SITE SYMMETRIES, BLEACHING BEHAVIOR, AND THERMAL-STABILITY OF HYDROGENIC CENTERS IN SRF2-PR3-PR3+( AND CAF2)

Five multihydrogenic Pr3+ centers are found in both SrF2 and CaF2. All of these centers exhibit permanent bleaching at 10 K following optica l excitation. This bleaching is caused by the migration of vibrational ly excited hydrogenic ions into different lattice positions. Some of t he excited centers are reoriented to produce equivalent centers. Alter natively, new centers with different excitation energies are created. All bleaching effects can be reversed by warming the whole crystal abo ve 120 K. Laser selective excitation spectroscopy was used to measure energy levels of the D-1(2), P-3(0), P-3(1), and I-1(6) multiplets of these multihydrogenic centers. The Pr3+ site symmetries for all five c enters were determined from the polarization behavior of their electro nic transitions. This required the derivation of tables of polarizatio n ratios for the C-4 upsilon, C-2 upsilon, and C-S symmetry groups. Mo dels are proposed for the multihydrogenic centers and their photoprodu ct centers which are consistent with these Pr3+ Site symmetries and ac count for the observed bleaching behavior, The D-1(2) fluorescence lif etimes of the centers were measured and support the models presented. The temperatures of fluorescence recovery for bleached centers and of fluorescence depletion for photoproduct centers were measured and foun d to be independent of the hydrogen isotope present. These temperature s were used to determine the barrier potentials which must be surmount ed when centers revert to their original configurations. [S0163-1829(9 8)01142-4].