ELECTRON-STIMULATED DESORPTION OF H- IONS VIA DISSOCIATIVE ELECTRON-ATTACHMENT IN CONDENSED METHANOL

Low-energy (0-20 eV) electron impact on condensed methanol films is observed to induce electron-stimulated desorption (ESD) of H- ions, via dissociative electron attachment (DEA). The onset for H- detection is at 6 eV, with a maximum yield centered near 8.7 eV and a shoulder at 7.3 eV. We also observe a gradual increase in H- intensity above 12 eV, which is attributed to the dipolar dissociation mechanism. Comparison of ESD data on isotopically labeled methanols CH3OD and CD3OH allows us to identify the originating sites of the H- ions. In contrast to the gas phase where the H- ions are known to arise exclusively from the OH group below 9 eV, the desorbing H- ions for condensed methanol are shown to originate in both the hydroxyl and the methyl groups. The release of H- ions from the hydroxyl group is shown to involve dissociative Rydberg anion states, in close similarity with ESD data of H- formation in amorphous ice. The kinetic energy distributions of the desorbing D- ions for both CD3OH and CH3OD are also reported. It is found that multiple electron scattering processes in the methanol films prior to DEA events enlarge the width of the peaks in the yield functions and that post-dissociation collisions at or near the surface may be involved in the reduction of the kinetic energy of the escaping anions.