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Scintillators with ultrafast response are of demand in medical imaging for time-of-flight positron emission tomography, in high-energy physics calorimetry, and in other applications. There are several fast luminescence processes in solid-state materials, which could be good candidates for fast timing applications. The development of new ultrafast silicon photomultipliers with increased vacuum-ultraviolet to ultraviolet (VUV-UV) sensitivity has renewed interest to one of those, cross-luminescence (CL), emitted in the VUV-UV region with subnanosecond decay time. Previous investigations have shown that CL is mostly situated in the VUV region in binary K and Ba compounds, but ternary fluorides with a complex valence band structure can provide ultrafast emissions in UV. According to the band structure calculations for K2SiF6, its complex valence band is composed of the F 2p, Si 2s, and Si 2p states. Transitions between the valence subbands and the outermost K 3p core band may extend CL from the VUV-UV to the visible region. This motivated us to perform spectroscopic study of phase-pure K2SiF6 micropowders with a high degree of crystallinity. The stationary (10 keV) and pulsed (100 keV, δ t ≈ 55 ps) electron beams were applied in the studies of relaxation processes of K2SiF6 in the temperature range of 77–300 K. At 300 K, the dynamics of ultrafast processes was studied at the FemtoMAX beamline (MAX IV Lab) providing 10-keV X-ray pulses of 100-fs duration. As a result, we revealed multiple CL bands extending from 9 to 4 eV. A nonexponential decay in the subnanosecond time range was revealed for the CL bands at 4.5 and 5.25 eV under both pulsed electron-beam and X-ray excitations. This opens prospects for a band structure engineering approach in developing new ultrafast scintillators with intrinsic emissions. This approach is based on a targeted modification of material composition to influence the density of states in complex valence bands and to provide a favorable shift of CL to lower energies. [ABSTRACT FROM AUTHOR]

Abstract: Absorption, emission and excitation spectra of 50 MeV electron beam irradiated and as-grown YAG single crystals were studied and compared in the 10–300 K temperature range using time-resolved luminescence spectroscopy under UV/VUV/XUV excitation by synchrotron radiation and cathodoluminescence. The emission spectra consist of intrinsic (excitonic) and defect related non-elementary bands in the VIS/UV range. It is shown that fast electrons create stable F and F+ color centers with characteristic emission and absorption bands in the visible/UV range. Induced absorption caused from these defects starts at 4.2 eV. Energy transfer from host to color centers is not an efficient process. [Copyright &y& Elsevier]

Hot intraband luminescence (IBL) was observed under excitation by high-power (120 keV, 10 A/cm 2 ) pulsed electron beam, low-energy (10 keV) and low-current (2 μA) continuous electron beam as well as pulsed x-rays. Thus, the fundamental possibility of IBL excitation by x-rays was confirmed, and for most studied materials the absence of the dependence of IBL spectral shape on excitation energy was revealed. For the first time, the intraband luminescence was monitored under low-power excitation, which confirmed the absence of a power threshold of its mechanism and completely ruled out the role of excitation density effects in IBL formation. The data obtained allow the prediction that the IBL can be excited by single photons of 511-keV energy, which is required for enhancing scintillation time resolution in TOF-PET. [ABSTRACT FROM AUTHOR]

Abstract: Luminescence spectroscopy in the VUV-visible range under electron-beam excitation and synchrotron radiation was applied to investigate electronic properties of alumina nanopowders, which were prepared using the combustion synthesis method. By varying reaction and post treatment conditions we were able to prepare phase pure samples and powders with mixtures of α- and γ-phases mainly. In addition to the well-known 7.6 eV luminescence of STE of α-alumina, all samples possessed complex emission bands in UV range (3–5 eV) which originate from intrinsic excitonic emissions and extrinsic electronic excitations. [Copyright &y& Elsevier]

Abstract: Interconfigurational 4f 105d ↔4f 11 transitions of the Er3+ ion in the YAG host were studied under both VUV photon (synchrotron radiation) and electron beam excitation. It was found that the lowest low-spin 5d level of the Er3+ ion has a rather large energy gap to the next lower 4f 2 D(2)5/2 crystal-field level, which results in a relatively low rate of nonradiative transitions from this 5d level leading to the appearance of weak spin-allowed 5d–4f luminescence at low temperature. The lowest high-spin 5d level, from which spin-forbidden 5d–4f radiative transitions could occur potentially, is situated only at ∼500cm−1 above the 4 D 1/2 level. Such close location allows fast depopulation of the 5d level resulting in the absence of spin-forbidden 5d–4f luminescence and appearance of 4 D 1/2 4f–4f luminescence. [Copyright &y& Elsevier]

• Crystalline K 2 GeF 6 particles were synthesized by co-precipitation. • Fast cross- and intraband luminescence with decay times 300–500 ps was observed. • Transitions between valence sub-bands (Ge 4s,4p, F 2p) cause intraband luminescence. • The decay of K 3p cation excitons results in the formation of Ge 4s,4p holes. • Electronic band structure parameters of K 2 GeF 6 were experimentally determined. Highly crystalline K 2 GeF 6 micropowders were prepared by co-precipitation in the hydrofluoric acid-water-alcohol solution and studied by means of the cathodoluminescence and time-resolved luminescence spectroscopy under synchrotron radiation excitation at 7 K. The nature of various intrinsic emissions was revealed. The luminescence band at 510 nm detected under excitation by photons with the energy above 9 eV is related to the radiative decay of self-trapped excitons. Fast emission bands with ~400 ps decay time found in the spectral range from VUV to visible (8–2.4 eV) are assigned to the cross-luminescence transitions from K 3p hole states and intraband luminescence transitions due to the presence of the Ge 4s, 4p valence states. The creation of cation excitons was detected in the energy range 18–20 eV. Their non-radiative decay results in the formation of hole states in the Ge energy bands, which finally recombine with electrons from the above lying valence states and provide fast intraband luminescence. Important electronic band structure parameters including the energy gap width of 11 eV, cation exciton formation energy 18.1 eV and ionization energy of cation states 20.0 eV were determined from the luminescence excitation spectra. [ABSTRACT FROM AUTHOR]