Chapter 3 State-specific dynamics of unimolecular dissociation

Abstract

This chapter discusses the state-specific dynamics of unimolecular dissociation. Unimolecular dissociation is one of the simplest types of irreversible chemical reactions. It takes place in a single isolated molecule with an internal energy that exceeds the first dissociation threshold. The chapter also discusses the concept of discrete metastable states (resonances) in the dissociation continuum. Resonances help to treat the spectroscopic and kinetic aspects of unimolecular dissociation on equal grounds—they are spectroscopically measurable states and, at the same time, the states in which a molecule can be temporally trapped, so that it can be stabilized in collisions with ‘bath’ particles. The chapter elaborates on the resonance formulation of unimolecular decay, discusses the principal aspects of this approach using simple one-dimensional models, and points out possible generalizations to the multidimensional case. It describes experimental approaches for measuring state-resolved dissociation rates, such as promotion to an excited electronic state along with an electronically nonadiabatic transition to the ground state and subsequent dissociation on the ground-state potential energy surface (PES), overtone pumping, and stimulated emission pumping. The practical aspects of computing resonance spectra in polyatomic molecules are also described.