Oxidation of a cesium-covered Ni (1 1 0) surface studied by metastable-induced electron spectroscopy

An initial stage of oxidation of a cesium-covered Ni (1 1 0) surface has been studied by metastable-induced electron spectroscopy (MIES) and low-energy electron diffraction (LEED). The MIES brought spectra with Cs 6s induced peak (P6s), Cs 5p (P5p), O 2p induced peak (Pox) and a structure related to the substrate Ni 3d states (P3d). The work function change Dw showed an oscillatory behavior in the progress of surface oxidation. The process is divided into three stages: (i) at low O2 exposures, Dw > 0 with unchanging P5p and P6s; (ii) at moderate exposures, Dw < 0 with a drastic decrease in the P6s intensity; (iii) at higher exposures, Dw > 0 with shifts of peaks P5p and Pox to higher energies, together with an appearance of peak P3d. A three-step model of initial oxidation of alkali-covered Ni (1 1 0) surfaces is presented.