PHOTOCHEMICAL-REACTIONS IN GEO2-SIO2 GLASSES INDUCED BY ULTRAVIOLET-IRRADIATION - COMPARISON BETWEEN HG LAMP AND EXCIMER-LASER

GeO2-SiO2 glasses prepared by vapor-phase axial deposition were expose d to ultraviolet (uv) radiation from a Hg discharge lamp (4.9 eV) and excimer lasers (KrF laser: 5.0 eV, XeCl laser: 4.0 eV). Two photochemi cal reaction channels were ascertained: (1) The exposure of the glasse s to the Hg lamp radiation (similar to 16 mW/cm(2)) induced Ge E' cent ers accompanied by bleaching of the absorption band due to oxygen-defi cient defects near 5 eV (5-eV band) and the emergence of an intense ba nd near 6.4 eV. (2) The irradiation with KrF and XeCl lasers (power de nsities of 10 and 90 mJ/cm(2)/pulse, respectively, pulse duration of 2 0 ns) generated two types of paramagnetic defects, electron trapped ce nters associated with fourfold coordinated Ge ions (GEC) and a self-tr apped hole center (STH: bridging oxygen trapping a hole). The former a nd the latter were considered to be caused via one-photon and two-phot on absorption processes, respectively. These alternative reactions pro ceeded independently depending on the power densities of uv photons. T he formation of GEC's was saturated easily by irradiation with KrF las er pulses, and then the conversion of GEC to Ge E' centers was caused by prolonged irradiation.