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.

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