http://arxiv.org/abs/2103.03890

The interaction of polycyclic aromatic hydrocarbons (PAHs) with vacuum ultraviolet (VUV) photons triggers the emission of the well-known aromatic infrared bands but other mechanisms such as fragmentation can be involved in this interaction. Fragmentation leads to selection effects favoring specific sizes and structures. We investigate the impact of aliphatic bonds on the VUV photo-stability of PAH cations under conditions applicable for photodissociation regions (PDRs). Cations of pyrene (C16H10) and coronene (C24H12) derivatives containing aliphatic bonds (methyl or ethyl sidegroups, superhydrogenation) are submitted to VUV (10.5 eV) photons over long timescales (~1000 s) in the cryogenic PIRENEA setup. The fragmentation cascades are analyzed with a simple kinetics model; fragmentation pathways, rates and branching ratios are derived. Aliphatic PAH derivatives are found to have a higher fragmentation rate and carbon loss compared to regular PAHs. The fragmentation of PAHs with alkylated sidegroups forms species with peripheral pentagonal cycles, which can be more stable than the bare PAH cations. This stability is quantified and the most stable species, for which there is an effective competition of fragmentation with isomerization and radiative cooling, are identified. This work supports a scenario in which the evaporation of nanograins with mixed aliphatic and aromatic composition followed by VUV photoprocessing results in both the production of the carriers of the 3.4um AIB by methyl sidegroups and in an abundant source of small hydrocarbons at the border of PDRs. An additional side effect is the efficient formation of stable PAHs containing some peripheral pentagonal rings. Our experiments also support the role of isomerization processes in PAH photofragmentation, including the H-migration process, which could lead to an additional contribution to the 3.4um AIB.

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A. Marciniak, C. Joblin, G. Mulas, et. al.
Tue, 9 Mar 21
4/68

Comments: subbitted for publication on Astronomy and Astrophysics

http://arxiv.org/abs/2101.11534

The evidence for benzonitrile (C$_6$H$_5$CN}) in the starless cloud core TMC-1 makes high-resolution studies of other aromatic nitriles and their ring-chain derivatives especially timely. One such species is phenylpropiolonitrile (3-phenyl-2-propynenitrile, C$_6$H$_5$C$_3$N), whose spectroscopic characterization is reported here for the first time. The low resolution (0.5 cm$^{-1}$) vibrational spectrum of C$_6$H$_5$C$_3$N} has been recorded at far- and mid-infrared wavelengths (50 – 3500 cm$^{-1}$) using a Fourier Transform interferometer, allowing for the assignment of band centers of 14 fundamental vibrational bands. The pure rotational spectrum of the species has been investigated using a chirped-pulse Fourier transform microwave (FTMW) spectrometer (6 – 18 GHz), a cavity enhanced FTMW instrument (6 – 20 GHz), and a millimeter-wave one (75 – 100 GHz, 140 – 214 GHz). Through the assignment of more than 6200 lines, accurate ground state spectroscopic constants (rotational, centrifugal distortion up to octics, and nuclear quadrupole hyperfine constants) have been derived from our measurements, with a plausible prediction of the weaker bands through calculations. Interstellar searches for this highly polar species can now be undertaken with confidence since the astronomically most interesting radio lines have either been measured or can be calculated to very high accuracy below 300 GHz.

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Z. Buchanan, K. Lee, O. Chitarra, et. al.
Thu, 28 Jan 21
56/64

Comments: 7 figures, 4 tables. Accepted for publication in J. Mol. Spec

http://arxiv.org/abs/2012.01234

The rates of numerous activated reactions between neutral species increase at low temperatures through quantum mechanical tunneling of light hydrogen atoms. Although tunneling processes involving molecules or heavy atoms are well known in the condensed phase, analogous gas-phase processes have never been demonstrated experimentally. Here, we studied the activated CH + CO2 -> HCO + CO reaction in a supersonic flow reactor, measuring rate constants that increase rapidly below 100 K. Mechanistically, tunneling is shown to occur by CH insertion into the C-O bond, with rate calculations accurately reproducing the experimental values. To exclude the possibility of H-atom tunneling, CD was used in additional experiments and calculations. Surprisingly, the equivalent CD + CO2 reaction accelerates at low temperature as zero point energy effects remove the barrier to product formation. In conclusion, heavy-particle tunneling effects might be responsible for the observed reactivity increase at lower temperatures for the CH + CO2 reaction, while the equivalent effect for the CD + CO2 reaction results instead from a submerged barrier with respect to reactants.

