Formation of subsurface carbon induced by oxygen adsorption on carbon-covered W(001)

The adsorption of oxygen on the carbon-covered W(001) surface was studied by AES and low energy ion scattering. At low carbon and oxygen coverages, both species can be accommodated on the surface. At higher coverages, the oxygen displaces the carbon into the near-surface (selvedge) region. When oxygen is adsorbed on the W(001)-p(5 × 1)C surface (formed by exposure to more than 50 L of ethylene at 1500 K), carbon is displaced in a nearly one for one manner. Annealing the oxygen-covered p(5 × 1)C surface to 950 K removes up to 0.5 ML oxygen as CO. Interestingly, the surface carbon coverage is unchanged by CO desorption at 950 K and subsurface carbon is partially replenished by diffusion from the bulk. Oxygen adsorption in excess of 0.5 ML suppresses carbon segregation from the bulk at 950 K. The additional oxygen does not desorb until 1300 K. Surface carbon is restored by annealing to 1500 K.. The degree and rate of carbon segregation depend on the initial ethylene exposure even though the resulting W(001)-p(5 × 1)C surfaces are identical according to AES, LEED, and ion scattering.