The phase space geometry underlying roaming reaction dynamics

TL;DR

This paper explores the phase space structures underlying the roaming dissociation phenomenon in formaldehyde and CH4+ models, revealing how complex invariant manifolds and heteroclinic intersections influence molecular dynamics.

Contribution

It uncovers the phase space geometry and invariant manifold interactions responsible for roaming, a dissociation pathway not associated with traditional transition states.

Findings

Roaming involves passage through three phase space bottlenecks.

Unstable periodic orbits define these bottlenecks, unrelated to potential energy saddle points.

Heteroclinic intersections are key to understanding roaming dynamics.

Abstract

Recent studies have found an unusual way of dissociation in formaldehyde. It can be characterized by a hydrogen atom that separates from the molecule, but instead of dissociating immediately it roams around the molecule for a considerable amount of time and extracts another hydrogen atom from the molecule prior to dissociation. This phenomenon has been coined roaming and has since been reported in the dissociation of a number of other molecules. In this paper we investigate roaming in Chesnavich's CH$_4^+$ model. During dissociation the free hydrogen must pass through three phase space bottleneck for the classical motion, that can be shown to exist due to unstable periodic orbits. None of these orbits is associated with saddle points of the potential energy surface and hence related to transition states in the usual sense. We explain how the intricate phase space geometry influences the shape and intersections of invariant manifolds that form separatrices, and establish the impact of these phase space structures on residence times and rotation numbers. Ultimately we use this knowledge to attribute the roaming phenomenon to particular heteroclinic intersections.