Diffuse structures and periodic orbits in the photodissociation of small polyatomic molecules

Abstract

The relation of diffuse vibrational structures in UV-absorption spectra of (small) polyatomic molecules and internal vibrational motion in excited electronic states are investigated. The method of choice is the propagation of time-dependent wavepackets with the autocorrelation function serving as the link between the energy dependence of the spectrum and the time dependence of the molecular motion in the excited electronic state. For the purpose of this paper we characterize diffuse structures as very short-lived resonances with ‘lifetimes’ of the order of at most one internal vibrational period. In particular, we study a model system for the photodissociation of symmetric triatomic molecules ABA such as H₂O, CO₂ and O₃, the photodissociation of H₂O in the second continuum, and the fragmentation of H₂S. The existence of unstable periodic orbits and their influence on the dissociation dynamics is especially elucidated. In the case of H₂S we demonstrate that the diffuse absorption structures are caused by symmetric stretch motion in a binding state which is strongly coupled to a dissociative state. Diffuse structures can be regarded as very broad resonances in excited electronic states. They manifest transition-state spectroscopy in the original sense of the word.