Unimolecular dissociation: A state-specific quantum mechanical perspective

The dynamics of unimolecular dissociation of small polyatomic molecules is discussed in the light of exact quantum mechanical calculations. The central quantities are vibrational resonances, that is quasi-bound states embedded in the continuum. The main concepts are illustrated with two prototypes, HCO and HNO, both in their ground electronic states. While HCO is a system with an extremely low density of states, a mostly regular internal vibrational dynamics and a 'tight' transition state, HNO has a higher density of states, behaves more chaotically and has a 'loose' transition state. In both cases the resonance widths and hence the dissociation rates strongly fluctuate, as functions of energy, about the average. In addition to the rates, the final state distributions of the products and their relationship to the dynamics beyond the transition state are also discussed in detail. These systems are well suited to elucidate the contrast between state- and mode-specificity in molecular breakup. An essential aspect is the test of approximations like classical mechanics and statistical models.