COMPETING PHOTODISSOCIATION PATHWAYS FOR PHYSISORBED O-2 GRAPHITE INVESTIGATED USING AR SPACER LAYERS/

We have studied photon stimulated ion desorption from O-2 physisorbed on graphite, using synchrotron radiation in the energy range 16-40 eV, Ar spacer layers were employed in order to distinguish between direct and indirect (photoelectron driven) dissociation channels, specifical ly, (i) direct dipolar dissociation, (ii) direct dissociative ionizati on, (iii) photoelectron driven dissociative attachment and (iv) photoe lectron driven dipolar dissociation. We find that the O- yield is main ly due to direct dipolar dissociation, while for O+ direct dipolar dis sociation and both direct and indirect dissociative ionization are ide ntified. The Ar layer reduces the image potential arising from the gra phite substrate. Some dissociation pathways in the energy region 24-27 eV are enhanced for O-2/graphite compared with the gas phase.