Forbidden electron attachment inO2

The electron-stimulated ${\mathrm{O}}^{\mathrm{-}}$ desorption yield from condensed ${\mathrm{O}}_2$ exhibits three peaks around 7, 8.5, and 13 eV, while that from ${\mathrm{O}}_2$ gas shows only the 7-eV peak. The 7-eV peak is known to arise from the ${\mathrm{O}}_2^{\mathrm{-}}$(1${\mathrm{\pi }}_{\mathrm{u}}^{\mathrm{-}1}$1${\mathrm{\pi }}_{\mathit{g}}^2$ ${)}^2$ ${\mathrm{\Pi }}_{\mathit{u}}$ compound state. In this paper the 8.5- and 13-eV peaks are shown to arise from the ${\mathrm{O}}_2^{\mathrm{-}}$(3${\mathrm{\sigma }}_{\mathit{g}}^{\mathrm{-}1}$1${\mathrm{\pi }}_{\mathit{g}}^2$ ${)}^2$ ${\mathrm{\Sigma }}_{\mathit{g}}^{+}$ and ${\mathrm{O}}_2^{\mathrm{-}}$(2${\mathrm{\sigma }}_{\mathrm{u}}^{\mathrm{-}1}$1${\mathrm{\pi }}_{\mathrm{g}}^2$ ${)}^2$ ${\mathrm{\Sigma }}_{\mathrm{u}}^{+}$ compound states, respectively. The absence of the 8.5- and 13-eV peaks in the gas-phase spectrum presents the first experimental evidence for a forbidden electron attachment, which is due to the ${\sigma }^{\mathrm{-}}$ selection rule [H. Sambe and D. E. Ramaker, Chem. Phys. Lett. 139, 386 (1987)].