performed and completed with molecular dynamics simulations at high internal energies to provide absolute values for the evaporation rate and distributions of kinetic energy released. The calculations used explicit atom-atom Lennard-Jones potentials in the rigid molecule approximation. A new method is proposed to take both inter- and intra-molecular vibrations into account. Without any parameter adjustment, the calculated evaporation rates agree well with available experimental data. We find that the non-rotation assumption has a limited impact on the evaporation rates. The photostability of PAH clusters increases dramatically with the size of molecules in the clusters, and to a lesser extent with the number of molecules in the clusters. For values of the UV radiation field that are typical of the regions where evaporating very small grains are observed, the smallest clusters in this study (~50 C-atoms) are found to be quickly photo-evaporated, whereas the largest clusters (~1000 C-atoms) are photostable. Our results support the idea that large PAH clusters are good candidates for evaporating very small grains.

Read this paper on arXiv…

J. Montillaud and C. Joblin
Wed, 9 Apr 14
1/67

Formation of silicon oxide grains at low temperature [GA]

http://arxiv.org/abs/1312.5517


The formation of grains in the interstellar medium, i.e., at low temperature, has been proposed as a possibility to solve the lifetime problem of cosmic dust. This process lacks a firm experimental basis, which is the goal of this study. We have investigated the condensation of SiO molecules at low temperature using neon matrix and helium droplet isolation techniques. The energies of SiO polymerization reactions have been determined experimentally with a calorimetric method and theoretically with calculations based on the density functional theory. The combined experimental and theoretical values have revealed the formation of cyclic (SiO)$_k$ ($k$ = 2–3) clusters inside helium droplets at $T$ = 0.37 K. Therefore, the oligomerization of SiO molecules is found to be barrierless and is expected to be fast in the low-temperature environment of the interstellar medium on the surface of dust grains. The incorporation of numerous SiO molecules in helium droplets leads to the formation of nanoscale amorphous SiO grains. Similarly, the annealing and evaporation of SiO-doped Ne matrices lead to the formation of solid amorphous SiO on the substrate. The structure and composition of the grains were determined by infrared absorption spectroscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. Our results support the hypothesis that interstellar silicates \textbf{can be formed} in the low temperature regions of the interstellar medium by accretion through barrierless reactions.

Read this paper on arXiv…

Fri, 20 Dec 13
10/56