Ultrafast pump-probe spectroscopy of linear molecular aggregates: Effects of exciton coherence and thermal dephasing

We model the pump-probe spectrum of linear molecular aggregates measured by using ultrafast laser pulses. We analyze in particular the effects of coherences between exciton states created by the pump pulse and show that these give rise to a small induced-absorption peak that is red-shifted relative to the main one-exciton bleaching peak. The new coherent peak survives common values of the static disorder. With increasing pump-probe delay time, its amplitude oscillates and decays with a rate that is inversely proportional to the width of the absorption band. We also consider coupling to vibrations and show that with increasing temperature the frequency separation between the main bleaching and induced-absorption peaks increases and provides a measure for the exciton coherence size imposed by scattering on the vibrations.