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

Abstract

We carried out wavelength-dispersed time-resolved absorption measurements of cis-stilbene to investigate the mechanism of the appearance of the approximately 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 Sn<--S1 transition moment, i.e., non-Condon effect. It was concluded that the vibronic coupling of the Sn 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 approximately 220 cm-1 vibration. The present work suggests a general importance of the non-Condon effect in spectroscopy involving highly excited electronic states.

Effects of Herzberg–Teller vibronic coupling on coherent excitation energy transfer

“…[10][11][12][13] In the meanwhile, however, the non-Condon vibronic coupling is also proposed to enhance the quantum coherence in many organic and biological molecular systems. [14][15][16] To obtain a better understanding of the origin of quantum coherence in EET dynamics, the interplay of the Condon and non-Condon vibronic coupling effects deserves more theoretical investigations.…”

Correlated vibration-solvent effects on the non-Condon exciton spectroscopy

“…The DEOM construction starts with an exponential expansion of correlation function satisfying Eq. (11) as…”

“…[6][7][8][9] It is found that the non-Condon vibronic coupling would affect this quantum coherence enhancement. [10][11][12] Spectroscopic studies on EET systems have been widely carried out to reveal the structures and underlying dynamic processes in these systems. [13][14][15][16][17][18] In reality, EET constitutes an open quantum process where intramolecular nuclear vibrations and the surrounding solvent would coordinatively play important roles.…”

Classical theory of two-dimensional time-domain terahertz spectroscopy

“…the description of ultrafast spectroscopic measurements. 55,56 A higher-order term is introduced to the dipole moment as…”