Kinetic-energy distributions of O- produced by dissociative electron attachment to O2 physisorbed on a Kr substrate.

We report measurements of the kinetic-energy (${\mathit{E}}_{\mathit{k}}$) distributions of ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ produced by low-energy electron impact (6--18 eV) on thin disordered films consisting of 0.15 ML of ${\mathrm{O}}_{2}$ physisorbed on 5 ML of Kr, all condensed on polycrystalline Pt. The present measurements confirm that, for incident electron energies E(e)\ensuremath{\le}12 eV, dissociative electron attachment (DEA) to physisorbed ${\mathrm{O}}_{2}$ involves the $^{2}\mathrm{\ensuremath{\Pi}}_{\mathit{u}}$, $^{2}\mathrm{\ensuremath{\Sigma}}_{\mathit{u}}^{+}$(I), and $^{2}\mathrm{\ensuremath{\Sigma}}_{\mathit{g}}^{+}$(I) resonances of ${\mathrm{O}}_{2}^{\mathrm{\ensuremath{-}}\mathrm{*}}$; of these, only the latter is found to dissociate to the second limit ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$${(}^{2}$P)+O${(}^{1}$D), via a nonadiabatic curve crossing. Above E(e)=12 eV, DEA to ${\mathrm{O}}_{2}$ is found to proceed mainly via the $^{2}\mathrm{\ensuremath{\Sigma}}_{\mathit{x}}^{+}$(II) [x=g and/or u] state(s), leading to ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$+O${(}^{1}$D) fragments. No evidence for ${\mathrm{O}}_{2}^{\mathrm{\ensuremath{-}}\mathrm{*}}$ resonance states which dissociate to the third limit ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$+O${(}^{1}$S) is found below E(e)=18 eV. The results also support the general notion that postdissociation interactions which lead to ion energy loss consist mainly of binary large-angle elastic collisions of the desorbing ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ with adsorbate particles in the van der Waals solid. Contributions of energy-loss electrons to the DEA ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ ${\mathit{E}}_{\mathit{k}}$ distributions are found to most likely originate in small ${\mathrm{O}}_{2}$ clusters at, or near, the surface of the Kr substrate.