Rainbow scattering in rotationally inelastic collisions of HCl and H$_2$

TL;DR

This study investigates rainbow scattering phenomena in rotationally inelastic collisions between HCl and H$_2$, using quantum and quasi-classical methods on an accurate potential energy surface, revealing distinct dynamical regimes and the limitations of classical approaches.

Contribution

It provides the first detailed analysis of rainbow scattering in HCl-H$_2$ collisions, highlighting the coexistence of different dynamical regimes and assessing classical methods' accuracy.

Findings

Rainbow scattering signatures are identified in cross sections.

Distinct dynamical regimes depend on collision energy and rotational state.

Quasi-classical methods approximate overall behavior but have limitations in capturing rainbow features.

Abstract

We examine rotational transitions of HCl in collisions with H$_2$ by carrying out quantum mechanical close-coupling and quasi-classical trajectory calculations on a recently developed globally accurate full-dimensional ab initio potential energy surface for the H$_3$Cl system. Signatures of rainbow scattering in rotationally inelastic collisions are found in the state resolved integral and differential cross sections as functions of the impact parameter (initial orbital angular momentum) and final rotational quantum number. We show the coexistence of distinct dynamical regimes for the HCl rotational transition driven by the short-range repulsive and long-range attractive forces whose relative importance depends on the collision energy and final rotational state suggesting that classification of rainbow scattering into rotational and $l$-type rainbows is effective for H$_2$+HCl collisions. While the quasi-classical trajectory method satisfactorily predicts the overall behavior of the rotationally inelastic cross sections, its capability to accurately describe signatures of rainbow scattering appears to be limited for the present system.