Luminescence of Tl<Superscript>+</Superscript> ions in a KZnF<Subscript>3</Subscript> crystal

The luminescence spectra of a KZnF₃: Tl⁺ crystal are investigated in the energy range from 4.75 to 5.9 eV at temperatures of 10–300 K upon excitation into the A absorption band (5.7–6.3 eV). At T=300 K, the luminescence spectra exhibit an intense band with a maximum at 5.45 eV, which is attributed to single Tl⁺ ions substituted for K⁺ ions. The 5.723-eV intense narrow band observed at T<20 K is assigned to the ³Γ_1u-¹Γ_1g zero-phonon transition, which is weakly allowed by the hyperfine interaction. The luminescence decay is studied as a function of temperature. The main characteristics of the luminescence spectra are adequately described in terms of the semiclassical theory based on the Franck-Condon principle and the Jahn-Teller effect for an excited sp configuration of the Tl⁺ ion with the use of the parameters obtained earlier from analyzing the absorption spectra of the system under investigation.     

Specific features of magneto-optical spectra of Tb<Subscript>3</Subscript>Ga<Subscript>5</Subscript>O<Subscript>12</Subscript>

The spectra of magnetic circular dichroism in the range of the ⁷ F ₆→⁵ D ₄ absorption band and the spectra of magnetic circular polarization of luminescence in the range of the ⁵ D ₄→⁷ F ₅ band in the terbium-gallium garnet Tb₃Ga₅O₁₂ are studied at a temperature of 80 K. The optical transitions between the Stark sublevels of the ⁷ F ₆, ⁷ F ₅, and ⁵ D ₄ multiplets are identified based on the analysis of the magneto-optical and optical spectra. It is shown that the experimentally determined symmetry and energy of the Stark sublevels of these multiplets confirm the results of numerical calculations of the energy spectrum of the Tb³⁺ rare-earth ion in terbium-gallium garnet.     

VUV spectroscopy of Ce<Superscript>3+</Superscript>-doped Na<Subscript>0.4</Subscript>Lu<Subscript>0.6</Subscript>F<Subscript>2.2</Subscript> single crystals

The short-wave transmission spectrum of Na_0.4Lu_0.6F_2.2 with the visible/ultraviolet transmission edge of 8 eV was studied. Absorption spectra of the 4f—5d transitions of the Ce³⁺ ion in the region of 4–8 eV were studied in Ce³⁺-doped Na_0.4Lu_0.6F_2.2 single crystals. Luminescence spectra in the ultraviolet and visible spectral regions, luminescence decay kinetics and reflection and luminescence excitation spectra in the visible/ultraviolet and ultraviolet regions (4–20 eV) were investigated at helium and room temperatures.     

Low-temperature time-resolved VUV luminescence spectroscopy of SrF<Subscript>2</Subscript>: Er<Superscript>3+</Superscript> crystals

Time-resolved excitation and emission spectra of SrF₂: Er³⁺ upon selective excitation with synchrotron radiation in the VUV and ultrasoft x-ray ranges at T = 8 K were studied. The VUV luminescence of SrF₂: Er³⁺ derives from high-energy interconfiguration 4f¹⁰5d-4f¹¹ transitions in the Er³⁺ ion. The VUV emission spectrum revealed, in addition to the 164.5-nm band (millisecond-range kinetics), a band at 146.4 nm (with a decay time of less than 600 ps). The formation of excitation spectra for the f-f and f-d transitions in the Er³⁺ ion is discussed.     

