Farbzentren und Kolloide in abschreckend kondensierten Schichten von NaJ, CaF2 und SrCl2

Abstract

By a simultaneous evaporation of NaI with excess alkali or alkaline earth metal onto a cooled substrate (T K =20–150 K) highly disordered thin films are produced, which contain electron excess centers up to a concentration of more than 1020 cm−3. A complex optical absorption band peaking at 600 and 530 nm is found. The 600 nm band is mainly due to unperturbed F centers, whereas the 530 nm band is ascribed to perturbed F centers, whose symmetry is lowered to C3v by a nearby Frenkel defect. Another optical transition of this F* center appears at about 600 nm in the dichroic spectrum, which is induced by polarized illumination at 20 K. The transition energies of the F* center are calculated by means of a semi-continuum model. A photochemical transformation of F* and F centers into one another is possible at 20 K. AboveT K =220 K the excess metal is forming colloidal centers. Two absorption peaks at about 600 and 400 nm, respectively, are ascribed to different colloids, which are either embedded in the crystalline material (600 nm) or only adsorbed to the crystallites (400 nm). By annealing up to 350 K a particle growth and transformation take place. Amorphous quenched films of CaF2 and SrCl2 with excess alkaline earth metal reveal mainly colloidal absorption (except a weak F band in CaF2). The colloids disappear by annealing above 150 K. This is believed to be closely connected with the crystallisation of the films.