Ultracold collisions between two light indistinguishable diatomic molecules: elastic and rotational energy transfer in HD+HD

TL;DR

This study performs quantum-mechanical calculations of ultracold HD+HD collisions, revealing detailed rotational energy transfer processes and a significant isotope effect at extremely low temperatures.

Contribution

It provides the first detailed quantum-mechanical analysis of rotational energy transfer in HD+HD collisions at ultracold temperatures using a global potential energy surface.

Findings

Pronounced isotope effect observed in collision cross sections.

Computed state-resolved cross sections and thermal rate coefficients at ultracold temperatures.

Comparison with H2+H2 collisions highlights differences in rotational transfer.

Abstract

A close coupling quantum-mechanical calculation is performed for rotational energy transfer in a HD+HD collision at very low energy, down to the ultracold temperatures: $T \sim 10^{-8}$ K. A global six-dimensional H$_2$-H$_2$ potential energy surface is adopted from a previous work [Boothroyd {\it et al.}, J. Chem. Phys., {\bf 116}, 666 (2002).] State-resolved integral cross sections $\sigma_{ij\rightarrow i'j'}(\varepsilon_{kin})$ of different quantum-mechanical rotational transitions $ij\rightarrow i'j'$ in the HD molecules and corresponding state-resolved thermal rate coefficients $k_{ij\rightarrow i'j'}(T)$ have been computed. Additionally, for comparison, H$_2$+H$_2$ calculations for a few selected rotational transitions have also been performed. The hydrogen and deuterated hydrogen molecules are treated as rigid rotors in this work. A pronounced isotope effect is identified in the cross sections of these collisions at low and ultracold temperatures.