S_1-S_2 vibronic coupling in cis-1,3,5-hexatriene.II.Theoretical investigation of absorption and resonance Raman spectra

摘要

A wave packet prepared on the 1 ~1B_1 potential-energy surface of cis-1,3,5-hexatriene (CHT) is characterized by a very short lifetime of ≈20 fs in this state. We present here model calculations of the excited-state dynamics of CHT that are consistent with the experimentally determined population decay time scale and yield an accurate description of the absorption, preresonance and resonance Raman (RR) spectroscopy of the 1 ~1B_1 state. The greater diffuseness and complexity of the free jet 1 ~1B~1 absorption band of CHT as compared to the 1~1A_g->1 ~1B_u transition of trans-1,3,5-hexatriene can be explained by a faster optical dephasing rate and more densely spaced vibronic level structure in the S_2 state of the cis isomer primarily due to the presence of two very active low-frequency S_1-S_2 coupling modes, v_(30) and v_(31). The first measurement of the one-photon 1 ~1A_1->1 ~1A_1 transition of CHT has been reported only ten years ago and the S_1 state has since been thoroughly studied by different techniques. The simulations of the excitation and RR emission profiles of the 2 ~1A_1 state performed for this work are shown to be in quantitative agreement with the observed spectra. One of the most important and controversial questions arising from the spectroscopic information about the 1 ~1A_1 state concerns the nature of the intensity carrier for the one-photon S_0->S_1 excitation process. It can be shown that the oscillator strength for one-photon transitions into the 2 ~1A_1 vibronic manifold is exclusively borrowed from the electronic 1 ~1B_1 configuration. One model Hamiltonian is defined for the representation of wave pakcet motion in the 1 ~1A_1,2 ~1A_1, and 1 ~1B_1 states and the nuclear coordinate space comprises eight dimensions. The relevant normal modes are either of a_1 or b_1 symmetry, i.e., only first-order intrastate or S_1-S_2 vibronic coupling effects are considered,and have been selected based on the electronic structure information compiled in the preceding paper.