Dissociative attachment from condensed O2: Violation of the selection rule Σ−⇆Σ+

Abstract

The formation of ${\mathrm{O}}^{\mathrm{-}}$ ions via dissociative attachment in electron-stimulated desorption from condensed ${\mathrm{O}}_2$ on a polycrystalline platinum surface is reported. The energy dependence of the ${\mathrm{O}}^{\mathrm{-}}$ yield exhibits two peaks at 7 and 13 eV. The 7-eV peak arises from the ${}_{}{}^2{}_{}{}^{}\mathrm{\Pi }_{\mathrm{u}}^{}{}_{}{}^{}$ state of ${\mathrm{O}}_2$ ${\mathrm{}}^{\mathrm{-}}$, whereas the 13-eV peak is ascribed to two distinct resonance states, the ${}_{}{}^2{}_{}{}^{}\mathrm{\Pi }_{\mathrm{u}}^{}{}_{}{}^{}$ and the ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{g}}^{+}{}_{}{}^{}$ ${\mathrm{O}}_2$ ${\mathrm{}}^{\mathrm{-}}$ states, from ion kinetic energy measurements. The ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{g}{}^{+}}^{}{}_{}{}^{}$ ${\mathrm{O}}_2$ ${\mathrm{}}^{\mathrm{-}}$ state is then accessible by attachment of an electron to the X ${}^3$ ${\Sigma }_{g^{\mathrm{-}}}$ ${\mathrm{O}}_2$ target via a violation of the selection rule ${\Sigma }^{\mathrm{-}}$⇆${\Sigma }^{+}$. .AE