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

This is the authors' abstract. We don't add key points for this paper.

The e. p. r. and optical absorption spectra of the self-trapped hole (Vk ) centres in CaF2, SrF2 and BaF2 have been measured and it is shown that the molecular axes of these centres are alined along <100>. Preferential alinement of the Vk centres along a particular cube edge is readily produced by bleaching into the optical absorption bands of the Vk 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 Vk centres is monitored by e. p. r. methods and it is shown that, in CaF2, linear (0°) motion of the Vk centres predominates over the entire decay range (85 to 138 K). In SrF2 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 BaF2 only random motion is observed. The kinetics of reorientation and decay of the Vk centre are investigated. A band observed in the X -ray fluorescence of CaF2 (at 279 nm), SrF2 (at 300 nm) and BaF2 (at 310 nm) is assigned to emission of the self-trapped exciton by comparison with a similar band in the recombination luminescence of the Vk centre (due to e-+ Vk ). 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 Vk 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 VH centre. Other centres with molecular axes close to <100> appear to be stabilized by impurity ions and are labelled VKA and VKA' centres. The VH centre occurs in CaF2, SrF2 and BaF2, the VKA centre in SrF2 and BaF2, and the VKA’ centre only in BaF2. The kinetics of reorientation and decay of the VKA centre have been investigated. We give a general discussion of the properties of hole centres, including the H centre, in alkaline earth fluorides.