A. V. Kruzhalov

Relaxation of electronic excitations in beryllium oxide: A time-resolved vacuum-UV spectroscopy study

Beryllium oxide crystals are studied by time-resolved optical and luminescence vacuum-UV spectroscopy. The low-temperature luminescence spectra and the luminescence decay kinetics (2.5–10 eV, 1–500 ns) upon selective photoexcitation, and also the luminescence excitation and reflectivity spectra (8–35 eV), are analyzed for BeO crystals with the optic axis aligned parallel and perpendicular to the electric vector of exciting polarized synchrotron radiation. It is found that the radiative relaxation of electronic excitations proceeds through a large number of channels. The excited states of self-trapped excitons are characterized by different multiplicity depending on the excitation energy and the sample orientation.

Luminescence excitation of pure and impure BeO single crystals using synchrotron radiation

Abstract Absorption, reflection, UV and VUV luminescence excitation and thermoluminescence excitation spectra have been measured for BeO and BeOZn crystals in the energy range from 8 to 35 eV, using synchrotron radiation. The nature of electronic excitations in the fundamental absorption edge region is discussed; the value of the forbidden-gap energy, E g , is estimated as 10.6 eV for BeO. Intrinsic luminescence with a 6.7 eV maximum for BeO and impurity luminescence with a 6.0 eV maximum for BeOZn are due to the relaxation of both optically created self-trapped or impurity-trapped excitons. The multiplication effect of electronic excitations with E >2 E g (or E ≥2 E ex for 6.7 eV luminescence) for beryllium oxide is due to the inelastic scattering of hot photoelectrons or hot photoholes.

Transient optical absorption and luminescence in Li2B4O7 lithium tetraborate

This paper reports on a study of the transient optical absorption exhibited by Li2B4O7 (LTB) in the visible and UV spectral regions. Using absorption optical spectroscopy with nanosecond time resolution, it is established that the transient optical absorption (TOA) in these crystals originates from optical transitions in hole centers and that the kinetics of the optical-density relaxation is controlled by interdefect tunneling recombination, which involves these hole centers and electronic Li0 centers representing neutral lithium atoms. At 290 K, the Li0 centers migrate in a thermally stimulated, one-dimensional manner, without carrier ejection into the conduction or valence band. The kinetics of the pulsed LTB cathodoluminescence is shown to be controlled by a relaxation process connected with tunneling electron transfer from a deep center to a small hole polaron migrating nearby, a process followed by the formation of a self-trapped exciton (STE) in an excited state. Radiative annihilation of the STE accounts for the characteristic σ-polarized LTB luminescence at 3.6 eV, whose kinetics is rate-limited by the tunneling electron transfer.

Electron excitation and luminescence in Bi4Ge3O12 and Bi4Si3O12 crystals

Abstract Reflection, luminescence excitation and thermoluminescence excitation spectra have been measured for Bi 4 Ge 3 O 12 and Bi 4 Si 3 O 12 crystals in the energy range from 3 to 35 eV using synchrotron radiation. The reflection data are evaluated using a modified Kramers-Kronig method for extracting the information on optical constants of these crystals. The nature of electronic excitations in the fundamental absorption edge region is discussed, the value of forbidden-gap energy E g is estimated as 5.0 eV for Bi 4 Ge 3 O 12 and 5.4 eV for Bi 4 Si 3 O 12 . Intrinsic luminescence with 2.5 eV maximum for Bi 4 Si 3 O 12 and 2.45 eV maximum for Bi 4 Ge 3 O 12 is due to both the relaxation of optically created excitons and recombination processes. The multiplication effect of electron excitations in E ⩾ 2 E g for these crystals is due to the inelastic scattering of hot photoelectrons and hot photoholes.

