Optical absorption spectra of P defects in vitreous silica

We present an investigation of the optical properties of diamagnetic P centers in P-doped silica by means of first-principles calculations, including many-body perturbation theory (GW and Bethe-Salpeter Equation) techniques. The calculated absorption spectra indicate that the 6.9 eV band is originated from the presence of a large number of [(O-)₃P(=O)]⁰ tetrahedra, while only a negligible number of [(O-)₂P(=O)₂]^- tetrahedra could occur. Furthermore we show that positively charged substitutional P atoms can affect the silica absorption spectrum only above ~8 eV, while three-fold P defects are not likely to occur as they should give rise to strong features, not observed, below ~6.5 eV.