KINETICS OF OXYGEN-ADSORPTION, ABSORPTION, AND DESORPTION ON THE CU(111) SURFACE

The interaction of oxygen with the Cu (111) surface in ultrahigh vacuum (UHV) has been studied in the temperature range 400 < T < 800 K using second-harmonic generation (SHG) and x-ray photoelectron spectroscopy (XPS). When the clean surface is exposed to oxygen at pressures between 5 X 10(-8) and 10(-5) Torr and for T < 500 K, the SHG intensity decreases monotonically with exposure by more than one order of magnitude to a value which has no measurable temperature dependence. For T > 500 K, the SHG intensity passes through a minimum before achieving this constant value. The observation of this minimum is interpreted in terms of an outward relaxation of the Cu (111) surface as oxygen penetrates the subsurface region. When UHV conditions are restored for T > 600 K, the SHG intensity reverses its temporal dependence. These observations are consistent with initial incorporation of atomic oxygen into the subsurface region at a rate which is dependent on surface temperature and oxygen pressure and subsequent backdiffusion in UHV, driven by the oxygen concentration gradient near the surface. From experiments performed at different oxygen pressures and sample temperatures we establish rate constants for oxygen incorporation and surface outward relaxation as a function of temperature. The kinetics of oxygen incorporation determined from changes in the SHG intensity are compared with those derived elsewhere from ellipsometry studies. Differences yielded by the two experimental techniques are related to differences in monitored depths. Complementary XPS experiments suggest that sites occupied by the subsurface oxygen are characterized by tetrahedral symmetry.