Photoabsorption of Dioxasilyrane and Silanone Groups at the Surface of Silica

Point defects in the bulk of amorphous and crystalline silica have received enormous attention in the past three decades. Much less work has been done to identify and characterize defects at the surface of silica. In this work, the lowest singlet->singlet and singlet->triplet transitions of two point defects observed at the surface of amorphous silica, the dioxasilyrane group, =Si(O sub 2), and the silanone group, =Si=O, have been computed with accurate first-principles quantrum 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(O sub 2) and =Si=O point defects.