Self-trapped holes in amorphous silicon dioxide

Electron-spin-resonance spectra tentatively attributed to self-trapped holes (STH’s) in amorphous silicon dioxide are reported and analyzed. Two types of STH’s were found in a numerical ratio ∼1:1 in all silica samples investigated (including high-OH flame hydrolytic, low-OH ${\mathrm{O}}_2$-plasma fused, sol-gel, and less pure isotopically enriched materials) following either 100-keV x-ray irradiation or 6.4-eV excimer-laser illumination at 77 K: ${\mathrm{STH}}_1$, stable below ∼180 K, is suggested to comprise a hole trapped on a ‘‘normal’’ bridging oxygen in the glass network. ${\mathrm{STH}}_2$, stable below ∼140 K, is provisionally ascribed to a hole trapped on two normal oxygens, in analogy with the $V_K$ center in alkali halides.