Transient absorption in excimer-exposed silica.

The transient absorption produced in high-purity fused silica by exposure to a 193-nm excimer laser is investigated as a function of exposure, dissolved molecular hydrogen content, and hydrogen-related processing. Long-term recovery of transmittance was found to correlate with the dissolved molecular hydrogen concentration, whereas short-term fade was due to geminate recombination of an E? center with an H radical. The redarkening process was shown to be the result of photolysis of SiH, which regenerates color centers. When the silica was processed in a hydrogen atmosphere at high temperature and subsequently exposed at 193 nm, the glass was found to produce an absorption spike, a fast, recoverable decrease in transmittance. The origin of the spike was linked to the creation of a precursor produced in the thermal reaction of silica with hydrogen. The precursor can be identified by its signal in the Raman spectrum. It is suggested that the precursor has absorption at 193 nm.