THE UNIMOLECULAR DISSOCIATION OF HCO - A SPECTROSCOPIC STUDY OF RESONANCE ENERGIES AND WIDTHS

We use dispersed fluorescence (DF) and stimulated emission pumping (SEP) spectroscopies on the B ($) over tilde (2)A'-X ($) over tilde (2)A' system of jet-cooled HCO to measure the vibrational energies, resonance widths, and relative fluorescence intensities of 73 bound and resonance states supported by the ground-state potential energy surface. The SEP experiments use both two-color resonant four-wave mixing (RFWM-SEP) and the more conventional technique in which SEP signals are obtained from fluorescence depletion (ED-SEP). Where applicable, RFWM-SEP provides superior spectra to those obtained with ED-SEP which is susceptible to saturation broadening that can prevent accurate determinations of resonance widths. The observed bound and resonance states span an energy range of 2000-21 000 cm(-1) and comprise a wide range of vibrational excitation among the three vibrational modes, including states with 1-12 quanta of excitation in the C-O stretch, 0-5 quanta of bending excitation, and 0-1 quanta of excitation in the C-H stretch. The widths are markedly mode-specific and often display striking, nonmonotonic variations with increasing C-O stretch excitation. We compare our results to those of previous DF and SEP studies and to recent dynamical calculations of resonance energies and widths that use realistic potential surfaces derived from ab initio calculations. The resonance widths are particularly sensitive gauges of the unimolecular dissociation dynamics and provide stringent tests of theoretical potential surfaces. (C) 1995 American Institute of Physics.