V.A. Pustovarov

Time-resolved luminescent VUV spectroscopy of F- and F+-centres in single BeO crystals

Abstract The time-resolved photoluminescence spectra in the energy region of 2.0–6.0xa0eV, as well as the excitation spectra (4.0–35.0xa0eV) and the decay kinetics of luminescence (the time region of 0.2–500xa0ns) were studied for F- and F + -centres in beryllium oxide crystals at T =10 and 295xa0K using selective vacuum ultraviolet excitation. The synchrotron radiation from the storage ring DORIS was used. The aim of the present work was to investigate and compare the stoichiometric BeO crystals with anion-imperfection samples. F- and F + -centres performed the role a of luminescent probe in a crystal lattice. The efficiency of energy transfer to F- and F + -centres was investigated on this basis. The role of the electron–hole and exciton mechanisms of energy transfer in BeO crystals and their dependence on temperature were determined. The photoconversion F + →F was found using time-resolved spectroscopy methods. A comparison of emission properties of F- and F + -centres in BeO and α-Al 2 O 3 crystals was made.

Luminescence of lithium triborate crystals under high intensity synchrotron radiation

The results of a preliminary study of the luminescence of LiB3O5 single crystals under high intensity synchrotron radiation are reported. The dependence of the luminescence intensity (λm = 290 nm) on the synchrotron radiation density and on the vacuum conditions is investigated. Two models for the observed phenomena are presented: (1) the formation of volume electrical charge under irradiation and (2) recombination processes with participation of the shallow trapping centers.

Kinetics of non-equilibrium processes in non-linear crystals of lithium borates excited with synchrotron radiation

Abstract The paper presents the results of a study of the LiB 3 O 5 and Li 2 B 4 O 7 crystals by the use of the luminescent spectroscopy with the sub-nanosecond time resolution under excitation of the high-power synchrotron radiation. The commonness in the origin of the non-equilibrium processes in these crystals as well as the observed differences in the luminescence manifestations is discussed.

Research of energy transfer in Y2SiO5Ce, Tb single crystals by time resolved luminescence spectroscopy

Abstract The energy transfer between the rare-earth ions in Y s SiO 5 ue5f8Ce, Tb crystals has been investigated with the use of the methods of luminescence spectroscopy with subnanosecond time resolution and excitation by pulsed synchrotron radiation (SR). Effective nonradiative energy transfer between Ce 3+ and Tb 3+ -ions which is realized by the inductive resonance mechanism has been observed. The absolute constant of the energy transfer speed K t and the probability of transfer have been determined. A dependence of the energy transfer parameters on the SR-excitation density has been discovered and investigated. Increasing of the density of the SR-excitation by a factor of 10 results in the increase of K t from 4.8 × 10 8 up to 1.7 × 10 10 mol −1 s −1 .

Low-temperature photoluminescence of ion-implanted SiO 2 :Sn + films and glasses

Low-temperature photoluminescence spectroscopy with pulsed synchrotron excitation is applied to study the regularities of excitation and relaxation of both point defects and nanoparticles formed by tin implantation into SiO2 films and glasses. It has been found that tin implantation followed by air and nitrogen annealing yields the formation of α-Sn nanoclusters and nonstoichiometric SnOx nanoparticles, while a stable phase of SnO2 does not appear. Alternative channels of luminescence excitation are revealed for nanoclusters, including energy transfer from excitons and electron-hole pairs of the host SiO2 matrix.

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.

A polarized fast luminescence of LiB3O5 single crystals excited by synchrotron radiation

The paper presents the results of study of luminescence for the oriented LiB3O5 crystals under excitation with a polarized synchrotron radiation over the X-ray spectral range. It was revealed that LiB3O5 crystal exhibits a polarized fast luminescence with the complex broad band peaked in the spectral region from 3.5 to 4.2 eV. Time-response of luminescence demonstrates both the fast exponential component with the time-constant of 1.2±0.2 ns (300 K) and 1.3±0.3 ns (80 K) and inertial components in the microsecond time-range. The intensities of the elementary sub-bands as well as the intensity of the fast component depend on orientation of the exciting electrical vector Eex to the main crystallographic axis Z, on the density and polarization of the exciting synchrotron radiation. The paper discusses the origin and excitation channels for the broad-band luminescence as well as mechanisms of excitation of the luminescence centers in LBO through the resonant energy transfers inheriting the initial polarization from the polarized synchrotron radiation.

Sub-nanosecond time-resolved spectroscopy of LiB3O5 under synchrotron radiation

Abstract This paper presents the results of study of luminescence of the LiB3O5 crystals excited with synchrotron radiation. It was revealed that LiB3O5 exhibits a polarized fast luminescence with the complex broad band peaked in the spectral region 3.5–4.2 eV. Time response of the luminescence demonstrates both the fast exponential component with the time constant of 1.2 ± 0.2 ns (300 K) or 1.3 ± 0.3 ns (80 K) and several slow components in the microsecond time range. The paper discusses the origin and excitation channels for the broad-band luminescence in LiB3O5 crystals.

Optical and electronic properties of undoped La 2 Be 2 O 5 single crystals in the far ultraviolet energy range

The optical and electronic properties of undoped La2Be2O5 single crystals were determined using low-temperature (T=10u2009u2009K) far ultraviolet (4.9–33xa0eV) synchrotron radiation spectroscopy, optical absorption spectroscopy (T=80 and 320xa0K), and calculations for the dispersions of the optical functions. On the basis of the obtained data we determined the Urbach’s formula parameters and the cut-off energy Ec=5.49u2009u2009eV at 80xa0K for the low-energy tail of the host absorption, the bandgap Eg=6.78u2009u2009eV at 10xa0K, the energy threshold for the creation of unrelaxed excitons Eex=6.28u2009u2009eV, and other electronic properties.

Radiation effects and defects in lithium borate crystals

The paper presents the results of a study of the formation and decay of lattice defects in wide band-gap optical crystals of LiB3O5 (LBO), Li2B4O7 (LTB) and Li6Gd(BO3)3 (LGBO) with a sublattice of mobile lithium cations. By means of thermoluminescence techniques, and luminescent and absorption optical spectroscopy with a nanosecond time resolution under excitation with an electron beam, it was revealed that the optical absorption in these crystals in the visible and ultraviolet spectral ranges is produced by optical hole-transitions from the local defect level to the valence band states. The valence band density of the states determines mainly the optical absorption spectral profile, and the relaxation kinetics is rated by the interdefect non-radiative tunnel recombination between the trapped-hole center and the Li0 trapped-electron centers. At 290 K, the Li0 centers are subject to thermally stimulated migration. Based on experimental results, the overall picture of thermally stimulated recombination processes with the participation of shallow traps was established for these crystals.