THE INTERACTION OF OXYGEN WITH RH STUDIED BY FIELD ELECTRON-MICROSCOPY

The initial process of the oxidation of Rh has been studied with field emission microscopy (FEM). This process is surface-structure-sensitiv e: the more open and rough the surfaces are, the more easily the oxida tion. The most stable Rh-oxygen bond is formed around the (210) orient ation. Heating a saturated oxygen layer in vacuum showed that two fact ors influence the desorption behaviour: (1) the heating rate and (2) t he diffusion rate of oxygen into the bulk below 700 K. At high heating rates (12-30 K/s) oxygen dissolves into the near-surface region to cr eate a surface oxide species. This oxide species decomposes and oxygen desorbs between 800 and 1200 K. On the other hand, at low heating rat es (< 0.3 K/s) or stepwise heating no oxide in the near-surface region is formed: the oxygen has diffused deeply enough into the bulk of the tip. The applied field disturbs this picture at low heating rates. Th e oxygen is forced to stay in the near-surface region and an oxide is formed above 700 K.