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


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

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J. Babb and B. McLaughlin
Tue, 28 Feb 17

Comments: 7 pages

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


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

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

Comments: 10 pages, 10 figures

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


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

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

Comments: 6 pages, 2 Figures

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


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

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

Comments: MNRAS in press

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


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

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

Comments: N/A

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


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

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D. Vieira and R. Krems
Tue, 10 Jan 17

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

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


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

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

Comments: Accepted by A&A