S1-S2 vibronic coupling in trans-1,3,5-hexatriene.: II.: Theoretical investigation of absorption and resonance Raman spectra

No direct absorption or emission signals of the 2 (1)A(g) state of trans-1,3,5-hexatriene (THT) have been detected so far. However, the ab initio calculations of the three valence singlet states of THT presented in the preceding paper (paper I) put the vertical excitation energy of the 2 (1)A(g) state ca. 0.5 eV below that of the 1 B-1(u) state. This result indicates possible strong vibronic coupling effects on the spectroscopy of the bright 1 B-1(u) state. We construct a quantum-mechanical three-state eight-mode model Hamiltonian operator for the microscopic description of the ultrafast S-2--> S-1 internal conversion dynamics following optical excitation of the 1 B-1(u) state based on the ab initio potential energy information for the S-0, S-1, and S-2 states of THT compiled in paper I. This dynamical model is shown to yield a reliable description of the absorption, preresonance and resonance Raman (RR) spectroscopy of the 1 B-1(u) state of THT. The homogeneous linewidth of 155 cm(-1) FWHM observed for the origin band of the 1 (1)A(g)--> 1(1)B(u) transition can be reproduced with an optical dephasing time T-2 of 90 fs. The strong enhancement of 1 B-1(u) RR bands involving the almost Franck-Condon inactive tuning mode nu(9) as well as the observed rapid 1 B-1(u) population decay indicate that the S-1 and S-2 states are probably nearly degenerate, the 2 (1)A(g) energy may also be slightly higher than that of the 1 B-1(u) state vertically. However, the parameter set that yields a realistic description of the RR spectroscopy and population dynamics within the eight-mode vibronic coupling model needs to be modified in order to reproduce the high-resolution 1 B-1(u) absorption profile, i.e., a significant reduction of the ab initio interstate coupling constants is required. A convergence of the two different parameter sets can be expected if the Hamiltonian is extended by the 28 weakly coupled modes that are considered by a phenomenological relaxation term in the present model. (C) 2000 American Institute of Physics. [S0021-9606(00)30402-0].