Pump-probe spectroscopy and the exciton delocalization length in molecularaggregates

Using numerical simulations, we investigate to what extent the pump-probe s pectrum can be used as a tool to determine the exciton delocalization lengt h in disordered molecular (J) aggregates. We compare the delocalization len gth obtained through heuristic arguments from the spectral separation betwe en the bleaching and the one- to two-exciton induced absorption features in this spectrum to the delocalization length obtained from the participation ratio at the J band center. We find that up to a certain saturation length these two delocalization lengths are indeed proportional. In the case of l ong-pulse two-color pump-probe spectra, the slope of this linear scaling is insensitive to the pump frequency; it is sensitive to the width of the pro be pulse. Beyond the saturation length, which is determined by the homogene ous line width of the exciton transitions, both the two-color and the short -pulse pump-probe spectra saturate and no longer can be used as a measure f or the delocalization length. The latter applies to the B850 ring of bacter ial antenna complexes.