No direct absorption or emission signals of the 2 1Ag 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 1Ag state ca. 0.5 eV below that of the 1 1Bu state. This result indicates possible strong vibronic coupling effects on the spectroscopy of the bright 1 1Bu state. We construct a quantum-mechanical three-state eight-mode model Hamiltonian operator for the microscopic description of the ultrafast S2→S1 internal conversion dynamics following optical excitation of the 1 1Bu state based on the ab initio potential energy information for the S0, S1, and S2 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 1Bu state of THT. The homogeneous linewidth of 155 cm−1 FWHM observed for the origin band of the 1 1Ag→11Bu transition can be reproduced with an optical dephasing time T2 of 90 fs. The strong enhancement of 1 1Bu RR bands involving the almost Franck–Condon inactive tuning mode ν9 as well as the observed rapid 1 1Bu population decay indicate that the S1 and S2 states are probably nearly degenerate, the 2 1Ag energy may also be slightly higher than that of the 1 1Bu 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 1Bu 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.