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D. Nunez-Reyes, K. Hickson, J. Loison, et. al.
Thu, 3 Dec 20
55/81

Comments: Accepted for publication in the Journal of Physical Chemistry A. 30 pages including supplementary materials

http://arxiv.org/abs/2011.12145

The phenomenon of life is discussed within a framework of its origin as defined by four hypotheses. The 1. hypothesis says: Life, as we know, is (H-C-N-O) based and relies on the number of bulk (Na-Mg-P-S-Cl-K-Ca) and trace elements (Cr-Mn-Fe-Co-Ni-Cu-Zn-Se-Mo-I-W, and possibly Li-B-F-Si-V-As). It originated when the element abundance curve of the living matter and of the Universe, coincided. The 2. hypothesis is: Life originated in an interstellar molecular cloud with the critical role of dust particles. The 3. hypothesis arises from the 1. and states: Because of the Universe ageing, life originated only once. The dust forming planetary system and stars already contained an excess of L-type amino acids and D-type sugars, therefore, the emerging life on any planet had to be chiral. Consequently, the 4. hypothesis has been formed: Chirality is a sine qua non-condition for the emergence of life. The arguments supporting these hypotheses are put forward based on numerous astrophysical observations and physics laws.

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V. Valkovic and J. Obhodas
Wed, 25 Nov 2020
9/65

Comments: 23 pages, 8 figures

http://arxiv.org/abs/2011.12226

Quantum chemical cluster calculations show that reactions of C$^+$ with HCN or HNC embedded in the surface of an icy grain mantle can account for the formation of a recently detected molecule, glycolonitrile, which is considered to be an important precursor to ribonucleic compounds. Reactions of cations deposited on ice mantles with minimal kinetic energy have been found theoretically to result in previously unknown pathways to significant organic compounds in protostellar systems and the interstellar medium. In density functional theory cluster calculations involving up to 24H$_2$O, C$^+$ reacts consistently with HCN embedded in ice to yield the neutral HOCHNC radical with no barrier, along with H$_3$O+ as a byproduct. If HOCHNC then reacts with H, three species can be formed: HOCH$_2$CN (isocyanomethanol), HOCH$_2$CN (glycolonitrile), and HOCHNCH. For the C$^+$ + HNC reaction on ice, the HOCHCN and H$_2$OCCN radicals form as intermediates, the first of which is another direct precursor to glycolonitrile via H addition. In addition to characterizing reaction pathways, predictions are provided of the vibrational and electronic spectra of the HCN and HNC starting clusters and the HOCHNC ice-bound intermediate.

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D. Woon
Wed, 25 Nov 2020
49/65

Comments: Accepted for publication in ApJ. 5 figures. Includes supplemental information (cartesian coordinates, energies, ZPE)

http://arxiv.org/abs/2007.15782

The surface processes on interstellar dust grains have an important role in the chemical evolution in molecular clouds. Hydrogenation reactions on ice surfaces have been extensively investigated and are known to proceed at low temperatures mostly below 20 K. In contrast, information about the chemical processes of molecules within an ice mantle is lacking. In this work, we investigated diffusive hydrogenation reactions of carbon monoxide (CO) embedded in amorphous solid water (ASW) as a model case and discovered that the hydrogenation of CO efficiently proceeds to yield H2CO and CH3OH even above 20 K when CO is buried beneath ASW. The experimental results suggest that hydrogen atoms diffuse through the cracks of ASW and have a sufficient residence time to react with embedded CO. The hydrogenation reactions occurred even at temperatures up to ~70 K. Cracks collapse at elevated temperatures but the occurrence of hydrogenation reactions means that the cracks would not completely disappear and remain large enough for penetration by hydrogen atoms. Considering the hydrogen-atom fluence in the laboratory and molecular clouds, we suggest that the penetration of hydrogen and its reactions within the ice mantle occur in astrophysical environments. Unified Astronomy

