Equilibrium geometry and dynamics of the valence excited states of 1,3-butadiene

Franck-Condon factors are calculated for alternative forms of the potential energy surface of the 1¹B_u⁺ excited state of butadiene. A comparison of these results with the experimental absorption spectrum indicates that the 1¹B_u⁺ state of butadiene may have a nonplanar minimum, but that the twist around a terminal bond is no larger than 30°; for the 2¹Ag^- excited state larger distortions from planarity may occur. In treating the transition from a planar ground state to nonplanar excited states, we have shown that a large number of modes and combination terms have significant Franck-Condon factors. The complex nature of the excited-state surfaces and the coupling between the 1¹B_u⁺ and 2¹A_g^- states are discussed and suggested as an explanation of some of the unusual photophysical properties of butadiene.