Adsorption and desorption and structural phase transition near T c on Ni surfaces with atomic-beam scattering

Abstract

The adsorption and desorption of hydrogen, oxygen, carbon monoxide, and ethylene on Ni(110) and of hydrogen on Ni(100) were monitored with He‐atomic beam scattering. The attenuation of the specular beam during gas adsorption initially shows a linear dependence on the adsorbate partial pressure and dose, which, when extrapolated, corresponds to a decrease of 100% for about one‐half a Langmuir (1 L=1.33×10⁻⁴ Pa s). This can be related to total collision cross sections of the adsorbed particles in the range of 30 to 100 Ų, indicating dominance of the attractive interaction of the He atoms with adsorbates in dilute concentrations. Statistical occupation or depletion of sites in the adlayer structures suffice to explain the changes in coherent specular intensity upon adsorption and desorption processes. Near the Curie temperature of Ni (T_c=631 K), diffracted intensities of H₂ and He show non‐Debye–Waller behavior in their temperature dependence. Connections to changes in carbon surface concentration and to a (structural) phase transition (‘‘surface roughening’’) near T_c are discussed.