On the Coupling of Vibronic States in NO2

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Abstract
A model of indirect near resonance coupling of two vibronic states belonging to different excited Born‐Oppenheimer electronic states of a molecular system is presented. In the absence of direct coupling terms the two states can interfer via a third “messenger” state or via a manifold of such states. If the coupling is not too strong the messenger states act only in a seeming catalytic capacity, i. e. the two originally not coupled vibronic states seem to be coupled directly. The 5935Å laser‐excited fluorescence of NO2 is interpreted in terms of indirectly coupled 2B1‐ and 2B2‐vibronic states via highly excited 2A1 vibrational states. The Franck‐Condon factors are calculated within the rigid bender model of Hougen, Bunker, Johns, and Stone. Best agreement with experiment is obtained if one assumes that the fluorescent state is a linear combination of 2B1 (ν = 4) and 2B2 (ν = 7) vibronic states. The spectra calculated with a pure 2B1 or 2B2 initial vibronic state disagree with experimental material.