Can the exciton delocalization length in molecular aggregates be determined by pump-probe spectroscopy?

Using numerical simulations, we study the possibility to determine the exci ton delocalization length in disordered molecular aggregates from the detun ing between the bleaching and induced absorption peaks in the pump-probe sp ectrum. We find that the thus determined delocalization length scales linea rly with the "real" delocalization length obtained from the participation r atio, as long as the static disorder dominates over homogeneous broadening of the exciton levels. However, if the delocalization length surpasses a sa turation length, imposed by the homogeneous line width, the pump-probe spec trum no longer gives information on the delocalization length. The latter s eems to be the case for the B850 ring in the bacterial LH2 system. (C) 1999 Elsevier Science B.V. All rights reserved.