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

Abstract

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( v 4) 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).