S. Janz

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 5×10−8 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...

Detection of subsurface oxygen on Cu(111): correlation of second-harmonic generation and Auger electron spectroscopy observations

The incorporation of atomic oxygen into the Cu(111) subsurface at pressures of 2.5 × 10−7 < P < 10−2 Torr and temperatures of 300 < T < 800 K is demonstrated using second-harmonic generation and Auger electron spectroscopy. We have also been able to use in situ second-harmonic generation to time-resolve the incorporation of oxygen into the surface as well as its evacuation when high vacuum conditions are restored. It is shown that the temperature at which subsurface to surface transfer becomes significant is around 600 K, below which it is possible to trap the oxygen.

Structural transformations in adsorbed oxygen layers on Al surfaces observed using optical second‐harmonic generation

Optical second‐harmonic generation is used in conjunction with Auger electron spectroscopy and low‐energy electron diffraction to study the evolution with temperature of oxygen adsorbed on Al (111), (100), and (110) surfaces. For O2 exposures ranging between 34 and 450 L, the adsorbed oxygen undergoes two distinct transformations. As the specimen is heated in vacuum to 350 °C after exposure to oxygen, the adsorbed oxygen reacts with the metal to form an oxide (Al2O3) phase. Above 350 °C the oxide overlayer breaks up into oxide islands. We propose that the island formation mechanism is triggered by the transformation of amorphous Al2O3 to crystalline Al2O3.