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

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Abstract

This paper presents velocity and angular distribution measurements of the products of N2O4 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 NO2 + NO2 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 NO2(X̃2A1) + NO2(14B2/14A2) and NO2(X̃2A1) + NO2(22B2). 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 N2O4. 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 3πσ* character. Branching to the NO2(X̃) + NO2(14B2/14A2) channel occurs upon intersystem crossing to the triplet surface, and formation of the ππ* diabatic products NO2(X̃) + NO2(22B2) occurs from the singlet ππ* state nonadiabatic dynamics. Finally, we note that the observed parallel photofragment anisotropy, unexpected for ππ* electronic excitation of N2O4, likely results from vibronic coupling with a σσ* electronic state.

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