Experimental results on a new O-2 center in CaF2 crystals

The model of a perturbed O-2 center in CaF2 is discussed on the basis of results from optical absorption, resonant Raman scattering, EPR and ENDOR measurements. Compared to the unperturbed center [4] the optical absorption of the presently discussed one is shifted to the visible and it exhibits a multiphonon structure. The O-2 stretching mode energy in the electronic excited state deduced from this structure is found to be 900 cm-1. A resonant Raman effect is observed up to 2nd order yielding the vibrational energy in the electronic ground state. The determined value amounts to 1 038 cm-1. The Raman selection rules are consistent with a molecular vibration along [l, 0, 0]. The ENDOR data are coherent when the center is assumed to be located near a fluorine cube edge probably substituting for two F- ions. At room temperature the center exhibits an orthorhombic g tensor in EPR. At higher temperatures, however, motional effects show up. The EPR spectrum becomes axial indicating vibrational motion of the center