Photochemical processes induced by 157-nm light in H₂-impregnated glassy SiO₂:OH

F₂ excimer-laser irradiation induces two major changes in SiO₂:OH glass impregnated with H₂ molecules. First, the vacuum-UV optical absorption edge is bleached, and the absorption at 157 nm decreases from 0.95 to 0.68 cm^-1. Second, preexisting free SiOH groups and interstitial H₂ are photochemically converted to hydrogen-bonded hydroxyl groups. It is suggested that the bleaching of the UV-absorption edge is caused by a change of OH groups from a free to a hydrogen-bonded state and by photolysis of distorted Si - O bonds that are absorbing in the edge region.