On intensity perturbations in vibronically induced radiative transitions

Abstract

Intensity perturbations in forbidden radiative transitions due to cubic anharmonic mixing involving the inducing mode and a totally symmetric progression-forming mode are discussed. It is shown that such mixing will lead to deviations from mirror symmetry of the absorption and fluorescence structure because the destructive-constructive interference pattern which prevails in absorption is different to that in emission. This analysis is applied to intensities in the 2600 Å system of benzene. Moderate cubic coupling energies (ca. 10 cm^-1) have marked effects on intensity distributions even when the interacting levels are 200 cm^-1 apart. The splitting of the progressions 1₀ ⁿ 6₁ ² and 1 _n ⁰6₂ ¹ into two sub-progressions (l=0, 2) is linearly dependent on the extent of vibronic excitation in the a _1g mode v ₁. These effects are amenable to experimental detection.