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An interaction of the Frenkel exciton with the molecular vibration has been studied using sub-5-fs multichannel real-time spectroscopy. The coherent molecular vibration with a 135 ± 4 fs period is observed in the time-frequency two-dimensional difference absorption spectrum. An analysis of the phase and amplitude of the oscillation has revealed that both of the negative (bleaching and photoinduced emission) and positive (photoinduced absorption) signals are modulated synchronously. The experimental result is well explained in terms of a transition dipole moment modulated by a dynamic intensity borrowing from an intense B transition to a weak Q transition through the vibronic interaction. The wave packet observed in the present study can be classified as a Herzberg−Teller type in contrast with the frequently studied conventional Franck−Condon type.
Kano et al. (Tue,) studied this question.