The luminescence spectra of fully oxidized porous silicon show two broad bands and one vibronic structure between 2 and 3.3 eV, all excited at 340 or 275 nm (3.65 and 4.51 eV). Two of these structures have not been described previously. Interestingly, these emissions are found in other forms of amorphous or disordered silica with high specific surfaces: frosted silica glass, silica gel, even various natural opals. These emissions disappear when the surface is passivated. They are, therefore, attributed to surface-related defects. The vibronic structure involves a nearly free SiO4 tetrahedron. Another vibronic structure found only in some opals is attributed to the uranyl group.

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