ELECTRON-STIMULATED DESORPTION OF O+ AND OH+ FROM CO COVERED RH(111)

The kinetic energy distributions and yield-time dependence of electron ically desorbed O+ have been examined from clean Rh(111) and CO covere d Rh(111). For the clean Rh(111) surface, we observe up to three peaks in the electron-stimulated desorption (ESD) O+ kinetic energy distrib utions dependent on the amount of CO adsorbed and the thermal history of the surface. The highest energy O+ peak appears at approximately 8. 0 eV and is present in the kinetic energy spectra only when CO is adso rbed on the surface. The intermediate energy O+ peak at approximately 3.5 eV appears only after CO exposures greater than approximately 1.0 L and requires extended electron irradiation. The lowest energy peak a t approximately 1.5 eV is present only after the sample has been annea led to T > 1200 K and can be seen regardless of whether the Rh(111) su rface has been exposed to CO. The intensities of both low energy peaks were electron beam dose dependent, indicating that the surface states detected were most likely electron beam induced. The high energy O+ k inetic energy distribution peak was apparently due to CO adsorbed in b oth atop and bridge adsorption sites. The low energy O+ kinetic energy distribution peak is thought to be due to electron-stimulated dissoci ation of adsorbed CO, which is supported by the similar behavior obser ved for the OH+ kinetic energy distributions. Finally, the lowest ener gy O+ kinetic energy distribution peak, which was present only after c leaning and annealing the Rh(111) crystal, may have been due to the pr esence of residual oxygen left over following the cleaning procedure.