The fluorescence spectrum of NO2: rotationally-forbidden bands and their interpretation

Fluorescence of NO2 excited near 5000 A at low gas pressure is predominantly 'parallel' in type, i.e., the values of the quantum number K in the initial state of the excitation, the intermediate (excited) state, and the final state(s)of the emission are all equal, Ki = K′ = Kf. However, a considerable number of the weaker fluorescence bands do not conform to this pattern; instead, they correspond to even-parity differences between the initial and final value of K, |Kf − Ki| = 2, 4, or 6, indicating that K (though not N) is a poor quantum number in the upper electronic state of the excitation–emission sequence. The observations are analyzed in terms of a mechanism in which the vibronic coupling between the 2B2 excited state and high vibrational levels of the ground state creates conditions where the asymmetry of the 2B2 basis state produces unexpectedly large couplings between hybrid states of the same parity in K.