Quantenausbeute für den photochemischen Abbau von O−−-Zentren in KCl
Potassium chloride crystals containing various amounts of O−− centers are irradiated with ultraviolet light into their bands at 2.92 eV, 4.25 eV, and 5.85 eV. It is shown that the photochemical reaction is the only one, which takes place. From the strong linear relation between the decrease of the O−− bands and the increase of the F band the concentration of the O−− centers can be derived. WhenK means the absorption constant in the maximum of the 4.25 eV band at 20 °C, the concentration [O−−]=(2.6± 0.1) · 1016 cm−2 ·K.Irradiation into the long wavelength band yields no decomposition. Irradiation into the shorter wavelength bands above 220 °K results into thermal activated ionisation of the O−− center. At higher temperatures the quantum yield for decomposition of the O−− center converges to 0.36 and not to one. This can be interpreted in such a way that immediately after the ionisation there is a high probability for radiationless retrapping the electron by the same center into its ground state. The thermal activation energies are 0.6±0.1 eV for the 4.25 eV band, and 0.3±0.1 eV for the 5.85 eV band.Long time irradiation shows that an equilibrium is reached after about 60% of the O−− centers have been decomposed. This is interpreted as coming from the inversion of the reaction: F centers are ionized simultaneously with the O−− centers by absorbing light in the L band region.