Nonadiabatic processes during the oxidation of Li layers.

Abstract

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 ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ 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 ${\mathrm{O}}_{2}$ $^{2\mathrm{\ensuremath{-}}}$ species. This species dissociates without a noticeable activation barrier and there is a finite (but rather low) probability that one of the ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ fragments formed near the surface is ejected into the gas phase. The ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ species at the Li surface forms, on the other hand, a hole state, which subsequently transforms into the ${\mathrm{O}}^{2\mathrm{\ensuremath{-}}}$ ground state. For excitations larger than the work function, the energy associated with its decay (g2 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 ${10}^{\mathrm{\ensuremath{-}}10}$ photons per reacting ${\mathrm{O}}_{2}$ molecule and indicates a short lifetime (100 fs) of the ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ species at a Li surface.