EPR AND LUMINESCENCE STUDIES OF LAF3 AND CEF3 UNDER X-RAY AND LASER IRRADIATION

After X-ray irradiation below 70 K, LaF3 exhibits a 3-line EPR resonan ce consistent with self-trapped holes (Vk centers) on the fluorine sub lattice. Specifically, we conclude that the V-k centers are formed on pairs of fluorine ions making 15 degrees angles to the c-axis. At temp eratures below 200 K, the spectrum of X-ray excited luminescence is do minated by 5d-4f transitions of Ce3+ impurity present at 4 ppm. Near r oom temperature, the transfer to the Ce3+ ceases within detectability, revealing a weak band suggested in other work to be self-trapped exci ton luminescence. Above 400 K, the 5d-4f transitions of Ce3+ again dom inate the spectrum, implying resumption of efficient energy transfer f rom the uniformly distributed X-ray excitations in the LaF3 crystal to the dilute impurity. In CeF3, a 7-line hyperfine EPR pattern at g = 2 , with isotropic angular dependence, is shown to be composed of indepe ndent 3-line and 4-line patterns possessing different thermal and micr owave power saturation properties. Such patterns arise from a single e lectron (or hole) interacting with 2 and 3 equivalent spin-1/2 nuclei, respectively.