The Line Shape of Optical and Electron Spin Resonance Absorption under the Influence of Exciton Motion

Abstract

The line shape of optical and electron spin resonance absorption under the influence of the coupled coherent and incoherent exciton motion is calculated analytically. The exciton motion is described by a stochastic Hamiltonian comprising both its coherent and its incoherent part. The coherent motion is represented by the transition matrix element and the incoherent one by the strengths of local and non-local fluctuations. Starting from this Hamiltonian, equations of motion of two-time correlation functions for two arbitrary operators are derived. Using these equations, the line shape of optical absorption of crystals containing two inequivalent molecules per unit cell is calculated. Furtheron an exact analytical expression for the ESR line shape of two differently oriented molecules is derived and discussed in detail as a function of the parameters of the model. From this exact expression for the line shape approximations for several limiting cases are obtained. A comparison between the exact and the approximate expressions is given.