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

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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); 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 σⱼ₁ⱼ₂→ ⱼ'₁ⱼ'₂(ε) and corresponding thermal rate coefficients kⱼ₁ⱼ₂→ ⱼ'₁ⱼ'₂(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 kⱼ₁ⱼ₂→ ⱼ'₁ⱼ'₂(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.