Non‐Linear Optical Susceptibilities of Molecular Crystals in the Region of Vibronic Excitations. II. Totally Symmetric Vibrations

Abstract

The quadratic non-linear optical susceptibility of molecular crystals in the vibronic region (one Frenkel exciton + one totally symmetric phonon) is calculated. A one-dimensional model is used to find an analytic expression for the non-linear susceptibility and numerical calculations of its frequency dependence are made. The two mechanisms of exciton-phonon interaction -frequency shift of the vibration and shift of the equilibrium positions -have: (1) a cummulative action for excitons of positive effective mass (one-particle resonances of the non-linear susceptibility appear) ; (2) a compensating effect for excitons of negative maw (many-particle resonances appear).