Ionic thermocurrent and optical absorption studies of S−−–vacancy dipoles in KI

The KI:S– system has three absorption bands. By irradiation at −30 °C into the two short wavelength bands in the uv, a photochemical reaction takes place. The decomposition of S−−–vacancy dipoles and the simultaneous formation of F-centres is observed by optical and by ITC measurements. No photochemical reaction is observed when the crystal is irradiated in the near uv absorption band (368 nm). However, this irradiation in the presence of an electic field increases the polarization due to S−−–vacancy dipoles, the orientation of which is already frozen in the lattice. The resulting increase of the ITC peak is a function of the irradiation temperature. It shows a maximum at 179 K which lies below the ITC peak (204 K). The effect is also studied as a function of the irradiation time and the strength of the electric field. A model is proposed which explains such an increase of the ITC peak.