Kinetic-energy distributions ofO−produced by dissociative electron attachment to physisorbedO2

We report measurements of the kinetic energy (${\mathit{E}}_{\mathit{k}}$) distributions of ${\mathrm{O}}^{\mathrm{-}}$ produced by low-energy electron impact (5.5–19.5 eV) on disordered multilayers of ${\mathrm{O}}_2$ physisorbed on a polycrystalline Pt substrate. The results confirm that dissociative electron attachment (DEA) proceeds via the formation of the ${}_{}{}^2{}_{}{}^{}\mathrm{\Pi }_{\mathit{u}}^{}{}_{}{}^{}$, ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathit{g}}^{+}{}_{}{}^{}$(I), and ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathit{x}}^{+}{}_{}{}^{}$(II) (x=g and/or u) states of ${\mathrm{O}}_2^{\mathrm{-}\mathrm{*}}$. We also find evidence for an additional resonance, namely the ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathit{u}}^{+}{}_{}{}^{}$(I), positioned at about 10 eV above the neutral ground state in the Franck-Condon region, and dissociating into ${\mathrm{O}}^{\mathrm{-}}$+O${(}^3$P). The measurements suggest that the autodetachment lifetimes of the ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathit{u}}^{+}{}_{}{}^{}$(I) and ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathit{g}}^{+}{}_{}{}^{}$(II) states may be longer than previously suggested. It is also observed that the effects of electron energy loss (EEL) in the solid prior to DEA, ${\mathrm{O}}^{\mathrm{-}}$ scattering in the solid after dissociation, and the charge-induced polarization energy of the solid, broaden the ${\mathit{E}}_{\mathit{k}}$ distributions, shift them to lower anion energies, and result in additional structure in them. The effects of EEL on the desorption dynamics of ${\mathrm{O}}^{\mathrm{-}}$ are estimated from high-resolution electron-energy-loss spectra and excitation functions for losses in the vicinity of the Schumann-Runge continuum of the physisorbed ${\mathrm{O}}_2$ molecules. We find indications for an enhancement of the optically forbidden X ${}_{}{}^3{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{g}}^{\mathrm{-}}{}_{}{}^{}$→A ${}_{}{}^3{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{u}}^{+}{}_{}{}^{}$ transition, and observe that the gas-phase Rydberg bands, for energy losses...