The role of hydrogen in excimer-laser-induced damage of fused silica

Hydrogen nuclear magnetic resonance (NMR) was used to probe defect reactions in fused silica during 193 nm ultraviolet irradiation and thermal treatment. Hydrogen concentration and Carr–Purcell measurements indicate that the majority of hydrogen atoms are isolated and do not segregate or evolve from the fused silica during annealing or irradiation. Previously, NMR spin-lattice relaxation measurements have revealed that a small fraction of hydrogen in fused silica exists as centers that undergo molecular motion and are consumed during annealing. Here, a detailed analysis of the NMR spin-lattice relaxation behavior is used to support the identification of this center as a pair of adjacent silanol groups. These silanol pairs react during annealing to form a strained bond, thus increasing the susceptibility of fused silica to E′ formation. Excimer laser irradiation, like thermal treatment, is shown to directly reduce the number of relaxation centers in the material. The measured dependence of spin-lattice relaxation as a function of irradiation for identically prepared samples is consistent with silanol pair center consumption by a two-photon process.