THE INFLUENCE OF LOCAL ELECTRONIC CHARACTER AND NONADIABATICITY IN THE PHOTODISSOCIATION OF NITRIC-ACID AT 193 NM

The dissociation of nitric acid upon pi(nb,O)-->pi(NO2) excitation at 193 nm has been studied in a crossed laser-molecular beam apparatus. The primary reaction channels are OH+NO2 and O+HONO. We measure the br anching ratio between these two competing processes and determine (OHNO2)/(O+HONO)=0.50+/-0.05. Our experiments provide evidence of a minor O+HONO pathway, which we assign to O(P-3) and HONO in its lowest trip let state. The dominant pathway correlates to O(D-1)+HONO(X (1)A'). Th e translational energy distributions reveal two distinct pathways for the OH+NO2 channel. One pathway produces stable NO2 fragments in the 1 B-2(2) electronic state. The second pathway produces unstable NO2 fra gments which undergo secondary dissociation to NO+O. We examine the in fluence of nonadiabaticity along the OH+NO2 reaction coordinate in ord er to explain the significant branching to this other channel. Finally , we introduce a new method for generating correlation diagrams for sy stems with electronic transitions localized on one moiety, in which we restrict the changes allowed in remote molecular orbitals along the r eaction coordinate. Analysis of previously measured X+NO2 photofragmen t pathways in nitromethane and methyl nitrate provides further support for using restricted correlation diagram to predict the adiabatic and nonadiabatic product channels. (C) 1997 American Institute of Physics .