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

A wave packet prepared on the 1 B-1(1) potential-energy surface of cis-1,3, 5-hexatriene (CHT) is characterized by a very short lifetime of approximate to 20 fs in this state. We present here model calculations of the excited- state dynamics of CHT that are consistent with the experimentally determine d population decay time scale and yield an accurate description of the abso rption, preresonance and resonance Raman (RR) spectroscopy of the 1 B-1(1) state. The greater diffuseness and complexity of the free jet 1 B-1(1) abso rption band of CHT as compared to the 1 (1)A(g) --> 1 B-1(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 ci s isomer primarily due to the presence of two very active low-frequency S-1 -S-2 coupling modes, nu (30) and nu (31). The first measurement of the one- photon 1 (1)A(1) --> 2 (1)A(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 (1)A(1) state performed for this work are shown to be in quantitative agreement with the observed spectra. One of the most important and controv ersial questions arising from the spectroscopic information about the 2 (1) A(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 (1)A(1) vibronic manifold is exclusiv ely borrowed from the electronic 1 B-1(1) configuration. One model Hamilton ian is defined for the representation of wave packet motion in the 1 (1)A(1 ), 2 (1)A(1), and 1 B-1(1) states and the nuclear coordinate space comprise s eight dimensions. The relevant normal modes are either of a(1) or b(1) sy mmetry, i.e., only first-order intrastate or S-1-S-2 vibronic coupling effe cts are considered, and have been selected based on the electronic structur e information compiled in the preceding paper. (C) 2001 American Institute of Physics.