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1. Fine-structure transitions of Si and S induced by collisions with atomic hydrogen

   https://doi.org/10.1093/mnras/stad1050  

   Yan, Pei-Gen; Babb, James F (April 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Using a quantum-mechanical close-coupling method, we calculate cross-sections for fine-structure excitation and relaxation of Si and S atoms in collisions with atomic hydrogen. Rate coefficients are calculated over a range of temperatures for astrophysical applications. We determine the temperature-dependent critical densities for the relaxation of Si and S in collisions with H and compare these to the critical densities for collisions with electrons. The present calculations should be useful in modelling environments exhibiting the [S i] 25 μm and [S i] 57 μm far-infrared emission lines or where cooling of S and Si by collisions with H is of interest. 

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2. Fine-structure Transitions of Ne ⁺ , Ar ⁺ , Ne ²⁺ , and Ar ²⁺ Induced by Collisions with Atomic Hydrogen

   https://doi.org/10.3847/1538-4357/ad0e77  

   Yan, Pei-Gen; Babb, James F (January 2024, The Astrophysical Journal)

   Abstract We calculate cross sections for fine-structure transitions of Ne⁺, Ar⁺, Ne²⁺, and Ar²⁺in collisions with atomic hydrogen by using quantum-mechanical methods. Relaxation rate coefficients are calculated for temperatures up to 10,000 K. The temperature-dependent critical densities for the relaxation of Ne⁺, Ar⁺, Ne²⁺, and Ar²⁺in collisions with H have been determined and compared to the critical densities for collisions with electrons. The present calculations will be useful for studies utilizing the infrared lines [Neii] 12.8, [Neiii] 15.6, [Neiii] 36.0, [Arii] 6.99, [Ariii] 8.99, and [Ariii] 21.8μm as diagnostics of, for example, planetary nebulae and star formation. 

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3. Fine-structure transitions of the carbon isoelectronic sequence C, N+, and O2 + induced by collisions with atomic hydrogen

   https://doi.org/10.1093/mnras/stac3471  

   Yan, Pei-Gen; Babb, James F (December 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Fine-structure transitions can be involved in various processes including photon absorption, charge transfer, and inelastic collisions between ions, electrons, and neutral atoms. We present fine-structure excitation and relaxation cross-sections for the collisions of the first few members of the carbon isoelectronic sequence (C, N+ and O2 +) with atomic hydrogen calculated using quantum-mechanical methods. For C, the scattering theory and computational approach is verified by comparison with previous calculations. The rate coefficients for the collisional processes are obtained. For N+ and O2 +, the transitions correspond to the lines [O iii] 52 μm, [O iii] 88 μm, [N ii] 122 μm, and [N ii] 205 μm, observed in the far-infrared in the local universe and more recently in high-redshift galaxies using radio interferometry. The influence of different potentials on the cross-sections and rate coefficients are demonstrated. 

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4. Branching Ratio for O + H ₃ ⁺ Forming OH ⁺ + H ₂ and H ₂ O ⁺ + H

   https://doi.org/10.3847/1538-4357/ac41ce  

   Hillenbrand, Pierre-Michel; Ruette, Nathalie de; Urbain, Xavier; Savin, Daniel W. (March 2022, The Astrophysical Journal)

   Abstract The gas-phase reaction of O + H 3 + has two exothermic product channels: OH + + H 2 and H 2 O + + H. In the present study, we analyze experimental data from a merged-beams measurement to derive thermal rate coefficients resolved by product channel for the temperature range from 10 to 1000 K. Published astrochemical models either ignore the second product channel or apply a temperature-independent branching ratio of 70% versus 30% for the formation of OH + + H 2 versus H 2 O + + H, respectively, which originates from a single experimental data point measured at 295 K. Our results are consistent with this data point, but show a branching ratio that varies with temperature reaching 58% versus 42% at 10 K. We provide recommended rate coefficients for the two product channels for two cases, one where the initial fine-structure population of the O( 3 P J ) reactant is in its J = 2 ground state and the other one where it is in thermal equilibrium. 

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5. Rate coefficients for rotational state-to-state transitions in H ₂ O + H ₂ O collisions for cometary and planetary applications, as predicted by mixed quantum-classical theory

   https://doi.org/10.1051/0004-6361/202245699  

   Mandal, Bikramaditya; Babikov, Dmitri (March 2023, Astronomy & Astrophysics)

   Aims. We present new calculations of collision cross sections for state-to-state transitions between the rotational states in an H 2 O + H 2 O system, which are used to generate a new database of collisional rate coefficients for cometary and planetary applications. Methods. Calculations were carried out using a mixed quantum-classical theory approach that is implemented in the code MQCT. The large basis set of rotational states used in these calculations permits us to predict thermally averaged cross sections for 441 transitions in para- and ortho-H 2 O in a broad range of temperatures. Results. It is found that all state-to-state transitions in the H 2 O + H 2 O system split into two well-defined groups, one with higher cross-section values and lower energy transfer, which corresponds to the dipole-dipole driven processes. The other group has smaller cross sections and higher energy transfer, driven by higher-order interaction terms. We present a detailed analysis of the theoretical error bars, and we symmetrized the state-to-state transition matrixes to ensure that excitation and quenching processes for each transition satisfy the principle of microscopic reversibility. We also compare our results with other data available from the literature for H 2 O + H 2 O collisions. 

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