Mechanism of photochromism in oxide glasses containing a large amount of CdO or ZnO

UV-induced optical and ESR absorptions of borosilicate glasses containing a large amount (⪯ 50 mol%) of CdO or ZnO were measured at room temperature and 77 K while changing the wavelength of the illuminating light. The induced optical absorption was one order of magnitude stronger at 77 k than at room temperature, and the most intense absorption was observed for 340 nm irradiation. At 77 K peak wavelengths of the induced band changed with changing illuminating wavelength; from 420 nm on 300 nm irradiation to 600 nm on 360 nm irradiation. Tailing of the bands to the near-infrared region was pronounced. The induced optical absorptions were photo-bleached at 77 K by illumination with light whose wavelength is in the induced absorption bands. The lineshape of the bleaching spectra, a difference between the spectra before and after illumination, changed with changing illumination wavelength. Three types of ESR signals, signals (a), (b) and (c), were induced simultaneously with the optical absorptions. The dependencies of intensities of the ESR absorptions on the illuminating temperature and wavelength were found to be similar to those of the induced optical absorptions. From the observed g-tensors of the ESR signals, signal (a) was assigned to a trapped hole on a nonbridging oxygen neighboring on Si, signal (b) was assigned to a trapped hole on oxygen(s) surrounded by several Cd ions and having no direct bonding to Si or B, and signal (c) was assigned to a trapped electron delocalized over several Cd ions. It was found that the hole centers give rise to the optical absorption in the visible region and the electron center absorptions were in the visible and near-infrafed regions.