High Resolution Electron Energy Loss Spectroscopy Study of the Interaction of Oxygen With A Mo(111) Surface

Abstract

Electron energy loss spectroscopy (EELS) is a sensitive tool for the characterization of the adsorbed states of atoms and molecules. Such approach has been utilized for the study of the interaction of oxygen with a Mo(111) surface at room temperature. An additional study by high resolution electron energy loss spectroscopy (HREELS) has been undertaken to obtain direct information on the adsorption sites and stages of interaction. EELS study has shown that with increasing exposure at room temperature, oxygen occupies three adsorption sites on the Mo(111) surface: three-fold, short-bridge, and on-top. This adsorption picture differs substantially from the model of oxygen adsorption on W(111) proposed on the basis of electron-stimulated desorption with ion angular distribution data. According to this model, oxygen atoms adsorbed at low exposures in bridge sites reside completely in on-top sites with increasing coverage. The populating of the three-fold and short bridge sites on the Mo(111) surface finishes at an exposure of about 4×10 −4 Pa.s, where two losses reach saturation. The intensity of the vibrational mode of on-top oxygen slowly increases with exposure and becomes commensurable with the other two modes at about 0.1 Pa.s. At this exposure, a new feature develops at 1432 cm −1 . It is identified as a combinational loss. After heating the oxygen-covered molybdenum surface up to 900 K, the loss at 1432 cm −1 disappears and loss assigned to on-top oxygen sharply decreases in intensity. This latter fact is because of desorption of most of the oxygen atoms adsorbed in top sites.