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M. Tsuge, H. Hidaka, A. Kouchi, et. al.
Mon, 3 Aug 20
-697/58

Comments: N/A

http://arxiv.org/abs/2007.08232

Interstellar complex organic molecules (iCOMs) are assumed to be mainly formed on dust-grain surfaces. However, neutral gas-phase reactions in the interstellar medium (ISM) can play an important role. In this paper, by investigating the reaction between aldehydes and the cyano radical, we show that both formaldehyde (CH$_2$O) and acetaldehyde (CH$_3$CHO) can lead to the formation of formyl cyanide (HCOCN). Owing to accurate quantum-chemical computations followed by rate constant evaluations, we have been able to suggest and validate an effective mechanism for the formation of HCOCN, one of the molecules observed in the ISM. Quite interestingly, the mechanism starting from CH$_2$O is very effective at low temperature, while that involving CH$_3$CHO becomes more efficient at temperatures above 200 K.

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F. Tonolo, J. Lupi, C. Puzzarini, et. al.
Fri, 17 Jul 20
-277/77

Comments: 13 pages, 7 figures, ApJ: accepted for publication

http://arxiv.org/abs/2005.09477

In this work, we present the results of our investigation into the chemistry of Z- and E-cyanomethanimine (HNCHCN), both of which are possible precursors to the nucleobase adenine. Ab initio quantum chemical calculations for a number of reactions with atomic hydrogen were carried out. We find that the reaction H + Z/E-HNCHCN leading both to H-addition as well as H$_2$-abstraction proceed via similar short-range barriers with bimolecular rate coefficients on the order of $\sim10^{-17}$ cm$^{3}$ s$^{-1}$. These results were then incorporated into astrochemical models and used in simulations of the giant molecular cloud G+0.693. The calculated abundances obtained from these models were compared with previous observational data and found to be in good agreement, with a predicted [Z/E] ratio of $\sim3$ – somewhat smaller than the previously derived value of $6.1\pm2.4$. We find that the [Z/E] ratio in our simulations is due mostly to ion-molecule destruction rates driven by the different permanent dipoles of the two conformers. Based on these results, we propose a general rule-of-thumb for estimating the abundances of isomers in interstellar environments.

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C. Shingledecker, G. Molpeceres, V. Rivilla, et. al.
Wed, 20 May 20
36/66

Comments: The Astrophysical Journal, accepted: 17 pages, 9 figures, 7 tables

http://arxiv.org/abs/2002.06101

Formation of interstellar complex organic molecules is currently thought to be dominated by the barrierless coupling between radicals on the interstellar icy grain surfaces. Previous standard DFT results on the reactivity between CH$_3$ and HCO on amorphous water surfaces, showed that formation of CH$_4$ + CO by H transfer from HCO to CH$_3$ assisted by water molecules of the ice was the dominant channel. However, the adopted description of the electronic structure of the biradical (i.e., CH$_3$/HCO) system was inadequate (without the broken-symmetry (BS) approach). In this work, we revisit the original results by means of BS-DFT both in gas phase and with one water molecule simulating the role of the ice. Results indicate that adoption of BS-DFT is mandatory to describe properly biradical systems. In the presence of the single water molecule, the water-assisted H transfer exhibits a high energy barrier. In contrast, CH$_3$CHO formation is found to be barrierless. However, direct H transfer from HCO to CH$_3$ to give CO and CH$_4$ presents a very low energy barrier, hence being a potential competitive channel to the radical coupling and indicating, moreover, that the physical insights ofthe original work remain valid.

