A new interpretation of the electronic spectrum of ethylene from 6–8 eV

The traditional assignment of the electronic spectrum of ethylene in terms of a π−π∗ N-V excitation and an overlapping π-3s Rydberg excitation is shown to be widely inconsistent with the oscillator strengths calculated for these transitions. A spectral intensity calculation based on intersecting N-V (B1u) and π-3py (B1g) surfaces, as proposed by Petrongolo et al., is shown to result in a vibronic spectrum in reasonably good agreement with experiment. This revised assignment provides much better overall agreement between theory and experiment than the traditional assignment for which, it is argued, there is no convincing evidence.