Visible to vacuum-UV range optical absorption of oxygen dangling bonds in amorphous SiO${}_2$

Synthetic silica glass with an optical absorption spectrum dominated by oxygen dangling bonds (nonbridging oxygen hole centers, or NBOHCs) and having negligible (<1%) contribution from the usually copresent Si dangling bonds (E′-centers), was prepared by room temperature ultraviolet photobleaching of high SiOH content (“wet”) silica, irradiated by F${}_2$ laser (7.9 eV) at $T$ = 80 K. This allowed us to obtain the up-to-now controversial optical absorption spectrum of NBOHC in the ultraviolet and vacuum-ultraviolet (UV-VUV) region of the spectrum and to show that it is semicontinuous from 4 to 7.8 eV and cannot be represented by a pair of distinct Gaussian bands. Since NBOHC is one of the main UV-VUV range optical absorbers in silica, its spectral shape provides a tool to disentangle contributions of different color centers to optical losses in this spectral region.

Figure 1

OA of wet silica glass, irradiated at $T$ = 80 K by F${}_2$ laser fluence of 8 J/cm${}^2$, measured after warming to $T$ = 295 K (spectrum A) and after subsequent photobleaching by UV light at room temperature (spectrum B). Spectrum B represents the as-measured data multiplied by 1.765 to match the 4.3- to 4.7-eV region of spectrum A. Spectrum C is the difference between spectra A and B.

Figure 2

EPR spectra of the F${}_2$ laser–irradiated and photobleached a–SiO${}_2$ sample described in Fig. 1. Spectra A and B (inset) were taken at $T$ = 295 K and microwave power of 1 μW before (A) and after (B) UV photobleaching. Spectrum B is redrawn with a 100 times gain (spectrum C) and compared to the spectrum of a reference sample (spectrum D) to show the negligible concentration (<3 × 10${}^{14}$ cm${}^{-3}$) of E′-centers remaining after photobleaching. Spectrum E, characteristic of NBOHCs, was recorded at 77 K and microwave power of 1 mW after photobleaching. Spectrum recorded in this mode before photobleaching (not shown), had an identical shape and ∼1.5 times higher intensity.

Figure 3

Comparison of the shape of the stable OA band remaining in the F${}_2$ laser–irradiated (80 K, 8 J/cm${}^2$) sample after photobleaching with UV light (spectrum D, lower panel). Shapes of the metastable bands induced by F${}_2$ laser irradiation at $T$ = 80 K with doses of 0.8 J/cm${}^2$ (spectrum E), 20 J/cm${}^2$ (spectrum F), and 95 J/cm${}^2$ (spectrum G) and measured at 80 K, without warming. Spectra D–G are all independently normalized to 1. The top panel shows the difference (spectrum C) between the shapes of the OA measured at 295 K (spectrum A) and 80 K (spectrum B) in a sample with a temperature-independent concentration of absorbing centers.

Figure 4

Correlated changes in IR (top panel) and VUV (bottom panel) absorption spectra of wet silica glass (a-SiO${}_2$) due to F${}_2$ laser irradiation and UV bleaching. Co-related spectra are denoted by the same letter in both panels: pristine sample (spectrum A), F${}_2$ laser–irradiated and UV-bleached sample (spectrum B), and difference sample (spectrum C). The data scatter in the 3600- to 3700-cm${}^{-1}$ region is due to humidity in air. Spectrum D represents the decrease of both IR and VUV absorption due to photolysis of SiOH groups. Their number was estimated from the concentration of created SiH groups (top panel, band at 2235 cm${}^{-1}$). Spectrum E shows the changes to IR absorption (top) and the related decrease of VUV absorption (bottom) caused by formation of H-bonded SiOH groups. The true VUV absorption spectrum of laser-induced color centers, corrected for the changes in overlapping SiOH absorption, equals C + D + E (see Fig. 5, spectrum A).

Figure 5

Joint visible to VUV range OA (spectrum A) of wet silica glass, containing dominant concentrations of NBOHCs, negligible amounts of E′-centers, and no other paramagnetic defects. Spectrum B is a scaled-down OA spectrum from Ref. 16. The vertical dashed line at 7.2 eV marks the energies above which the corrections for the destruction of hydroxyl groups and formation of additional hydrogen bonding become significant (see Fig. 4). The inset shows a more accurate shape of the 1.96-eV absorption band measured in the sample, irradiated by a higher dose (95 J/cm${}^2$) of 7.9-eV photons. We suggest that spectrum A represents the current best available approximation to the VIS-UV to VUV absorption spectrum of NBOHC in a-SiO${}_2$.

Figure 6

Spectral shape of UV-VUV range OA of oxygen dangling bonds in a-SiO${}_2$ (circles, replotted from Fig. 5, spectrum A) fitted by a sum of five Gaussian functions (solid lines), whose parameters are given in Table . They serve merely as a convenient reference representation of the experimental band shape and are not associated with distinct electronic transitions at their respective peak energies. The top spectrum shows a magnified spectrum of the difference between the experimental spectrum and the fit. The dashed line (peak = 4.80 eV, FWHM = 1.10 eV) illustrates the commonly used representation of the low-energy wing by a single Gaussian function.