Nonlinear resonance and stochasticity in intramolecular energy exchange

The dynamics of intramolecular vibrational energy transfer is studied, with particular reference to its effect on the unimolecular dissociation rates of isolated molecules. The molecule is represented as a set of classical, coupled, nonlinear (anharmonic) oscillators which transfer energy through resonant interactions. We suggest that isolated nonlinear resonances (which predominate at low energies) lead to trapping of the vibrational energy of the system, hence to slow vibrational relaxation, while resonance overlap at higher energies leads to rapid energy exchange and the random lifetime distribution assumed by RRKM theory. Results are presented for some simple model systems, and the approach is compared with other recent theoretical descriptions of vibrational relaxation in isolated molecules.