Competing photodissociation pathways for physisorbed O2/graphite investigated using Ar spacer layers

We have studied photon stimulated ion desorption from O2 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, specifically, (i) direct dipolar dissociation, (ii) direct dissociative ionization, (iii) photoelectron driven dissociative attachment and (iv) photoelectron driven dipolar dissociation. We find that the O− yield is mainly due to direct dipolar dissociation, while for O+ direct dipolar dissociation and both direct and indirect dissociative ionization are identified. The Ar layer reduces the image potential arising from the graphite substrate. Some dissociation pathways in the energy region 24–27 eV are enhanced for O2/graphite compared with the gas phase.