Charge-transfer bands in crystals of alkaline earth fluorides with Eu<Superscript>3+</Superscript> and Yb<Superscript>3 + 1</Superscript>

The absorption, luminescence, and excitation spectra of CaF₂, SrF₂, and BaF₂ crystals with EuF₃ or YbF₃ impurity have been investigated in the range 1–12 eV. In all cases, strong wide absorption bands (denoted as CT₁) were observed at energies below the 4f ⁿ -4f ^{n ? 1}5d absorption threshold of impurity ions. Weaker absorption bands (denoted as CT₂) with energies 1.5–2 eV lower than those of the CT₁ bands have been found in the spectra of CaF₂ and SrF₂ crystals with EuF₃ or YbF₃ impurities. The fine structure of the luminescence spectra of CaF₂ crystals with EuF₃ impurities has been investigated under excitation in the CT bands. Under excitation in the CT₁ band, several Eu centers were observed in the following luminescence spectra: C _4v , O _h , and R aggregates. Excitation in the CT₂ bands revealed luminescence of only C _4v defects.     

Electronic excitations and defects in nanostructural Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

A time-resolved cathodo-and photoluminescence study of nanostructural modifications of Al₂O₃ (powders and ceramics) excited by heavy-current electron beams, as well as by pulsed synchrotron radiation, is reported. It was found that Al₂O₃ nanopowders probed before and after Fe⁺ ion irradiation have the same phase composition (the γ-phase/δ-phase ratio is equal to 1), an average grain size equal to ～17 nm, and practically the same set of broad cathodoluminescence (CL) bands peaking at 2.4, 3.2, and 3.8 eV. It was established that Al₂O₃ nanopowders exhibit fast photoluminescence (PL) (a band at 3.2 eV), whose decay kinetics is described by two exponential stages (τ₁ = 0.5 ns, τ₂ = 5.5 ns). Three bands, at 5.24, 6.13, and 7.44 eV, were isolated in the excitation spectrum of the fast PL. Two alternate models of PL centers were considered, according to which the 3.2-eV luminescence either originates from radiative relaxation of the P^? centers (anion-cation vacancy pairs) or is due to the formation of surface analogs of the F⁺ center (F _S ⁺ -type centers). In addition to the fast luminescence, nano-Al₂O₃ was found to produce slow luminescence in the form of a broad band peaking at 3.5 eV. The excitation spectrum of the 3.5-eV luminescence obtained at T = 13 K exhibits two doublet bands with maxima at 7.8 and 8.3 eV. An analysis of the luminescent properties of nanostructural and single-crystal Al₂O₃ suggests that the slow luminescence of nanopowders at 3.5 eV is due to radiative annihilation of excitons localized near structural defects.     

Anisotropy of the Complex Permittivity of the Kagome-Staircase Compounds Co<Subscript>3</Subscript>V<Subscript>2</Subscript>O<Subscript>8</Subscript> and Ni<Subscript>3</Subscript>V<Subscript>2</Subscript>O<Subscript>8</Subscript>: Experiment and Ab Initio Calculations

The anisotropy of the components of the complex permittivity of vanadate Co₃V₂O₈ and Co₃V₂O₈ single crystals in the paramagnetic phase are studied by optical ellipsometry in the spectral region 0.5–5.0 eV. Our experimental results support the weak anisotropy of the optical response detected earlier for axes a and c. The optical properties are also investigated along axis b. The properties of both compounds are compared. The optical spectra of both compounds along axis b are shifted toward low energies as compared to axes a and c. The maximum of the main interband absorption band of Co₃V₂O₈ is shifted toward low energies by 0.25–0.3 eV as compared to Co₃V₂O₈. The electronic structure parameters of both compounds are determined. Optical function spectra are analyzed using the results of ab initio band calculations.     

Spectroscopic studies of erbium-doped potassium-lead double chloride crystals KPb<Subscript>2</Subscript>Cl<Subscript>5</Subscript>:Er<Superscript>3+</Superscript>: 1. Optical spectra and relaxation of excited states of the erbium ion in potassium-lead double chloride crystals

Optical spectra, intensities of radiative and nonradiative transitions, and luminescence kinetics in erbium-doped potassium-lead double chloride crystals KPb₂Cl₅:Er³⁺s(KPC:Er³⁺) were investigated. The crystals were grown by the Bridgman-Stockbarger method. Their absorption and luminescence spectra were studied experimentally. The crystal-matrix absorption edge was determined at 80 and 300 K. Intensity parameters, radiative transition probabilities, branching ratios, and nonradiative relaxation rates were estimated by the Judd-Ofelt method. The luminescence kinetics from the emitting levels ⁴ G _11/2, ² G _9/2, ⁴ S _3/2, and ⁴ F _9/2 upon selective excitation was studied.     

