On the role of dynamical barriers in barrierless-reactions at low energies: S($^1$D) + H$_2$

TL;DR

This study investigates how different potential energy surfaces influence the low-energy reactivity of S($^1$D) with H$_2$, revealing the impact of dynamical barriers and topography features on reaction probabilities.

Contribution

It compares two ab initio PESs using quantum and classical methods, highlighting how PES topography affects low-energy reaction dynamics and cross-sections.

Findings

Reactivity varies significantly between the two PESs.

Dynamical barriers on the DMBE/CBS PES limit reactivity.

Resonance features are observed on the RKHS PES.

Abstract

Reaction probabilities as a function of total angular momentum and the resulting reaction cross-sections for the collision of open shell S($^1$D) atoms with para-hydrogen have been calculated in the kinetic energy range 0.09--10 meV (1--120 K). The quantum mechanical (QM) hyperspherical reactive scattering method and quasi--classical trajectory (QCT) and statistical quasiclassical trajectory (SQCT) approaches were used. Two different ab initio potential energy surfaces (PESs) have been considered. The widely used RKHS PES by Ho et al. (J. Chem. Phys. 116, 4124, 2002) and the recently published DMBE/CBS PES by Song and Varandas (J. Chem. Phys. 130, 134317, 2009). The calculations at low collision energies reveal very different dynamical behaviors on the two PESs. The reactivity on the RKHS PES is found to be considerably larger than that on the DMBE/CBS PES. The observed differences have their origin in two major distinct topography features. Although both PESs are essentially barrierless for equilibrium H--H distances, when the H--H bond is compressed the DMBE/CBS PES gives rise to a dynamical barrier which limits the reactivity of the system. This barrier is completely absent in the RHKS PES. In addition, the latter PES exhibits a van der Walls well in the entrance channel which reduces the height of the centrifugal barrier and is able to support resonances. As a result a significant larger cross section is found on this PES, with marked oscillations attributable to shape resonances and/or to the opening of partial wave contributions.