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

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 $\langle $100$\rangle $. Preferential alinement of the V$_{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 degrees) 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 degrees) 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 $\langle $111$\rangle $, involves an interstitial fluorine ion and is named the V$_{H}$ centre. Other centres with molecular axes close to $\langle $100$\rangle $ appear to be stabilized by impurity ions and are labelled V$_{KA}$ and V$_{KA^{\prime}}$ 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$_{KA^{\prime}}$, 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.