The cross-luminescence (CRL) resulting from radiative electronic transitions from the mainly anion-related valence band to the uppermost cation core band has been studied in a number of complex halides containing CsCl, RbF, KF, and BaF2. The energy of the CRL photons is determined by the energy difference of the two bands, and the shapes of the spectra reflect the grouping of the molecular orbitals in the clusters involving cations with a hole in the core shell and nearest-neighbour halide ions. For crystals with more than one type of cation, the spectrum reveals information about the 'active' cation which contains the core hole and also about the other cations. Correlations have been established between the local symmetry of the clusters in different crystals and the number and position of the subbands in the CRL spectra.

The introduction of Gas+ ions into cc-Al,O, crystals results in a new complex luminescence band at about 5 eV. The main optjcal excitation maximum is observed a t 8.5 eV indicating the bound exciton nature of the excited state. I n the cathodoluminescence spectra a t 80 K three sub-bands can be detected with maxima at 4.61, 5.15, and 6.58 eV. The luminescence decay in all the three bands has two exponential components -a short one with T = (15 & 1) ns and a long one with T about 1 4 s . The Ga-related luminescence can be observed under the excitation of X-rays up to 600 K; a drop in intensity takes place at about 210 K corresponding to the Ga-related TSL peak. The activation energy of the release of electrons a t this peak as determined by the fractional-

The vacuum ultraviolet luminescence of pure and doped MgO crystals excited by 10 keV electrons is studied in the temperature region 80 to 450 K. At 80 K the narrow edge luminescence band at, 7.65 eV due t o radiative decay of free excitons dominates in the emission spectrum of undoped crystals. At thermal quenching of this luminescence with a n activation energy of (0.037 5 0.005) eV, another luminescence band with maximum a t 6.9 eV increases. The activation energy of this increase is (0.039 & 0.005) eV. No relation of the 6.9 eV luminescence band with the presence of impurities and defects could be established and it is suggested to be due to radiative decay of relaxed (self-trapped) excitons. MCCJIeAOBaHa BaKyyMHaR yJIbTpa#MOJIeTOBaH