Photon stimulated ion desorption from polyatomic molecules in the VUV: C2H4-graphite

We have investigated photon stimulated ion desorption from C2H4 physisorbed on highly oriented pyrolytic graphite using VUV synchrotron radiation in the energy range 13-40 eV. In contrast with gas phase photodissociation measurements, only H+ ions are detected. This result is attributed to the image potential barrier to desorption from the surface and the unequal distribution of kinetic energy amongst the molecular photofragments. The yield of desorbed H+ ions shows a threshold at 20.5 eV and a resonance at 24 eV and appears to be enhanced in the monolayer regime. This behaviour differs significantly from that of the gas phase, and is attributed to selective quenching of excited molecular electronic states on the surface and to chemical reactions between specific molecular dissociated fragments and the substrate.