Photoabsorption of dioxasilyrane and silanone groups at the surface of silica

The lowest singlet→singlet and singlet→triplet transitions of two point defects observed at the surface of amorphous silica, the dioxasilyrane group, =Si(O2), and the silanone group, =Si=O, have been computed by means of first-principles quantum chemical techniques using cluster models of various sizes. The lowest allowed transitions of the dioxasilyrane ring are found at ≈3 and ≈5 eV; the first excitation in silanone is computed at 5.7 eV. These values are in quantitative agreement with the observed bands and allow the unambiguous assignment of the observed optical transitions in the surface of activated silica to the =Si(O2) and =Si=O point defects.