Equations of motion for excitons in molecular aggregates obtained from stochastic Hamiltonian models are known to be applicable only at infinite temperatures. It is argued that the problem lies in the omission of dissipative contributions that must be present if the excitons are to achieve thermal equilibrium. Stochastic equations of motion are constructed that are applicable at finite temperatures. As an application of the model optical line shapes are considered.

• Received 29 April 1983

DOI:https://doi.org/10.1103/PhysRevLett.51.1370