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Excitation spectra of CaF2, SrF2 and BaF2 crystals doped by CdF2, LaF3, YF3 in vacuum ultraviolet region were investigated at liquid-helium temperature and at room temperature. Spectra of doped BaF2 at low temperature and CaF2, SrF2 at room temperature show significant decrease of excitation intensity against those in undoped crystals while the intensities in exciton regions remain nearly the same. The common reason of the suppression of exciton emission by impurity in alkaline-earth fluorides is unradiative electron-hole re- combination on impurity sites. [ABSTRACT FROM AUTHOR]

Abstract: A comparative analysis of the electron–hole (e–h) excitation mechanism of impurity luminescence has been performed for wide-gap oxides (WGO) and alkali halides crystals (AHC). A low quantum yield of scintillation in WGO with respect to that in AHC is connected with the high value of the average energy needed for the creation of an e–h pair in WGO. In AHC and many WGO, the width of a valence band is smaller than the energy gap and hot valence holes are not able to create any secondary e–h pairs. The ratio that describes “hole losses” in scintillators is 1.5–2.0 times higher in WGO than in CsI. The scenario of the two-step transformation of an e–h pair or an anion exciton into two emitted photons in α-Al2O3:Mg2+,Ti4+ has been considered. [Copyright &y& Elsevier]

Characteristic dimensions and evolution times of regions of secondary electronic excitations created by the interaction of ionizing radiation with matter cannot be measured directly. At the same time these are essential parameters both for engineering of nanostructured composite materials defining optimal layer thickness and nanoparticles radii and for the development of optimized scintillators. The paper demonstrates how such spatial and temporal data can be extracted from luminescence decay kinetics excited by vacuum ultraviolet (VUV) and X-ray photons at modern sources of synchrotron radiation MAX IV and PETRA III. Specific features of energy-band structure of self-activated crystal CeF3 are discussed, and its potential for a super-fast detection of ionizing radiation evaluated. Diffusion-controlled dipole–dipole interaction of Frenkel excitons is demonstrated to account well for the luminescence non-exponential decay kinetics providing information on the scales of excited regions created by photons of different energy. For 20 eV photons the radius of excited regions is estimated to be 10 nm, and for 200 eV photons it increases to 18 nm. Effective radius of excited regions of complicated shape created by 19 keV is as large as 80 nm and the diffusion length of Frenkel excitons over radiative time is 14 nm. [ABSTRACT FROM AUTHOR]

Mixed nanopowders of transition alumina prepared by combustion synthesis and phase pure ultra-porous α-alumina by oxidation method were investigated using low temperature time-resolved cathodoluminescence and photoluminescence spectroscopy under VUV-XUV excitation. In all samples along with the 7.6 eV emission of self-trapped excitons of α-alumina, luminescence bands due to F, F + centres with maxima at 3 and 3.8 eV and other UV–visible luminescence bands of intrinsic and extrinsic origin with varying intensity depending on sample preparation method and thermal treatment were studied. In alumina nanopowders the excitonic excitation peak at ∼9.1 eV near fundamental absorption edge is shifted to the higher energies by 0.15 eV in comparison with the same feature in single crystals. The nanostructure of alumina is responsible for this shift. [ABSTRACT FROM AUTHOR]

CdWO 4 :Sm single crystals were studied by optical, electron-paramagnetic and thermoluminescence spectroscopy in the temperature range of 4–750 K. A hopping diffusion of self-trapped excitons and energy transfer to Sm3+ centres is demonstrated. The mechanism of energy transfer includes the formation of a Sm3+ bound exciton either by direct excitation or in the result of a thermally stimulated hopping diffusion of self-trapped excitons as well as a sequential trapping of charge carriers near the Sm3+ ions. The energy transfer from the perturbed excitons to Sm3+ ions is controlled by an electric dipole-dipole or exchange interaction and it is terminated in the temperature range of 450–650 K due to the thermal destruction of Sm3+-perturbed excitons. In the case of near-surface excitations created in the energy region of high absorption coefficients by 5-eV photons, the Sm3+ emission is additionally quenched by surface-induced quenching centres. • Energy transfer processes were studied in Sm3+ doped CdWO4 crystals. • Self-trapped excitons are thermally quenched due to hopping diffusion in CdWO 4. • Self-trapped excitons transfer energy to Sm3+ ions via Sm3+-bound excitons. • Sm3+-bound excitons are formed by trapping self-trapped excitons or charge carriers. • Energy transfer is terminated due to the destruction of bound excitons at T > 400 K. [ABSTRACT FROM AUTHOR]