ELECTRON-STIMULATED ION-MOLECULE REACTIONS IN O2 FILMS ON GRAPHITE

We have investigated the production of O-, O2- and O3- ions by electro n-stimulated desorption from well-ordered films of physisorbed O2 on g raphite. Resonances observed in the yield of all products as a functio n of electron impact energy are attributed to dissociative electron at tachment, generating O- ions which can react with neighbouring O2 mole cules in the film. We interpret characteristic differences in the ion yield profiles from one product to another in terms of a simple binary collision model, which estimates the kinetic energy of the ion produc ts and thus the probability of escape from the polarisation potential induced in the O2 film. The model is also consistent with the measured angular distributions of desorbed ions.