Resonance Raman spectra of acetylene with excitation at 204 and 178 nm in the region X̃→Ã(1Σg→1Σ-u in D∞h) and X̃→B̃(1Σ+g→1Δu in D∞h) transitions exhibit very strong activity in both even and odd quanta of the πg(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 1Πu and 1Σ+u symmetry).
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry