V. Yu. Ivanov

Low-energy charge transfer excitations in NiO

Comparative analysis of photoluminescence (PL) and photoluminescence excitation (PLE) spectra of NiO poly- and nanocrystals in the spectral range 2-5.5 eV reveals two PLE bands peaked near 3.7 and 4.6 eV with a dramatic rise in the low-temperature PLE spectral weight of the 3.7 eV PLE band in the nanocrystalline NiO as compared with its polycrystalline counterpart. In frames of a cluster model approach we assign the 3.7 eV PLE band to the low-energy bulk-forbidden p-d (t1g(π)-eg) charge transfer (CT) transition which becomes the allowed one in the nanocrystalline state while the 4.6 eV PLE band is related to a bulk allowed d-d (eg-eg) CT transition scarcely susceptible to the nanocrystallization. The PLE spectroscopy of the nanocrystalline materials appears to be a novel informative technique for inspection of different CT transitions.

Luminescence properties of Li6GdB3O9:Ce crystal fibers upon their excitation in the range of 4d → 4f core transitions

We have investigated UV luminescence with a subnanosecond time resolution of Li6GdB3O9:Ce crystal fibers upon their ultrasoft X-ray selective excitation at 10 and 293 K in the range of 4d → 4f core transitions. We have revealed an intense fast-decaying subnanosecond luminescence component, which is caused by a high local density of electronic excitation and Auger core hole relaxation processes, and modulation of the luminescence excitation spectrum by an absorption band of the 4d–4f photoionization giant resonance in the energy range 135–160 eV.

Investigation of luminescent properties of 6Li-based fibers at soft X-ray excitation

The investigation of luminescent and scintillation properties of 6Li20-MgO-SiO2-Ce fibers is presented. The probable mechanisms of energy transfer to activator centers are discussed. The estimation of fibers absolute scintillation efficiency was implemented. The modeling of neutron registration efficiency in substitution of monocrystalline detector by a fiber one was carried out.

The influence of temperature on narrow I1 and I2 lines in the luminescence spectrum of Ni0.6Zn0.4O

The behavior of the luminescence spectrum of solid solution Ni0.6Zn0.4O, in which two intense narrow lines were recently discovered, is investigated as a function of temperature. It is shown that the intensity of one of the lines drops in accordance with the Mott law with increasing temperature in the range between 10 and 50 K. The lines experience broadening, and the ratio of their intensities changes. In addition, the lines shift toward lower energies. This shift and broadening of both lines occur differently, suggesting that they are of different nature.

Time-resolved photoluminescence of LaBr3:Ce scintillation crystals under ultrasoft X-ray excitation

Time-resolved X-ray-induced photoluminescence (PL) of LaBr3:Ce scintillation spectrometer crystals excited by synchrotron radiation with photon energies within 45–290 eV has been studied at 7.5 and 295 K. It is established that the PL response is essentially nonproportional to the exciting photon energy in the regions of photoionization of the Br 3d and La 4d levels and especially in the vicinity of the giant resonance. The PL decay kinetics in the spectral region corresponding to the d → f luminescence of Ce3+ ions is nonexponential. The decay time is smaller than 15 ns and depends on the exciting photon energy and radiation wavelength. The influence of the products of interaction of these hygroscopic crystals with atmosphere on the spectral and kinetic characteristics of X-ray-induced PL has been determined.

Self-trapping of the d-d charge transfer exciton in bulk NiO evidenced by X-ray excited luminescence

Soft X-ray (XUV) excitation did make it possible to avoid the predominant role of the surface effects in luminescence of NiO and revealed a bulk luminescence with a puzzling well isolated doublet of very narrow lines with close energies near 3.3 eV which is assigned to recombination transitions in self-trapped d-d charge transfer (CT) excitons formed by coupled Jahn-Teller Ni+ and Ni3+ centers. The conclusion is supported both by a comparative analysis of the CT luminescence spectra for NiO and solid solutions NixZn1 − xO, and by a comprehensive cluster model assignment of different p-d and d-d CT transitions, their relaxation channels. To the best of our knowledge, it is the first observation of the luminescence due to self-trapped d-d CT excitons.

Luminescence of LaBr3:Ce,Hf scintillation crystals under UV-VUV and X-ray excitation

The present study was carried out by means of the low temperature time-resolved luminescence UV–VUV spectroscopy as well as the optical and thermally activation spectroscopy. The Ce3+ centers in regular lattice sites and located in the vicinity of the point defects of crystal structure were observed. Spectral and kinetic characteristics of the luminescence of these centers were defined. A photoluminescence of new point defects of the crystal structure are also manifested. We evaluated the energy of the interband transitions as Eg ≈ 6.2 eV in LaBr3. At the excitation energies Eexc>13 eV (higher than 2Eg) the effect of the electronic excitations multiplication was detected.

Time-resolved luminescence of LaBr3-Ce scintillation crystals upon selective UV-VUV-XUV excitation

The time-resolved photoluminescence (PL) of LaBr3-Ce scintillation spectrometric crystals produced in Russia is measured upon excitation using synchrotron radiation with photon energies of 3.7–21 and 45–290 eV at temperatures of 295 and 7.5 K. The PL spectra and decay curves are measured for excitation in the transparency range, at the edge of fundamental absorption, at the interband transitions, and in the range of inner-shell absorption. It is demonstrated that the PL yield is not proportional to the excitation energy, and that the PL decay curves are modified in the range of photoionization of the 3d (Br) and 4d (La) inner shells and, especially, in the range of giant resonance.

Vacuum ultraviolet and X-ray emission spectroscopy of anion and cation excitons in oxide crystals

The results of a combined experimental investigation into the radiative relaxation of anion and cation excitons during selective vacuum ultraviolet-(up edge) and soft X-ray (core) excitation of berylliumcontaining BeO, Be2SiO4 and La2Be2O5 crystals are presented. They indicate that the relaxation of anion and cation excitons in multi-component oxides occurs in the same local fragments of the crystal lattice, due to their higher predisposition (in comparison with other fragments) to short-lived distortions. It has been found that in La2Be2O5 crystals characterized by a considerable difference in the ionic radii of cations (beryllium and lanthanum), the beryllium-oxygen tetrahedra do not participate in exciton relaxation processes.

Low-temperature luminescence and thermally stimulated luminescence of BeO: Mg single crystals

Luminescence and thermally stimulated luminescence (TL) of BeO: Mg crystals are studied at T = 6–380 K. The TL glow curves and the spectra of luminescence (1.2–6.5 eV), luminescence excitation, and reflection (3.7–20 eV) are obtained. It is found that the introduction of an isovalent magnesium impurity into BeO leads to the appearance of three new broad luminescence bands at 6.2–6.3, 4.3–4.4, and 1.9–2.6 eV. The first two are attributed to the radiative annihilation of a relaxed near-impurity (Mg) exciton, the excited state of which is formed as a result of energy transfer by free excitons. The impurity VUV and UV bands are compared with those for the intrinsic luminescence of BeO caused by the radiative annihilation of self-trapped excitons (STE) of two kinds: the band at 6.2–6.3 eV of BeO: Mg is compared with the band at 6.7 eV (STE1) of BeO, and the band at 4.3–4.4 eV is compared with the band at 4.9 eV (STE2) of BeO. In the visible region, the luminescence spectrum is due to a superposition of intracenter transitions in an impurity complex including a magnesium ion. The manifestation of X-ray-induced luminescence bands at T = 6 K in BeO: Mg indicates their excitation during band-to-band transitions and in recombination processes. The energy characteristics of the impurity states in BeO: Mg are determined; the effect of the isovalent impurity on the fluctuation rearrangement of the BeO: Mg structure in the thermal transformation region of STE1 → STE2 is revealed.