PHOTON-STIMULATED ION DESORPTION FROM POLYATOMIC-MOLECULES IN THE VUV- C2H4-GRAPHITE

We have investigated photon stimulated ion desorption from C2H4 physis orbed on highly oriented pyrolytic graphite using VUV synchrotron radi ation in the energy range 13-40 eV. In contrast with gas phase photodi ssociation measurements, only H+ ions are detected. This result is att ributed to the image potential barrier to desorption from the surface and the unequal distribution of kinetic energy amongst the molecular p hotofragments. 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 monolay er regime. This behaviour differs significantly from that of the gas p hase, and is attributed to selective quenching of excited molecular el ectronic states on the surface and to chemical reactions between speci fic molecular dissociated fragments and the substrate.