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J. Enrique-Romero, S. Álvarez-Barcia, F. Kolb, et. al.
Mon, 17 Feb 20
28/53

Comments: Submitted to MNRAS main journal. For associated supporting material refer to the publication in MNRAS. Accepted 2020 February 14. Received 2020 February 14

http://arxiv.org/abs/1912.11605

The sources behind numerous key diffuse interstellar bands (DIBs) remain elusive, and thus evidence is presented here that a family of seven anionic hydrogen atomic clusters (H(-)2n+1, n = 1 to 7) are pertinent contributors. Configuration interaction calculations show hat these charge-induced dipole clusters are stable at temperatures characteristic of the interstellar medium, and emerge from the most abundant element in the Universe. The clusters’ spectra yield 25 absorption optical lines that align with observed DIBs to within the computational uncertainties. The absorption bands are due to excitations from the ground states of the clusters to metastable states.

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L. Huang, C. Matta, D. Majaess, et. al.
Mon, 30 Dec 19
8/51

Comments: 20 pages in total including 3 figures, 3 tables, and Supporting Information

http://arxiv.org/abs/1912.11605

The sources behind numerous key diffuse interstellar bands (DIBs) remain elusive, and thus evidence is presented here that a family of seven anionic hydrogen atomic clusters (H(-)2n+1, n = 1 to 7) are pertinent contributors. Configuration interaction calculations show hat these charge-induced dipole clusters are stable at temperatures characteristic of the interstellar medium, and emerge from the most abundant element in the Universe. The clusters’ spectra yield 25 absorption optical lines that align with observed DIBs to within the computational uncertainties. The absorption bands are due to excitations from the ground states of the clusters to metastable states.

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L. Huang, C. Matta, D. Majaess, et. al.
Mon, 30 Dec 19
32/51

Comments: 20 pages in total including 3 figures, 3 tables, and Supporting Information

http://arxiv.org/abs/1912.08426

Aims. Due to the limitations of current computational technology, the fragmentation and isomerization products of vibrationally-excited polycyclic aromatic hydrocarbon (PAH) molecules and their derivatives are poorly studied. In this work, we investigate the intermediate products of PAHs and their derivatives as well as the gas-phase reactions relevant to the interstellar medium, with coronene as a case study.
Methods. Based on the semi-empirical method of PM3 as implemented in the CP2K program, molecular dynamics simulations are performed to model the major processes (e.g., vibrations, fragmentations, and isomerizations) of coronene and its derivatives (e.g., methylated coronene, hydrogenated coronene, dehydrogenated coronene, nitrogen-substituted coronene, and oxygen-substituted coronene) at temperatures of 3000 K and 4000 K.
Results. We find that the anharmonic effects are crucial for the simulation of vibrational excitation. For the molecules studied here, H2, CO, HCN, and CH2 are the major fragments. Following the dissociation of these small units, most of the molecules could maintain their ring structures, but a few molecules would break completely into carbon chains. The transformation from hexagon to pentagon or heptagon may occur and the heteroatomic substitutions (e.g., N- or O-substitutions) facilitate the transformation.

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T. Chen, Y. Luo and A. Li
Thu, 19 Dec 19
35/82

Comments: 9 pages, 10 figures; accepted for publication in A&A

http://arxiv.org/abs/1912.03137

Polycyclic aromatic hydrocarbons (PAHs) are key species in astrophysical environments in which vacuum ultraviolet (VUV) photons are present, such as star-forming regions. The interaction with these VUV photons governs the physical and chemical evolution of PAHs. Models show that only large species can survive. However, the actual molecular properties of large PAHs are poorly characterized and the ones included in models are only an extrapolation of the properties of small and medium-sized species. We discuss here experiments performed on trapped ions including some at the SOLEIL VUV beam line DESIRS. We focus on the case of the large dicoronylene cation, C48H20+ , and compare its behavior under VUV processing with that of smaller species. We suggest that C2H2 is not a relevant channel in the fragmentation of large PAHs. Ionization is found to largely dominate fragmentation. In addition, we report evidence for a hydrogen dissociation channel through excited electronic states. Although this channel is minor, it is already effective below 13.6 eV and can significantly influence the stability of astro-PAHs. We emphasize that the competition between ionization and dissociation in large PAHs should be further evaluated for their use in astrophysical models.

