Pronounced Non-Condon Effect as the Origin of the Quantum Beat Observed in the Time-Resolved Absorption Signal from Excited-State cis-Stilbene

We carried out wavelength-dispersed time-resolved absorption measurements of cis-stilbene to investigate the mechanism of the appearance of the ∼220 cm-1 oscillation, which has been assigned to the nuclear wavepacket motion of the S1 state. The observed oscillatory pattern showed almost same amplitude and phase across the absorption peak at 645 nm, which indicates that the modulation of the transition intensity gives rise to the quantum beat. We also carried out a semiquantitative numerical simulation of the time-resolved absorption spectra based on the effective linear response theory, in which we newly incorporated the Herzberg−Teller coupling model by introducing a coordinate-dependence of the transition moment. The results of these experiments and simulation clearly showed that the intensity of the quantum beat arises from a significant coordinate dependence of the S n ←S1 transition moment, i.e., non-Condon effect. It was concluded that the vibronic coupling of the S n state with other electronic states is so large that the Herzberg−Teller coupling predominantly contributes to the intensity of the quantum beat of the totally symmetric ∼220 cm-1 vibration. The present work suggests a general importance of the non-Condon effect in spectroscopy involving highly excited electronic states.