The vacuum ultraviolet excited electronic states of 1,3-butadiene: Selective enhancement of vibrational modes in resonant Raman transitions

The assignments of many of the vacuum ultraviolet valence and Rydberg states of trans-1,3-butadiene are investigated using resonance Raman spectroscopy which provides data on distortions and vibronic activity. The ¹B_g(π-3s) Rydberg state (50 144 cm^-1) is confirmed to be vibronically coupled to the 1 ¹ B_u(π π*) valence state by modes of a_u symmetry, especially V ₁₂ (CH₂ twist). The higher energy Rydberg states were also examined and an analysis was made of both their assignments and their behavior when resonantly enhanced. Selective excitation into the sharp vibrational structure of these Rydberg states provides a striking example of mode specific resonance enhancement. Evidence was sought for the location of the high energy ¹A_g (ππ*) state, said to be near 60 000 cm^-1, but the results were indefinite.