Neutral-atom scattering from random isolated adsorbate atoms on clean metal surfaces: Oxygen and carbon monoxide on nickel (001)

The intensity $I$ of the specular beam of a helium nozzle beam scattered from a Ni(001) surface has been measured as a function of adsorbate coverage $\Theta$ for both oxygen and CO exposures at 350 K for different angles of incidence. A linear relationship is found between ln ($\frac{I}{I_0}$) ($I_0$, the intensity of the specular beam from the clean surface) and $\Theta$ up to $\Theta =0.15\mathrm{}$ monolayer of O on Ni and $\Theta =0.1\mathrm{}$ monolayer of CO on Ni. A model is proposed in which the scattering is governed by the repulsive part of the gas-surface potential, the latter being described by a hard-wall corrugation. A constant attractive well depth and a temperature-dependent vibration amplitude of the atoms are also incorporated into the model. The adsorbate atoms are treated as a shot noise on a flat metal surface. By means of suitable averaging, a formula is found that explains the linear dependence indicated. From the best fit of the model to the experimental data, a set of parameters describing the corrugation of a single adsorbate is derived. Cross sections for the helium-adatom scattering are 65 and 26 ${\mathrm{\AA }}^2$ for CO and O, respectively. The corresponding corrugations have been fitted with Gaussians of height 0.62 Å (for CO) and 0.32 Å (for O).