An investigation of trapped holes and trapped excitons in alkaline earth fluorides

Abstract

The e. p. r. and optical absorption spectra of the self-trapped hole (V k ) centres in CaF 2 , SrF 2 and BaF 2 have been measured and it is shown that the molecular axes of these centres are alined along <100>. Preferential alinement of the V k centres along a particular cube edge is readily produced by bleaching into the optical absorption bands of the V k centre at 77 K with linearly polarized light. The anisotropy created in the crystals by preferential alinement is detected in both the e.p.r. and optical spectra. The thermally induced decay of the optically alined V k centres is monitored by e. p. r. methods and it is shown that, in CaF 2 , linear (0°) motion of the V k centres predominates over the entire decay range (85 to 138 K). In SrF 2 linear motion predominates in the early stages of decay ( ca . 110 K) but random (90°) motion sets in at higher temperatures ( ca . 120 K). In BaF 2 only random motion is observed. The kinetics of reorientation and decay of the V k centre are investigated. A band observed in the X -ray fluorescence of CaF 2 (at 279 nm), SrF 2 (at 300 nm) and BaF 2 (at 310 nm) is assigned to emission of the self-trapped exciton by comparison with a similar band in the recombination luminescence of the V k centre (due to e - + V k ). The temperature dependence of the X -ray fluorescence intensity is described. The polarization properties of the recombination luminescence are investigated on crystals containing optically alined V k centres. The e. p. r. and optical absorption spectra of trapped hole centres involving other lattice defects are also described. One such centre, with its molecular axis close to <111>, involves an interstitial fluorine ion and is named the V H centre. Other centres with molecular axes close to <100> appear to be stabilized by impurity ions and are labelled V KA and V KA' centres. The V H centre occurs in CaF 2 , SrF 2 and BaF 2 , the V KA centre in SrF 2 and BaF 2 , and the V KA’ centre only in BaF 2 . The kinetics of reorientation and decay of the V KA centre have been investigated. We give a general discussion of the properties of hole centres, including the H centre, in alkaline earth fluorides.