Electronic excitations in BeAl2O4, Be2SiO4, and Be3Al2Si6O18 crystals

Low-temperature (T = 7 K) time-resolved selectively photoexcited luminescence spectra (2–6 eV) and luminescence excitation spectra (8–35 eV) of wide-bandgap chrysoberyl BeAl2O4, phenacite Be2SiO4, and beryl Be3Al2Si6O18 crystals have been studied using time-resolved VUV spectroscopy. Both the intrinsic luminescence of the crystals and the luminescence associated with structural defects were assigned. Energy transfer to impurity luminescence centers in alexandrite and emerald was investigated. Luminescence characteristics of stable crystal lattice defects were probed by 3.6-MeV accelerated helium ion beams.

Electron excitations in LiB3O5 crystals with defects: Low-temperature time-resolved luminescence VUV spectroscopy

The dynamics of electron excitations and luminescence of LiB3O5 (LBO) single crystals was studied using low-temperature luminescence vacuum ultraviolet spectroscopy with a subnanosecond time resolution under photoexcitation with synchrotron radiation. The kinetics of the photoluminescence (PL) decay, the time-resolved PL emission spectra, and the time-resolved PL excitation spectra of LBO were measured at 7 and 290 K, respectively. The PL emission bands peaking at 2.7 eV and 3.3 eV were attributed to the radiative transitions of electronic excitations connected with lattice defects of LBO. The intrinsic PL emission bands at 3.6 and 4.2 eV were associated with the radiative annihilation of two kinds of self-trapped electron excitations in LBO. The processes responsible for the formation of localized electron excitations in LBO were discussed and compared with those taking place in wide-gap oxides.

Recombination-assisted creation of cation excitons and cross-luminescence quenching in CsCl crystals at high excitation densities

At high excitation densities, recombination-assisted creation of cation excitons, which transfer energy efficiently to the anion sublattice to initiate the luminescence of anion excitons and impurity centers, has been observed in CsCl crystals. At the same time, the creation of cation excitons competes with the electron recombination with cation holes and quenches the cross-luminescence. The intensity ratio of the cross-luminescence to exciton-impurity luminescence is different for crystal irradiation with γ rays and heavy particles.

Metastable defects in beryllium oxide crystals

The metastable luminescence centers of regular lattice are investigated in binary beryllium oxide crystals. Beryllium oxide hexagonal crystals are the simplest among low-symmetry oxide scintillators and serve as a model system. The anisotropy of energy transformation and transfer is analyzed.

Luminescence excitation of colour centers in beryllium oxide

Abstract Synchrotron radiation is used in the measurement of the spectral luminescence characteristics of F-centers in BeO. The electron-hole and the exciton mechanism of energy transfer to F-centers is discussed. The photon multiplication effect (E > 25 eV) is connected with the creation of secondary carriers by hot photoelectrons. The anomalous absorption of polarized light by F-centers in BeO is due to the splitting of the 1p-excitation level of the F-centers in BeO under the influence of a C3v crystal symmetry field.

Low-temperature time-resolved vacuum UV spectroscopy of potassium pentaborate crystals

For the first time, subnanosecond time resolution is attained in the low-temperature (at 7 K) measurements of the photoluminescence (PL) spectra (2–6 eV), the PL excitation spectra (4–32 eV), the PL kinetics, and the reflection spectra (4–21 eV) of undoped potassium pentaborate KB5O8·4H2O (KB5) crystals under selective photoexcitation by synchrotron radiation. The PL peaks associated with the intrinsic defects of the KB5 lattice are detected. The PL bands resulting from radiative annihilation of the localized and self-localized electron excitations are singled out; these excitations are most efficiently photogenerated at the fundamental absorption edge in the region where the free exciton formation is expected. The difference between the PL spectra of the fast and slow components is revealed. An effective low-temperature energy transport over the KB5 hydrogen sublattice is deduced from a drop in efficiency of PL excitation in the interband-transition region as a result of nonradiative energy loss. Long-term vacuum UV irradiation of a KB5 crystal at 7 K gives rise to defects in the hydrogen sublattice, which facilitate localization of the electron excitations and reduce the effective length of their diffusion. This leads to a decrease in the nonradiative energy loss, thus enhancing the efficiency of the PL photoexcitation in the band-to-band transition region.