Intrinsic ultraviolet luminescence of LiB<Subscript>3</Subscript>O<Subscript>5</Subscript> single crystals under inner-shell excitation

Ultraviolet photoluminescence (PL) of LiB₃O₅ (LBO) crystals has been studied under selective excitation by photons in the vacuum ultraviolet and ultrasoft x-ray regions, including the K-absorption edges of the Li and B cations and O anion. Radiative recombination of electron-hole pairs was established to be the main channel of the intrinsic PL excitation at 4.2 eV. Features were observed in the PL excitation spectra near the lithium and boron K-absorption edges originating from excitation of the cation 1s core excitons. Experimental evidence of the multiplication of Li 1s excitons in LBO was obtained. It is shown that excitation of the O 1s core excitons does not affect the PL yield noticeably. The differences in the appearance of the Li, B, and O 1s excitons in the excitation spectra of the LBO ultraviolet PL are discussed.     

High-frequency EPR of Cr<Superscript>2+</Superscript> ions in CdGa<Subscript>2</Subscript>S<Subscript>4</Subscript>

High-frequency broad-band (65–240 GHz) EPR is used to study impurity centers of bivalent chromium in a CdGa₂S₄ crystal. It is found that the EPR spectra correspond to tetragonal symmetry. The spin Hamiltonian H = βB · g · S + B ₂ ⁰ O ₂ ⁰ + B ₄ ⁰ O ₄ ⁰ + B ₄ ⁴ O ₄ ⁴ with the parameters B ₂ ⁰ =23659±2 MHz, B ₄ ⁰ =1.9±1 MHz, |B ₄ ⁴ |=54.2±2 MHz, g_‖=1.93±0.02, and g_⊥=1.99±0.02 is used to describe the observed spectra. It is concluded that chromium ions occupy one of the tetrahedrally coordinated cation positions.     

Electronic excitations in BeAl<Subscript>2</Subscript>O<Subscript>4</Subscript>, Be<Subscript>2</Subscript>SiO<Subscript>4</Subscript>, and Be<Subscript>3</Subscript>Al<Subscript>2</Subscript>Si<Subscript>6</Subscript>O<Subscript>18</Subscript> 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.     

Tunneling of 6<Emphasis Type="Italic">s</Emphasis><Superscript><Emphasis Type="Italic">2</Emphasis></Superscript> electrons in PbO and Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>: XANES spectroscopy and DFT calculations

The results of XANES spectroscopy and DFT calculations are evidence for the occurrence of anomalous 2p_2/3→6s_1/2 electronic transitions in lead(II) and bismuth(III) oxides. The observed transitions may be due to the tunneling of outer 6s² electrons.     

Study of absorption spectra of Er<Superscript>3+</Superscript> in KTaO<Subscript>3</Subscript> crystals

The first results of the study of optical absorption spectra of KTaO₃: Er³⁺ crystals are presented. In the 350–660-nm region, lines are observed deriving from intraconfigurational electronic transitions from the ⁴ I _15/2 ground state to levels of the ⁴ F _9/2, ⁴ S _3/2, ² H _11/2, ⁴ F _7/2, ⁴ F _5/2(⁴ F _3/2), ² G _9/2, and ⁴ G _11/2 excited states of the Er³⁺ ions. A comprehensive study of transitions to the ⁴ F _9/2, ⁴ S _3/2, ² H _11/2, and ⁴ F _7/2 levels at 77 K is carried out. The number of lines observed for the above transitions fits the theoretically possible number for ?-? electronic transitions in Er³⁺ ions in the cubic crystal field. In the case of a differently charged substituted ion, this situation occurs only under nonlocal impurity charge compensation. The energies of the excited state levels for the transitions under study are determined.     

