F-Aggregate Centers in Sodium Fluoride. I

Publisher Website
Google Scholar
Abstract
Thermal aggregation of color centers in a sample of NaF, x irradiated 67 h at 200°K, was studied by observation of the optical‐absorption spectrum during warming. At 200°K, a very intense F′ band (470 mμ) is formed and obscures the M1 band of any M centers which may be present. During warming the F′ band decreases to half‐intensity at 280°K but some remains even at 320°K. M centers begin to appear at ∼270°K, preceded by the appearance of a band at 725 mμ which forms and then thermally bleaches. The 725‐mμ band is assigned to M+ centers resulting from capture of α centers by F centers. R+ centers (λ=max520 mμ) are also observed to form during warming. Thermal aggregation to yield M centers is not nearly as extensive in NaF as in KCl, nor does the M1 band grow on standing in the dark at room temperature. Bleaching the sample with F‐band light at room temperature produced large R1, R2, R+, and M1 bands as well as six smaller bands (576, 604, 630, 736, 870, and 1040 mμ), measured at 65°K. Zero‐phonon lines are associated with the R2, R+, 604‐, 870‐, and 1040‐mμ bands. Additional experiments on samples containing M+ centers indicate that the M+ band exhibits a dichroism parallel to that of the M1 band and may be bleached at low temperature by x‐ray‐released electrons. The M+‐band maximum shifts from 726 mμ (273°K) to 736 mμ (80°K) as temperature is decreased, while the half‐width (0.18 eV at 80°K) behaves in the usual way.