Onset of sliding friction in incommensurate systems

Interstitial O3 molecules in 7.9 eV photon-irradiated silica are identified. Their optical absorption band at 4.8 eV nearly coincides with the 4.8 eV band of nonbridging oxygen hole centers. The O3-related band is distinguished by a smaller halfwidth (0.84 vs 1. 05 eV), by susceptibility to ultraviolet bleaching, by lack of correlation to the 1.9 eV luminescence band, and by rise of a singlet O2 luminescence band at 0.974 eV during photobleaching. This identification solves a long controversy on the nature of optical bands in silica and gives a tool for studying the mobility of atomic oxygen in SiO2.