Rotation–vibration state resolved unimolecular dynamics of highly excited CH3O (X̃ 2E). II. Intramolecular vibrational dynamics of excited ‘‘C–O stretch’’ states

Abstract

The stimulated emission pumping (SEP) spectra of highly excited CH3O (X̃ 2E) reported in the preceding paper [A. Geers, J. Kappert, F. Temps, and J. Wiebrecht, J. Chem. Phys. 101, 3618 (1994); paper I] are analyzed to extract quantitative data on the collision-free intramolecular vibrational dynamics of the molecules. Attention is focused on the spectra and dynamics of the excited C–O stretch vibration (ν3) states at energies between 3 000 cm−1≤E≤10 000 cm−1. The spectra are found to exhibit a rather sudden transition at E≊5000 cm−1 from the regular, assignable properties at low energies to the ‘‘statistical’’ region at high energies. This IVR ‘‘threshold’’ is approximately 2000 cm−1 below the asymptotic H–H2CO dissociation limit of the molecule. The observed densities of states at these energies approach the full J- and symmetry-sorted rovibronic densities estimated from harmonic state counts. The nearest-neighbor level spacing distribution in the spectra and the magnitude of the inferred root mean square level coupling matrix elements support the assumption of nearly ‘‘ergodic’’ molecular properties in this region. From computed time autocorrelation functions, the IVR lifetimes of the highly excited C–O stretch states are found to be of the order of τ≊0.2–0.3 ps (v3=5 to 6). Evidence is obtained from some spectra for two-tier IVR processes with characteristic time scales of 0.2 and 2 ps, respectively. The highest excited C–O stretch states at energies above ≊8000 cm−1 (v3≥8) appear to be coupled only weakly to neighboring background states. This last observation hints at the existence of quasistable periodic orbits, which are embedded in the quasicontinuum of dissociative states at these energies.