UNIMOLECULAR DISSOCIATION DYNAMICS OF HIGHLY VIBRATIONALLY EXCITED DCO((X)OVER-TILDE(2)A') .1. INVESTIGATION OF DISSOCIATIVE RESONANCE STATES BY STIMULATED-EMISSION PUMPING SPECTROSCOPY

The vibrational level structure and unimolecular dissociation dynamics of highly vibrationally excited (X) over tilde(2)A' DCO were investig ated using the method of stimulated emission pumping spectroscopy (SEP ). Single vibration-rotation states were probed with excitation energi es up to E((X) over tilde)=18 200 cm(-1), approximate to 12 700 cm(-1) above the asymptotic D-CO dissociation limit. The vibrational level s tructure of the molecule was found to be determined by distinctive pol yads arising from a 1:1:2 resonance between the CD stretching, CO stre tching, and DCO bending vibrations. Anharmonic coupling mechanisms giv e rise to considerable level mixings, especially regarding the CD and CO stretching motion. Thus, only a minority of vibrational states can be unambiguously assigned. The spectral line shape profiles of approxi mate to 100 highly excited ''resonance states'' in the continuum above the D-CO dissociation limit were measured at high resolution. The pro files are homogeneously broadened. The unimolecular decay rates, obtai ned from the observed line widths, were observed to fluctuate by more than two orders of magnitude in a strikingly state specific manner. Th e decay rates on average increase with increasing vibrational excitati on energy. The state resolved experimental data are compared to predic tions of the microcanonical specific unimolecular rate coefficients ca lculated from different statistical models. Serious problems were enco untered considering the calculation of the density of states of the mo lecules in the continuum region of the potential energy surface regard ing the contribution of the disappearing oscillator. Despite tentative corrections, the calculated rate coefficients were to too high by one to two orders of magnitude. Overall, the unimolecular dynamics of DCO appears to conform to an intermediate case between the strictly vibra tionally ''mode specific'' and the ''statistical'' limits. (C) 1997 Am erican Institute of Physics.