Oxygen related defect center formation in MeV energy boron implanted silica

Silica samples were implanted with 1.6 and 4 MeV B²⁺ at nominal doses ranging from 0.5 to 5.0×10¹⁵ ions/cm². Sample temperatures during implantation were either 100, 300 or 673 K. Optical absorption was measured from 2.7 to 6.5 eV and in all spectra two maxima were resolved at 5.0 and 5.8 eV. Based on the literature we assumed bands at 4.8, 5.01, 5.17, 5.88, and 7.15 eV comprised the observed spectra. Assuming that the shapes of these bands were Gaussian functions, the data was fitted by allowing the maximum amplitude to be a free parameter. The fit between 2.5 and 5.5 eV was within ±3%. At larger energies the fit was worse. Assuming that there was a band at 6.35 eV reduced the difference between the fit and the data to <±2% from 2.5 to 6.5 eV. The same accuracy of fit was achieved with all spectra, irrespective of energies, doses or sample temperatures. Each of the bands had a different dependence on dose, on energy, and on sample temperature. A band at 5.5 eV which has been observed in the spectra of some silicas when irradiated and photons of differing energies was not required for the fitting. This band is absent from the spectra of as implanted B samples.