Vibronic spectra of crystals at arbitrary shifts of the atomic equilibrium positions in the molecules

The vibronic spectra of molecular crystals are investigated for the case of a combination of the electronic excitation with the totally symmetrical intramolecular vibration at arbitrary shifts ξ of the atomic equilibrium positions in molecules. It is shown that three types of excitations can correspond to the vibronic excitations of molecules in the crystal: a) the vibronic excitons, i.e. the one‐particle excitations with definite value of the wave vector; b) the two‐particle excitations corresponding to a simultaneous formation of free excitons and intramolecular vibrations in the crystal; c) the unstable one‐particle excitations decaying into free excitons and intramolecular vibrations. The relative disposition of the corresponding absorption bands and their intensities depend on the three parameters ξ2, ΔΩ and L.