Spectroscopic study of double sodium-yttrium fluoride crystals doped with erbium Na<Subscript>0.4</Subscript>Y<Subscript>0.6</Subscript>F<Subscript>2.2</Subscript>:Er<Superscript>3+</Superscript>: II. Luminescence self-quenching and microparameters and macrorates of energy transfer

Na_0.4Y_0.6F_2.2:Er³⁺ (NYF:Er³⁺) crystals with an Er concentration up to 15% were grown by the Bridgman-Stockbarger method. The luminescence kinetics was investigated for a series of NYF:Er³⁺ crystals (0.5–15% Er), as well as the concentration and temperature quenching of the luminescence from radiative Er levels upon selective laser excitation. It is shown that the luminescence from the ⁴S_3/2 level is quenched significantly with increasing temperature and concentration. The luminescence from the ⁴G_11/2, ²G(H)_9/2, ⁴F_9/2, and ⁴I_9/2 levels is quenched mainly due to nonradiative multiphonon transitions. The concentration quenching of the luminescence from the ⁴I_11/2 and ⁴I_13/2 levels was not observed. Possible schemes of the self-quenching of excited levels of erbium are considered and the microparameters and macrorates of self-quenching are estimated by model quantum-mechanical calculation. Based on the comparison of the calculated and experimental self-quenching rates, the most probable mechanisms and schemes of self-quenching are determined. The self-quenching of the ⁴S_3/2 level of erbium was investigated experimentally and theoretically. Good agreement is obtained between the experimental and the calculated kinetic curves and the dependences of the self-quenching rates on Er concentration. It is concluded that NYF:Er³⁺ crystals are promising as active media for tunable lasers with laser diode pumping.     

Spectroscopic study of double sodium-yttrium fluoride crystals doped with erbium Na<Subscript>0.4</Subscript>Y<Subscript>0.6</Subscript>F<Subscript>2.2</Subscript>:Er<Superscript>3+</Superscript>: I. Intensities of luminescence spectra and kinetics of luminescence

Na_0.4Y_0.6F_2.2:Er³⁺ (NYF:Er) crystals with an erbium concentration as high as 100 at. % (Na_0.4Er_0.6F_2.2) were grown by the Bridgman-Stockbarger method. The optical spectra were investigated at low (6 K) and room temperatures. It is shown that the absorption spectrum of NYF:Er crystals contains wide bands (790–801 and 965–980 nm) corresponding to the emission range of laser diodes. The peak absorption cross section σ_a for the band peaked at λ=970.4 nm is 0.15×10^?20 cm². On the basis of the analysis of the absorption and luminescence spectra at low (6 and 12 K) temperatures, the structure of the Stark splitting of erbium levels was determined as a structure of quasi-centers for which Stark components are inhomogeneously broadened. The oscillator strengths of the transitions from the ground state of erbium to excited multiplets were calculated from the absorption spectra measured at T=300 K, and the intensity parameters Ω_t were determined by the Judd-Ofelt method: Ω₂=1.65×10^?20 cm², Ω₄=0.56× 10^?20 cm², and Ω₆=1.01×10^?20 cm². These values of the intensity parameters were used to calculate the probabilities of radiative transitions and the branching ratios. The rates of multiphonon nonradiative transitions in NYF: Er were estimated. The luminescence decay kinetics for radiative levels of erbium ions upon their selective excitation by nanosecond laser pulses was studied. The intracenter lifetimes of radiative levels of erbium ions were determined from the luminescence kinetics upon selective ion excitation by low-intensity light in a sample with a low erbium concentration (0.5%). It is demonstrated that, with an increase in temperature from 6 to 300 K, luminescence from the ⁴ G _11/2, ² G(H)_9/2, and ⁴ F _9/2 levels is quenched as a result of multiphonon nonradiative transitions. Luminescence from the ⁴ I _9/2 level is quenched only insignificantly with increasing temperature, and no quenching of luminescence from the ⁴ I _11/2 and ⁴ I _13/2 levels is observed. The spectra of steady-state luminescence of NYF:Er(0.5–15%) crystals were investigated upon broadband excitation by UV and UV-visible lamp light and selective time-resolved laser excitation. It is shown that low-lying levels of erbium ions separated by an energy gap smaller than 2500 cm^?1 are populated via cascade mechanisms. On the basis of the results obtained, it is concluded that NYF:Er ³⁺ crystals are promising candidates for active media of tunable diode-pumped lasers.     

