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

TL;DR

This study uses quantum mechanical calculations with modified potential energy surfaces to analyze low-temperature HD+o-/p-H2 collisions, providing data crucial for astrophysical models of primordial gas cooling.

Contribution

It offers new quantum collision data for HD+H2 systems using a modified DJ potential, improving accuracy over previous models for astrophysical applications.

Findings

Good agreement with recent PES models

Significant differences in specific rotational transition results

Data applicable for primordial gas cooling calculations

Abstract

The Diep and Johnson (DJ) H$_2$-H$_2$ 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+$o$-/$p$-H$_2$ 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 $\sigma_{j_1j_2\rightarrow j'_1j'_2}(\epsilon)$ and corresponding thermal rate coefficients $k_{j_1j_2\rightarrow j'_1j'_2}(T)$. A rather satisfactory agreement has been obtained between our results computed with the modified DJ PES and with the newer H$_4$ 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 H$_2$/HD molecules between our results and previous calculations. The low temperature data for $k_{j_1j_2\rightarrow j'_1j'_2}(T)$ 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.