Optical line shapes of one-dimensional excitions: influence of a heat bath with coloured noise

Optical absorption line shapes for Frenkel excitons moving on a linear chain are calculated. The model takes into account coherent nearest neighbour interaction and the influence of the phonons by a random modulation of the local excitation energies. The energy modulations are described by a stochastic process with coloured noise and exponentially decaying correlation functions. With increasing rate of the energy fluctuations the model describes a transition from static to dynamic disorder. The calculated optical absorption lines are asymmetric for slowly fluctuating energies and become symmetric and Lorentzian in shape for fast energy modulations