Thermal Formation of F2+ Centers in KCl Crystals

Thermal formation of F 2 + centers in KCl crystals has been investigated in temperature region between 210°K and 270°K. KCl crystals containing electron traps such as Tl + ions were X-rayed at room temperature and was illuminated with F light at 120°K. Negative ion vacancies thus produced from F centers began to move about 220°K and combined with F centers to from F 2 + centers. The activation energy for the F 2 + -center formation was estimated to be 0.52±0.03 eV. An absorption band at 1.35µ at room temperature was confirmed to arise from F 2 + centers and to shift to 1.40µ at liquid helium temperature. The frequency of the lattice vibration influencing the ground state of the F 2 + center was 2.9×10 12 cps, which was almost the same value with that of the M center. The F 2 + -center formation is discussed by assuming the covalent force acting between an F center and a negative ion vacancy.