CONTINUUM RESONANCE IN ETHYLENE - EVIDENCE FROM VIBRATIONALLY RESOLVED CORE PHOTOIONIZATION

The carbon Is photoelectron spectrum of gas-phase ethylene has been me asured with high resolution in order to study changes in vibrational s tructure arising from core photoionization through a continuum resonan ce. By evaluating the changes in vibrational intensity distribution we are able to determine that the ionization is non-Franck-Condon for en ergies close to the maximum of the broad resonance in the ethylene abs orption spectrum. From these measurements it is possible to investigat e the specific effect on the vibrational intensity distribution arisin g from the resonance. We find that the intensity of the C-H stretch vi brational 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 re sonance which, according to the scattering model of the shape resonanc e, must be primarily associated with the C-C bond rather than the C-H bond.