Resonance Raman scattering of ethylene: Evidence for a twisted geometry in the V state

The resonance Raman spectra of ethylene obtained with 212.8 and 193.4 nm excitation are presented. With these excitation frequencies, on the red edge of the N→V absorption system, strong enhancement of the even overtones of the torsional coordinate is found. In addition, scattering to the carbon-carbon double bond stretching and the symmetric CH₂ scissors modes show enhancement. These results demonstrate that the CH₂groups lie in mutually perpendicular planes in the nuclear equilibrium configuration of the V state, while the C-C bond distance has been lengthened and the HCH angles reduced relative to those of the ground state. These results are discussed within the context of the G₁₆(2) dynamic point group and recent calculations.