PHOTOCHROMIC MATERIALS RESEARCH.

The photoconductive properties of CaF2 and SrTiO3 and the optical properties of CaF2 were investigated in order to better characterize photochromic behavior. It has been found that there is charge transfer during switching that is detectable as a photocurrent in both systems however, the photoconductive sensitivity is quite low due to the large density of photochromic trapping centers. Room temperature photo-Hall measurements on CaF2Tb show that charge transfer is via the conduction band with a Hall mobility of 30 sq cmV.sec. The thermal depth of the metastable centers has been found to be between 1.10 and 1.20 eV for CaF2Ce, CaF2Tb, and CaF2Gd. The emission cross section the product of the photogeneration efficiency and the absorption cross section for the metastable center at the peak of its absorption band appears to be about a decade smaller than that of the stable center in the 30,000cm region. Photoconductive models for the metastable and stable centers are discussed. The effect of an externally applied magnetic field upon the absorption of circularly polarized light circular dichroism has been used to study the physical nature of the thermally stable photochromic center in rare earth doped CaF2. The results are discussed in terms of the orbital and paramagnetic states of the center. The principal result is that the center involves a rare earth ion. Author