F-Aggregate Centers in Sodium Chloride. I

Abstract

Thermal aggregation of electron excess color centers in NaCl, x irradiated at 80°K, was studied by observation of changes in optical absorption in pulse annealing experiments. An initial 80 h x irradiation introduced large concentrations of F, F′ and positive hole centers. As annealing temperatures were raised (80 → 235°K) recombination of thermally released positive holes with electrons in F centers produced α centers. Thermal aggregation of F centers occurred above 240°K when α centers become mobile. M+ centers (F2+) form and disappear and M centers (F2) increase rapidly in number near 265°K as a result of the reactions F + α → F2+ and F2+ + e → F2. R+ centers (F3+) form by the reaction F2 + α → F3+ below 320°K and disappear slowly by the reactions F3+ + e → F3 and F3+ + heat → F2 + α, above 330°K. Optical absorption band maxima at 80°K are: M+, 1.22 eV (1015 nm); M1, 1.74 eV (713 nm); R+, 1.80 eV (690 nm); R1, 2.28 eV (543 nm) and R2, 2.07 eV (600 nm). N1 and N2 band systems were observed but were not studied in any detail. M+ centers also were formed by direct ionization of M centers by 300-nm light. The concentration, however, was low because of a counterbalancing of photoionization and electron capture processes. R+ centers were formed in x-irradiated NaCl by excitation with light absorbed in the R2 band. They could be transformed back to R centers by x-irradiation or F center bleaching.