Herzberg‐Teller Interaction and Vibronic Coupling in Molecular Crystals IV. Interaction of Two Exciton States via a Totally Symmetric Vibration

Abstract

AbstractThe three‐level model of a molecular crystal with two exciton bands coupled by a totally symmetric vibration according to the Herzberg‐Teller interaction mechanism is discussed. The absorption spectrum to a lower, narrow exciton band is investigated. The formula for distribution of intensity among different vibronic lines is obtained and discussed. It is shown that this distribution depends not only on the Herzberg‐Teller coupling constant, the distortion parameter in the lower band, and the ratio of the intensities of the two coupled bands, but it is also very sensitive to the difference between distortion parameters in the two bands, as well as to the angle between the transition dipole moments in the molecule. The one‐phonon region of the spectrum is discussed with respect to the existence of vibronic exciton and dissociated exciton—phonon states. General conditions for the existence of different numbers of vibronic excitons in this region of a spectrum are obtained. The changes introduced by the Herzberg‐Teller coupling in the one‐phonon region of the spectrum are illustrated by an example. The general formulas obtained reduce for zero distortion parameters to the formulas describing the coupling of two exciton states by a non‐totally symmetric coordinate, and for zero Herzberg‐Teller coupling constant they reduce to the formulas describing the simple vibronic coupling in a molecular crystal.