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 branching ratio between these two competing processes and determine (OH+NO2)/(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 triplet state. The dominant pathway correlates to O(D-1)+HONO(X (1)A'). The 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 fragments which undergo secondary dissociation to NO+O. We examine the influence of nonadiabaticity along the OH+NO2 reaction coordinate in order to explain the significant branching to this other channel. Finally, we introduce a new method for generating correlation diagrams for systems with electronic transitions localized on one moiety, in which we restrict the changes allowed in remote molecular orbitals along the reaction coordinate. Analysis of previously measured X+NO2 photofragment 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.