Photon energy dependence of vacuum ultraviolet light-induced stress in silica glass

Abstract

The photon energy dependence of vacuum ultraviolet light-induced stress was investigated for two types of silica glass, type I (GE214) and type III (Suprasil P20). Samples of both glass types were irradiated under three conditions: (1) with xenon (Xe) arc light directly, (2) through three kinds of filtered Xe arc light, and (3) with a high-pressure mercury (Hg) lamp. To evaluate the samples, the absorption spectra were measured in vacuum ultraviolet (VUV), UV, and infrared regions. The irradiation-induced stress was also measured. The relative shape of the Xe arc spectrum was derived, and the absorption spectra of GE214 and Suprasil P20 were determined in the VUV region at the Xe lamp operation temperature. The measured stress was converted to strain energy, which was then corrected with respect to the hydroxyl group (≡ SiOH) content and the irradiation time. The results revealed that the corrected strain energy was linearly proportional to the amount of absorbed light within the measurement error. The photon energy threshold for inducing the strain was evaluated to be 5.6 eV for GE214, at which the absorption coefficient was ∼10 cm−1. In contrast, light absorption occurred at photon energy not less than 6.5 eV for Suprasil P20. Therefore, there is a photon energy dependence on the VUV induced stress. This dependence varies not only with the type of silica glass but also its ambient temperature which, in turn, controls the absorption coefficient. In fact, the estimated absorption coefficient at 7.5 eV and at 743 K for GE214 was about two orders of magnitude higher than that measured at room temperature. The bond cleavage and rearrangement of the silica glass network should be enhanced by heat. The analysis of these photon energy dependences also suggested that Suprasil P20 was strained more effectively than GE214, probably due to absorption of VUV light by ≡ SiOH s contained in silica glass.