Continuum resonance in ethylene: Evidence from vibrationally resolved core photoionization

The carbon $1s$ photoelectron spectrum of gas-phase ethylene has been measured with high resolution in order to study changes in vibrational structure arising from core photoionization through a continuum resonance. By evaluating the changes in vibrational intensity distribution we are able to determine that the ionization is non-Franck-Condon for energies close to the maximum of the broad resonance in the ethylene absorption spectrum. From these measurements it is possible to investigate the specific effect on the vibrational intensity distribution arising from the resonance. We find that the intensity of the C-H stretch vibrational mode is constant as the photon energy is scanned through the resonance, while the intensity of vibrations associated with the C-C bond varies with photon energy. This shows the existence of a shape resonance which, according to the scattering model of the shape resonance, must be primarily associated with the C-C bond rather than the C-H bond.