Fundamental radiation-induced defect centers in synthetic fused silicas: Atomic chlorine, delocalized E' centers, and a triplet state.

Abstract

A series of synthetic fused silicas of diverse OH contents was subjected to 100-keV x irradiations at 77 K and investigated by electron-spin-resonance techniques at \ensuremath{\sim}110 K or higher temperatures. Spectra were recorded at X-band frequencies (\ensuremath{\sim}9.2--9.3 GHz) both as the first derivative of absorption and in the high-power second-harmonic mode in order to bring out features not fully accessible by using one of these methods alone. In addition to the previously known ${E}_{\ensuremath{\alpha}}^{\mathcal{'}}$, ${E}_{\ensuremath{\gamma}}^{\mathcal{'}}$, and oxygen-associated hole centers, three new defects were detected and characterized by computer line-shape simulation methods. These were atomic chlorine, a delocalized E' center (denoted ${E}_{\ensuremath{\delta}}^{\mathcal{'}}$), and the first biradical to be reported in a-${\mathrm{SiO}}_{2}$. A sample-to-sample correlation of the radiation yields of these three new centers has been noted, leading to the suggestion that all three find their origins in specific chlorine-decorated precursor sites in the unirradiated glasses. Although significant chlorine impurities (g100 ppm) may be ubiquitous in both type-III (high OH) and type-IV (low OH) fused silicas, the occurrence of chlorine-associated radiation-induced defects appears to be anticorrelated with the OH contents of the materials. Some possible technological implications of these findings are discussed.