A comparative study of the low energy HD+o-/p-H2 rotational excitation/de-excitation collisions and elastic scattering

The Diep and Johnson (DJ) H2-H2 potential energy surface (PES) obtained from the first principles [P. Diep, K. Johnson, J. Chem. Phys. 113, 3480 (2000); P. Diep, K. Johnson, J. Chem. Phys. 114, 222 (2000)], has been adjusted through appropriate rotation of the three-dimensional coordinate system and applied to low-temperature (T < 300 K) HD+o-/p-H2 collisions of astrophysical interest. A non-reactive quantum mechanical close-coupling method is used to carry out the computation for the total rotational state-to-state cross sections \documentclass[12pt]{minimal}\begin{document}$\sigma _{j_1j_2\rightarrow j^\prime _1j^\prime _2}(\epsilon )$\end{document}σj1j2→j1′j2′(ε) and corresponding thermal rate coefficients \documentclass[12pt]{minimal}\begin{document}$k_{j_1j_2\rightarrow j^\prime _1j^\prime _2}(T)$\end{document}kj1j2→j1′j2′(T). A rather satisfactory agreement has been obtained between our results computed with the modified DJ PES and with the newer H4 PES [A. I. Boothroyd, P. G. Martin, W. J. Keogh, M. J. Peterson, J. Chem. Phys. 116, 666 (2002)], which is also applied in this work. A comparative study with previous results is presented and discussed. Significant differences have been obtained for few specific rotational transitions in the H2/HD molecules between our results and previous calculations. The low temperature data for \documentclass[12pt]{minimal}\begin{document}$k_{j_1j_2\rightarrow j^\prime _1j^\prime _2}(T)$\end{document}kj1j2→j1′j2′(T) calculated in this work can be used in a future application such as a new computation of the HD cooling function of primordial gas, which is important in the astrophysics of the early Universe.