Distinctive features of diffusion-controlled radiation defect recombination in stoichiometric magnesium aluminate spinel single crystals and transparent polycrystalline ceramics.

MgAl2O4 spinel is important optical material for harsh radiation environment and other important applications. The kinetics of thermal annealing of the basic electron (F, F+) and hole (V) centers in stoichiometric MgAl2O4 spinel irradiated by fast neutrons and protons is analyzed in terms of diffusion-controlled bimolecular reactions. Properties of MgAl2O4 single crystals and optical polycrystalline ceramics are compared. It is demonstrated that both transparent ceramics and single crystals, as well as different types of irradiation show qualitatively similar kinetics, but the effective migration energy Ea and pre-exponent D0 are strongly correlated. Such correlation is discussed in terms of the so-called Meyer-Neldel rule known in chemical kinetics of condensed matter. The results for the irradiated spinel are compared with those for sapphire, MgO and other radiation-resistant materials.

Resonant inelastic x-ray scattering and UV-VUV luminescence at the Be 1s edge in BeO.

We carried out a combined study of UV-VUV luminescence and resonant x-ray emission from BeO single crystals with incident photon energies in the vicinity of the Be 1s absorption edge. The x-ray emission spectra show that at the Be 1s photoabsorption edge the lattice relaxation processes in the excitation site take place already on the timescale of the radiative decay of the core excitation. Comparison of the x-ray emission and the luminescence spectra indicates that the maximum energy loss of the process of lattice relaxation during the decay of inner-shell holes is similar to the loss that occurs in the self-trapping process of valence excitons. The possible decay channels of core excitations have been discussed and the mechanism for the creation of 5.2 eV luminescence at the photoabsorption resonances has been suggested.

Luminescence properties of undoped LiBaAlF6 single crystals.

This paper presents the results of the study of electronic excitations in undoped LiBaAlF(6) single crystals by means of luminescence spectroscopy and complimentary optical methods. The intrinsic emission at 4.2 eV due to self-trapped excitons was identified. The fast nanosecond defect-related luminescence was revealed at 3.0 eV. Both emissions degrade under electron beam irradiation, the most probable reason of which is defect creation introducing an additional non-radiative relaxation channel prohibiting energy transfer to luminescence centers. These defects can be recovered and luminescence intensity restored at higher temperatures (>200 K). The permanent damage by electron beam irradiation results only in overall growth of the absorption coefficient in the whole 1.5-6.5 eV spectral region studied. The analysis of thermally stimulated luminescence glow curves in the temperature range of 5-410 K revealed two shallow charge carrier traps with the activation energies of 0.22 and 0.33 eV, respectively. The luminescence of an impurity peaked at 2.5 eV was found and tentatively assigned to an oxygen-related emission center.

Luminescence of singlet self-trapped excitons in MgF(2).

Fast (τ∼1.7 ns) broadband (full width at half-maximum = 1.1 eV) vacuum ultraviolet (VUV; hν = 8.4 eV) luminescence from MgF(2) crystals has been detected at low temperature (T<80 K) and analyzed with techniques of cathodoluminescence and time-resolved VUV spectroscopy using synchrotron radiation. The VUV emission discovered has been attributed to the luminescence of spin-singlet self-trapped excitons (STEs). In contrast to the case for alkaline earth fluoride crystals with fluorite structure, the luminescence of the singlet STEs in MgF(2) shows smaller Stokes shift than that of the triplet STEs, which is similar to 'typical' behavior observed for alkali halide crystals.