S 1 –S 2 vibronic coupling in cis-1,3,5-hexatriene. II. Theoretical investigation of absorption and resonance Raman spectra

Abstract

A wave packet prepared on the 1 1B1 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 1B1 state. The greater diffuseness and complexity of the free jet 1 1B1 absorption band of CHT as compared to the 1 1Ag→1 1Bu 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 S2 state of the cis isomer primarily due to the presence of two very active low-frequency S1–S2 coupling modes, ν30 and ν31. The first measurement of the one-photon 1 1A1→2 1A1 transition of CHT has been reported only ten years ago and the S1 state has since been thoroughly studied by different techniques. The simulations of the excitation and RR emission profiles of the 2 1A1 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 2 1A1 state concerns the nature of the intensity carrier for the one-photon S0→S1 excitation process. It can be shown that the oscillator strength for one-photon transitions into the 2 1A1 vibronic manifold is exclusively borrowed from the electronic 1 1B1 configuration. One model Hamiltonian is defined for the representation of wave packet motion in the 1 1A1, 2 1A1, and 1 1B1 states and the nuclear coordinate space comprises eight dimensions. The relevant normal modes are either of a1 or b1 symmetry, i.e., only first-order intrastate or S1–S2 vibronic coupling effects are considered, and have been selected based on the electronic structure information compiled in the preceding paper.