Nonadiabatic processes during the oxidation of Li layers

The first stages of oxidation of thin Li films (leading to the formation of ${\mathrm{Li}}_2$O) were found to be accompanied by emission of ${\mathrm{O}}^{\mathrm{-}}$ ions as well as electrons, reflecting the participation of electronically highly excited states in this reaction. A model is proposed and supported by the results of local-spin-density-approximation cluster calculations whereby electron transfer from the metal onto the impinging ${\mathrm{O}}_2$ molecule leads to formation of a transient ${\mathrm{O}}_2$ ${}_{}{}^{2\mathrm{-}}{}_{}{}^{}$ species. This species dissociates without a noticeable activation barrier and there is a finite (but rather low) probability that one of the ${\mathrm{O}}^{\mathrm{-}}$ fragments formed near the surface is ejected into the gas phase. The ${\mathrm{O}}^{\mathrm{-}}$ species at the Li surface forms, on the other hand, a hole state, which subsequently transforms into the ${\mathrm{O}}^{2\mathrm{-}}$ ground state. For excitations larger than the work function, the energy associated with its decay (>2 eV) may be released in an Auger process associated with electron emission. The yield of light emission was found to be below the detection limit of about ${10}^{\mathrm{-}10}$ photons per reacting ${\mathrm{O}}_2$ molecule and indicates a short lifetime (<100 fs) of the ${\mathrm{O}}^{\mathrm{-}}$ species at a Li surface.