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C. Joblin, G. Wenzel, S. Castillo, et. al.
Mon, 9 Dec 19
15/53

Comments: N/A

http://arxiv.org/abs/1911.01239

In this work, we reexamine sulfur chemistry occurring on and in the ice mantles of interstellar dust grains, and report the effects of two new modifications to standard astrochemical models; namely, (a) the incorporation of cosmic ray-driven radiation chemistry and (b) the assumption of fast, non-diffusive reactions for key radicals in the bulk. Results from our models of dense molecular clouds show that these changes can have a profound influence on the abundances of sulfur-bearing species in ice mantles, including a reduction in the abundance of solid-phase H$_2$S and HS, and a significant increase in the abundances of OCS, SO$_2$, as well as pure allotropes of sulfur, especially S$_8$. These pure-sulfur species – though nearly impossible to observe directly – have long been speculated to be potential sulfur reservoirs and our results represent possibly the most accurate estimates yet of their abundances in the dense ISM. Moreover, the results of these updated models are found to be in good agreement with available observational data. Finally, we examine the implications of our findings with regard to the as-yet-unknown sulfur reservoir thought to exist in dense interstellar environments.

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C. Shingledecker, T. Lamberts, J. Laas, et. al.
Tue, 5 Nov 19
23/72

Comments: ApJ, accepted: 37 pages, 8 figures

http://arxiv.org/abs/1909.05718

Photoelectron photoion coincidence spectroscopy has been used to measure the threshold photoelectron spectrum of 1-ethynylpyrene and to obtain the breakdown graph describing the dissociation of the 1-ethynylpyrene cation. The threshold photoelectron measurement has allowed us to improve the determination of the ionization energy of 1-ethynylpyrene at 7.391 $\pm$ 0.005 eV. Concerning the main dissociation channels, the analysis of the breakdown graph has given 3.70 $\pm$ 0.60 eV as the activation energy for the loss of one H atom and 2.98 $\pm$ 1.80 eV for the loss of a second independent H atom. The corresponding entropies of activation are affected by large errors as observed in similar studies of other polycyclic aromatic hydrocarbon cations. Minor dissociation channels were also detected and identified as the loss of the C$_2$H group and the loss of a C$_2$H$_2$ unit and/or that of an H atom plus the C$_2$H group. The activation energies and the entropies of activation of these minor pathways could not be derived from the measurements. It is found that the cation of 1-ethynylpyrene behaves like the cation of pyrene and is consequently more photostable than the cation of 1-methylpyrene. We conclude that photodissociation is not the leading cause of the low abundance, if not the absence, of ethynyl-substituted polycyclic aromatic hydrocarbon species in the interstellar medium.

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G. Rouillé, M. Steglich, P. Hemberger, et. al.
Fri, 13 Sep 19
46/70

Comments: 13 pages, 9 figures, accepted for publication in The Astrophysical Journal

http://arxiv.org/abs/1908.07787

Dust particles play a major role in the formation, evolution and chemistry of interstellar clouds, stars and planetary systems. Commonly identified forms include amorphous and crystalline carbon-rich particles and silicates. Also present in many astrophysical environments are polycyclic aromatic hydrocarbons (PAHs), detected through their infrared emission, and which are essentially small flakes of graphene. Astronomical observations over the past four decades have revealed a widespread unassigned Extended Red Emission (ERE) feature which is attributed to luminescence of dust grains. Numerous potential carriers for ERE have been proposed but none has gained general acceptance. In this Letter it is shown that there is a strong similarity between laboratory optical emission spectra of graphene oxide and ERE, leading to this proposal that emission from graphene oxide nanoparticles is the origin of ERE and that these are a significant component of interstellar dust. The proposal is supported by infrared emission features detected by the Infrared Space Observatory (ISO) and the Spitzer Space Telescope.

