Exciton dynamics in disordered molecular aggregates: dispersive dephasing probed by photon echo and Rayleigh scattering

Results are presented of absorption, fluorescence, accumulated photon echo, and resonance Rayleigh scattering experiments on aggregates of a thiacarbocyanine (TC) dye. In the echo experiments the dephasing time constant is found to vary strongly with excitation wave length; a similar effect is observed in resonance Rayleigh scattering. All data can be, interpreted using a one-dimensional Frenkel exciton Hamiltonian with diagonal disorder. In this model the dispersion of the dephasing time constant is due to an energy-dependent exciton-phonon scattering process. The effective frequency-dependent electron-phonon coupling constant is found to be a fractal power of frequency.