THE ROLE OF PROTON NUCLEAR-MAGNETIC-RESONANCE SPIN-LATTICE RELAXATION CENTERS IN THE STRONG ABSORPTION TRANSITION AT 210 NM IN FUSED-SILICA

The performance of fused silica for deep ultraviolet optical applications is adversely affected by a radiation-induced absorbance centered at 210 nm which is attributed to the formation of E' centers. In this work, the H-1 nuclear magnetic resonance spin-lattice relaxation rates, 1/T1 of various types of unirradiated fused silica were shown to be correlated to variations in the transmission at 210 nm (T210) that occurs upon irradiation. High concentrations of spin-lattice relaxation centers correlate with the ability to withstand larger numbers of 248 nm laser pulses before a sudden drop in T210, known as the strong-absorption transition (SAT), occurs. If irradiation is halted prior to the SAT, higher concentrations of these centers also correlate with faster rates of partial T210 recovery. We propose that these centers are diamagnetic defects consisting of an adjacent pair of silanol groups that release mobile hydrogen upon irradiation. This hydrogen can reversibly passivate E' centers, thus accounting for the differences in partial recovery rates of T210 prior to SAT. We also propose the onset of the SAT corresponds to the consumption of all the irradiation susceptible silanol pair defects, after which no partial recovery of T210 is observed when laser irradiation has ceased.