Intersystem crossing and nonadiabatic product channels in the photodissociation of N{sub 2}O{sub 4} at 193 nm

This paper presents velocity and angular distribution measurements of the products of N{sub 2}O{sub 4} photodissociated at 193 nm. The data show evidence for only N-N bond fission, with no significant branching to N-O bond fission or NO elimination products. The translational energy distribution of the N-N bond fission products is bimodal, indicating that at least two different NO{sub 2} + NO{sub 2} product channels contribute significantly to the observed products. Both product channels have an anisotropy parameter of {beta} = 1.7 {+-} 0.2. Using a Franck-Condon-like sudden analysis, the authors tentatively assign the two fragmentation channels observed as NO{sub 2}({tilde X}{sup 2}A{sub 1}) + NO{sub 2}(1{sup 4}B{sub 2}/1{sup 4}A{sub 2}) and NO{sub 2}({tilde X}{sup 2}A{sub 1}) + NO{sub 2}(2{sup 2}B{sub 2}). To further characterize the system the authors present ab initio calculations (at the level of configuration interaction with single excitations) of the relevant excites states of N{sub 2}O{sub 4}. The data considered together with the calculations suggest a model for the product branching in which there is spin-orbit coupling in the Franck-Condon region between the excited state, which has mixed singlet {pi}{pi}* and n{sigma}* character, and a state with {sup 3}{pi}{sigma}* character. Branching to the NO{sub 2}({tilde X}) + NO{sub 2}(1{sup 4}B{sub 2}/1{sup 4}A{sub 2}) channel occurs upon intersystem crossing to the triplet surface, and formation of the {pi}{pi}* diabatic products NO{sub 2}(X) + NO{sub 2}(2{sup 2}B{sub 2}) occurs from the singlet {pi}{pi}* state nonadiabatic dynamics. Finally, the authors note that the observed parallel photofragment anisotropy, unexpected for {pi}{pi}* electronic excitation of N{sub 2}O{sub 4}, likely results from vibronic coupling with a {sigma}{sigma}* electronic state.