A blue colored surface observed during high temperature oxidation of aluminum (111)

The oxidation of the Al(111) surface at 850K under clean, ultrahigh vacuum conditions produces a sapphire blue coloration of the surface. The blue color is observable for oxygen exposures in the range 400L to 3200L. XPS, ELS and LEED measurements indicate that a crystalline aluminum oxide forms on Al(111) upon initial exposure to oxygen and then subsequently grows as islands on the surface. The Al(2p) core level binding energy measured by XPS combined with previously reported soft-x-ray absorption measurements of the Al(2p) core level-conduction band energy separation suggests that the blue color arises from an optically excited electron transfer between the aluminum metal Fermi energy and the aluminum oxide conduction band.