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

We use dispersed fluorescence (DF) and stimulated emission pumping (SE P) spectroscopies on the B ($) over tilde (2)A'-X ($) over tilde (2)A' system of jet-cooled HCO to measure the vibrational energies, resonan ce widths, and relative fluorescence intensities of 73 bound and reson ance states supported by the ground-state potential energy surface. Th e SEP experiments use both two-color resonant four-wave mixing (RFWM-S EP) and the more conventional technique in which SEP signals are obtai ned from fluorescence depletion (ED-SEP). Where applicable, RFWM-SEP p rovides superior spectra to those obtained with ED-SEP which is suscep tible to saturation broadening that can prevent accurate determination s 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 vibra tional excitation among the three vibrational modes, including states with 1-12 quanta of excitation in the C-O stretch, 0-5 quanta of bendi ng excitation, and 0-1 quanta of excitation in the C-H stretch. The wi dths are markedly mode-specific and often display striking, nonmonoton ic variations with increasing C-O stretch excitation. We compare our r esults to those of previous DF and SEP studies and to recent dynamical calculations of resonance energies and widths that use realistic pote ntial surfaces derived from ab initio calculations. The resonance widt hs are particularly sensitive gauges of the unimolecular dissociation dynamics and provide stringent tests of theoretical potential surfaces . (C) 1995 American Institute of Physics.