Resonance Raman activity in odd quanta of the trans bending vibration of acetylene: Strong vibronic coupling in the X̃ to à and X̃ to B̃ transitions

Resonance Raman spectra of acetylene with excitation at 204 and 178 nm in the region X̃→Ã(¹Σ_g→¹Σ^-_u in D_∞h) and X̃→B̃(¹Σ⁺_g→¹Δ_u in D_∞h) transitions exhibit very strong activity in both even and odd quanta of the π_g(v₄) trans bending vibration. Enhancement of the two-quantum even overtone is expected from linear vibronic activity in these forbidden transitions resulting in quadratic (C-term) Raman activitiy. The higher even quanta overtone activity is consistent with strongly bent excited states or nonlinear vibronic coupling. The induction of intensity in the odd overtones of this non-totally symmetric vibrational mode, however, requires mixed linear and quadratic vibronic coupling involving two different allowed excitations (to states of ¹Π_u and ¹Σ⁺_u symmetry).