Molecular vibrational energy flow and dilution factors in an anharmonic state space.

A fourth-order resonance Hamiltonian is derived from the experimental normal-mode Hamiltonian of SCCl2. The anharmonic vibrational state space constructed from the effective Hamiltonian provides a realistic model for vibrational energy flow from bright states accessible by pulsed laser excitation. We study the experimentally derived distribution PE(sigma) of dilution factors sigma as a function of energy. This distribution characterizes the dynamics in the long-time limit. State space models predict that PE(sigma) should be bimodal, with some states undergoing facile intramolecular vibrational energy redistribution (small sigma), while others at the same total energy remain "protected" (sigma approximately 1). The bimodal distribution is in qualitative agreement with analytical and numerical local density of states models. However, there are fewer states protected from energy flow, and the protected states begin to fragment at higher energy, shifting from sigma approximately 1 to sigma approximately 0.5. We also examine how dilution factors are distributed in the vibrational state space of SCCl2 and how the power law specifying the survival probability of harmonic initial states correlates with the dilution factor distribution of anharmonic initial states.