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

Using numerical simulations, we study the possibility to determine the exciton delocalization length in disordered molecular aggregates from the detuning between the bleaching and induced absorption peaks in the pump-probe spectrum. We find that the thus determined delocalization length scales linearly with the "real" delocalization length obtained from the participation ratio, as long as the static disorder dominates over homogeneous broadening of the exciton levels. However, if the delocalization length surpasses a saturation length, imposed by the homogeneous line width, the pump-probe spectrum no longer gives information on the delocalization length. The latter seems to be the case for the B850 ring in the bacterial LH2 system. (C) 1999 Elsevier Science B.V. All rights reserved.