Resonances: Bridge between Spectroscopy and Dynamics

Abstract

Resonances are metastable, quasibound states of a molecular complex. They are formed predominantly by vibrational excitation of a molecular complex above a dissociation threshold. Resonances share a number of features in common with bound states, including the possibility of making spectroscopic assignments of them. Thus, resonances can be viewed as the bridge between the bound state spectrum, conventionally the domain of spectroscopy, and the continuum, which is the domain of dynamics. I review a variety of methods from a number of articles to calculate and characterize resonances, with a special focus on resonances in HCO, which have been extensively studied both theoretically and experimentally. HCO represents an extreme case, where most resonances are isolated and nonoverlapping. The effect of overall rotation on resonance positions and widths of HCO is examined in detail, and I present tests of several approximate treatments of rotation. I also point out the role that resonances play in the dynamics of unimolecular reactions, radical−radical reactions, and recombination/dissociation reactions, again using HCO as the key example. The use of “reduced dimensionality” ideas to obtain full dimensional reaction probabilities for a resonance-dominated reaction is illustrated for the OH + CO → H + CO2 reaction, with special attention to the role of the “spectator” CO-stretch.