Nonadiabatic processes during the oxidation of Li layers

The first stages of oxidation of thin Li films (leading to the formation of Li₂O) were found to be accompanied by emission of O^- 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 O₂ molecule leads to formation of a transient O₂ ^2- species. This species dissociates without a noticeable activation barrier and there is a finite (but rather low) probability that one of the O^- fragments formed near the surface is ejected into the gas phase. The O^- species at the Li surface forms, on the other hand, a hole state, which subsequently transforms into the O^2- 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^-10 photons per reacting O₂ molecule and indicates a short lifetime (100 fs) of the O^- species at a Li surface.