Nonlinear optical line shapes of disordered molecular aggregates: Motional narrowing and the effect of intersite correlations

We theoretically investigate nonlinear optical line shapes of linear molecular aggregates with Gaussian disorder in the molecular transition frequencies. A perturbative treatment in the disorder is used, within which the joint stochastic distribution function of the frequencies of all multiexciton states of an aggregate can be determined analytically. It is shown that motional narrowing, which is characteristic for the linear absorption spectra of aggregates, also occurs for nonlinear line shapes. An important aspect of our disorder model is that it allows for general correlations between the transition frequencies of molecules within one aggregate, thereby interpolating between continuous energy disorder and a segment or kink model. The general theory is applicable for nonlinearities of any order. Specific applications are discussed for linear absorption, nonlinear absorption, and two-color pump-probe spectra. Our theory suggests that pump-probe experiments provide a novel and very promising approach to obtain microscopic information on aggregate systems; in particular, this technique can be used to determine both the magnitude of the molecular disorder and its degree of intersite correlation within aggregates