Photochemical processes induced by 157-nm light in H_2-impregnated glassy SiO_2:OH

${\mathrm{F}}_2$ excimer-laser irradiation induces two major changes in ${\mathrm{SiO}}_2:\mathrm{OH}$ glass impregnated with ${\mathrm{H}}_2$ molecules. First, the vacuum–UV optical absorption edge is bleached, and the absorption at 157 nm decreases from 0.95 to $0.68 {\mathrm{cm}}^{-1}$. Second, preexisting free SiOH groups and interstitial ${\mathrm{H}}_2$ 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.