Exchange narrowing in dynamically disordered molecular aggregates

We study the absorption lineshape for one-dimensional molecular aggregates with colored dynamic disorder. We show that exchange narrowing of the absorption spectrum, which is well-known in the limit of static disorder, also occurs for fast fluctuations, provided the fluctuation rate lambda is not large compared to the intermolecular excitation transfer interaction J. For small aggregates, the narrowing factor is found to equal the number of molecules in the aggregate, while for large aggregates it saturates at root 8J/lambda(lambda/J << 4). These results are derived for dichotomic noise, but we argue that they also hold in the fast-fluctuation limit of other Markov processes. (C) 1998 Elsevier Science B.V.