Intersystem Crossing and Nonadiabatic Product Channels in the Photodissociation of N2O4 at 193 nm

This paper presents velocity and angular distribution measurements of the products of N 2 O 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 2 + NO 2 product channels contribute significantly to the observed products. Both product channels have an anisotropy parameter of β = 1.7 ± 0.2. Using a Franck−Condon-like sudden analysis, we tentatively assign the two fragmentation channels observed as NO 2 (X̃ 2 A 1 ) + NO 2 (1 4 B 2 /1 4 A 2 ) and NO 2 (X̃ 2 A 1 ) + NO 2 (2 2 B 2 ). To further characterize the system we present ab initio calculations (at the level of configuration interaction with single excitations) of the relevant excited states of N 2 O 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 ππ* and nσ* character, and a state with 3 πσ* character. Branching to the NO 2 (X̃) + NO 2 (1 4 B 2 /1 4 A 2 ) channel occurs upon intersystem crossing to the triplet surface, and formation of the ππ* diabatic products NO 2 (X̃) + NO 2 (2 2 B 2 ) occurs from the singlet ππ* state nonadiabatic dynamics. Finally, we note that the observed parallel photofragment anisotropy, unexpected for ππ* electronic excitation of N 2 O 4, likely results from vibronic coupling with a σσ* electronic state.