THE VIBRATIONAL PREDISSOCIATION OF CIS-METHYL NITRITE IN THE S1 STATE- A COMPARISON OF EXACT QUANTUM-MECHANICAL WAVE-PACKET CALCULATIONS WITH CLASSICAL TRAJECTORY CALCULATIONS AND DETAILED EXPERIMENTAL RESULTS

We present quantum mechanical wave packet calculations for the vibrati onal predissociation of cis-CH3ONO in the S1 state including three deg rees of freedom-the CH3O-NO dissociation bond, the N=O stretching coor dinate, and the CH3O-N-O bending angle. We calculate the autocorrelati on function, the absorption spectrum, the lifetimes of the excited com plex as a function of the internal excitation, and the final vibration al-rotational state distributions of the NO fragment. The lifetimes an d the product state distributions are compared with experimental data as well as with previous results obtained from classical trajectory ca lculations. The calculated vibrational state distributions of the NO p roduct satisfactorily reproduce the systematic variation with the init ially prepared quasibound state of the CH3ONO(S1) complex found experi mentally; however, they are considerably narrower than the experimenta l distributions. The theoretical rotational state distributions of NO, all being highly inverted and having the overall shape of a Gaussian, agree well with the experimental data; this is the case for several q uasibound vibrational states of CH3ONO(S1) as well as several final vi brational states of the NO product. In general, the classical trajecto ry calculations parallel the quantum mechanical results. The existing differences have to be attributed to the inability of the purely class ical treatment in reproducing subtle quantum effects if the dissociati on proceeds through a relatively long-lived complex. While the calcula tions yield satisfactory agreement with the experimental NO state dist ributions including the envelope of the absorption spectrum, they disa gree with the experiment in that the resonance widths are about one or der of magnitude narrower than in the measured spectrum. Additional ca lculations for which the torsional angle of NO with respect to the int ermolecular dissociation vector R is approximately taken into account as a fourth coordinate reveals that dephasing by out-of-plane motion c an explain most of this discrepancy.