The surface effects of alkali halides in the infrared

The surface state extension of the shallow donor theory is reviewed. The surface state theory and its applications to F-centers of alkali halides is presented. Experimental infrared data show an absorption which seems to correspond with the ground state to first excited state transition. Surface state concentrations of F-centers as a function of depth into the crystal are given. The ratio of surface to bulk cross sections in KBr and experimental calculations of effective masses in various alkali halides are in excellent agreement with theory. The theoretical cross sections are derived from transition probabilities. However, lack of excited states, the apprearance [sic] of a side band at slightly higher energy and the angular polarization measurements of absorption cannot be supported by surface state theory and alternate explanations involving thin films of alkali halides are presented. The surface bands do not appear if the alkali halides are irradiated under vacuum. The bands can only be formed if atmospheric gases are present. It is found that only irradiation of the gas near the surface of the alkali halide is necessary to form the surface bands and that these bands are independent of color center formation. The band with the largest absorbance, the main band, appears approximately at 1370 cm⁻¹. The small side band appears at approximately 1440 cm⁻¹. A model with the bands as the longitudinal-optic and transverse-optic infrared modes associated with thin films of alkali halides is presented. Angular polarization results agree fairly well for the main band but contradicted the theory for the side band. A model with adsorbed molecules on the surface is presented but fails to give quantitative agreement with the angular polarization dependence observed. However, it does give reasonable values for the refractive index of the thin film. A model of an absorbing thin film on a transparent substrate is presented. This model explains the angular polarization dependence of the absorption as a function of incident angle very accurately. It also yields reasonable values for the index of refraction of the thin film, film thickness and absorption coefficient --Abstract, pages ii-iii