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P. Sarre
Thu, 22 Aug 19
8/70

Comments: 5 pages, 3 figures

http://arxiv.org/abs/1908.04356

The work is devoted to the adaptation of the results of laboratory studies of the laser-induced dissociation of molecules of benzene adsorbed on a quartz substrate to the conditions of the interstellar medium. Adsorption was performed under conditions of low temperature and deep vacuum. The difference between the photolysis of adsorbed molecules and molecules in the gas phase is identified. Significance of process of photolytic desorption in the interstellar conditions is analyzed, in particular, in the conditions of photodissociation regions. It is shown that the efficiency and dissociation channels of photolysis of adsorbed and gas phase benzene differ substantially. It is concluded that the photolysis of aromatic hydrocarbons adsorbed on the interstellar dust grains contributes a negligible fraction to the abundance of small hydrocarbons in the interstellar medium.

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M. Murga, V. Varakin, A. Stolyarov, et. al.
Wed, 14 Aug 19
15/60

Comments: The original paper is published in Astronomy Reports. The paper includes 17 pages, 3 figures

http://arxiv.org/abs/1908.00268

A very low ortho-to-para ratio (OPR) of 0.1-0.5 was previously reported in the Orion Bar photon-dominated region (PDR), based on observations of two optically thin $\mathrm{H_2^{18}O}$ lines which were analyzed by using a single-slab large velocity gradient model. The corresponding spin temperature does not coincide with the kinetic temperature of the molecular gas in this UV-illuminated region. This was interpreted as an indication of water molecules being formed on cold icy grains which were subsequently released by UV photodesorption. A more complete set of water observations in the Orion Bar, including seven $\mathrm{H_2^{16}O}$ lines and one $\mathrm{H_2^{18}O}$ line, carried out using Herschel/HIFI instrument, was reanalyzed using the Meudon PDR code to derive gas-phase water abundance and the OPR, taking into account the steep density and temperature gradients present in the region. The model line intensities are in good agreement with the observations assuming that water molecules formed with an OPR corresponding to thermal equilibrium conditions at the local kinetic temperature of the gas and when solely considering gas-phase chemistry and water gas-grain exchanges through adsorption and desorption. Gas-phase water is predicted to arise from a region deep into the cloud, corresponding to a visual extinction of $A_{\mathrm{V}} \sim 9$, with a $\mathrm{H_2^{16}O}$ fractional abundance of $\sim 2\times 10^{-7}$ and column density of $(1.4 \pm 0.8) \times 10^{15}$ cm$^{-2}$ for a total cloud depth of $A_{\mathrm{V}}=15$. A line-of-sight average ortho-to-para ratio of $2.8 \pm 0.1$ is derived. The observational data are consistent with a nuclear spin isomer repartition corresponding to the thermal equilibrium at a temperature of $35 \pm 2$ K, much higher than the spin temperature previously reported for this region and close to the gas kinetic temperature in the water-emitting gas.

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T. Putaud, X. Michaut, F. Petit, et. al.
Fri, 2 Aug 19
20/61

Comments: N/A

http://arxiv.org/abs/1907.09351

Aims. In this work we determine the effects of anharmonicity on the mid-infrared spectra of the linear polycyclic aromatic hydrocarbons (PAHs) naphthalene, anthracene, tetracene and pentacene recorded using the free electron laser FELIX. Methods. Comparison of experimental spectra obtained under supersonic jet conditions with theoretically predicted spectra was used to show if anharmonicity explicitly needs to be taken into account. Results. Anharmonic spectra obtained using second-order vibrational perturbation theory agree on average within 0.5% of the experimental frequencies. Importantly, they confirm the presence of combination bands with appreciable intensity in the 5-6 $\mu$m region. These combination bands contain a significant fraction of the IR absorption, which scales linearly with the size of the PAH. Detection and assignment of the combination bands are a preliminary indication of the accuracy of far-IR modes in our anharmonic theoretical spectra. Detailed analysis of the periphery-sensitive CH out-of-plane band of naphthalene reveals that there is still room for improvement of the VPT2 approach. In addition, the implications of our findings for the analysis of the aromatic infrared bands are discussed.

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A. Lemmens, D. Rap, J. Thunnissen, et. al.
Tue, 23 Jul 19
38/72

Comments: 10 pages, 3 figures and Appendix. Accepted in Astronomy and Astrophysics