COLLECTIVE COORDINATES FOR NUCLEAR SPECTRAL DENSITIES IN ENERGY-TRANSFER AND FEMTOSECOND SPECTROSCOPY OF MOLECULAR AGGREGATES

A theory for Frenkel exciton dynamics in molecular aggregates which in corporates coupling to vibrational motions (intramolecular, intermolec ular and solvent) with multiple spectral densities of arbitrary nature and interpolates between the coherent and the incoherent limits is de veloped. A rigorous procedure for identifying the relevant collective nuclear coordinates necessary to represent a given set of spectral den sities is obtained. Additional coordinates are required as the tempera ture is lowered. Exciton dynamics is calculated by following the evolu tion of wavepackets representing the electronic density matrix in the collective coordinates phase space. The signatures of excitonic and nu clear motions in ultrafast fluorescence spectroscopy are explored usin g a hierarchy of reduction schemes with varying numbers of collective coordinates. (C) 1996 American Institute of Physics.