Nature of low-temperature phase transitions in CaMn<Subscript>7</Subscript>O<Subscript>12</Subscript>

New data on the specific heat, thermal expansion, and magnetization of the CaMn₇O₁₂ phase require a revision of the current concepts of the sequence of phase transitions in this compound. It is found that a spin-glass phase transition occurs in CaMn₇O₁₂ at T _M = 49 K, whereas the transition at T _S = 89 K exhibits the features of a first-order phase transition and thereby is apparently of structural origin. In the range T _M < T< T _S , the CaMn₇O₁₂ compound exhibits negative thermal expansion, which is also indicative of structural changes.     

Spectroscopy of NaLa(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript>: Tm<Superscript>3+</Superscript> and NaGd(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript>: Tm<Superscript>3+</Superscript> crystals as advanced laser materials

Single crystals of double sodium-containing lanthanum and gadolinium molybdates doped with Tm³⁺ ions were synthesized by the Czochralski method. The spectroscopic properties of these crystals were investigated from the viewpoint of their use as active media in diode-pumped lasers. The polarized spectra of absorption on the ³ H ₄ and ³ F ₄ levels and the polarized spectra of luminescence due to the ³ F ₄-³ H ₆ laser transition were recorded, and the lifetimes of the ³ H ₄ and ³ F ₄ excited states of the Tm³⁺ ions were determined. The luminescence cross sections were calculated using the Füchtbauer-Ladenburg formula. The simulation of the decay curve of the ³ H ₄ excited state according to the Golubov-Konobeev-Sakun method revealed that, in the crystals under investigation, the interaction between Tm³⁺ ions predominantly occurs through the dipole-dipole mechanism.     

Experimental Study and Analysis of Absorption Spectra of Ni<Superscript>2+</Superscript> Ions in Nickel Orthoborate Ni<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>2</Subscript>

The results of studies of the absorption spectra of nickel orthoborate Ni₃(BO₃)₂ in the range of electronic d–d-transitions are reported. The obtained data are analyzed in the framework of the crystal field theory. The Ni²⁺ ions are located in two crystallographically nonequivalent positions 2a and 4f with point symmetry groups C_2h and C₂, respectively, surrounded by six oxygen ions forming deformed octahedra. The absorption spectra exhibit three intense bands corresponding to spin-resolved transitions from the ground state of nickel ion ³A_2g (³F) to the sublevels of the ³T_2g (³F), ³T_1g (³F) and ³T_1g (³P) triplets split by the spinorbit interaction and the rhombic component of the crystal field. At temperatures below 100 K, the spectra exhibit a thin structure, in which phonon-free lines can be distinguished. Comparison of the calculated frequencies of the zero-phonon transitions with the experimental data allows estimating parameters of the crystal field acting on the nickel ions in the 2a- and 4f-positions, as well as the parameters of electrostatic interaction between the 3d electrons and spin-orbit interaction constants.     

Effect of the chemical state of silver on the luminescence properties of GeO<Subscript>2</Subscript>-Eu<Subscript>2</Subscript>O<Subscript>3</Subscript>-Ag films

A sol-gel method is used to prepare GeO₂-Eu₂O₃-Ag films in which the luminescence efficiency of Eu³⁺ ions during UV excitation is comparable to that in films activated by organic europium complexes. The luminescence spectra of these films are recorded, and the films are also studied using EPR and x-ray diffraction. The main origin of this effect is found to be complex Eu-Ag centers with a high quantum yield of the intracenter transfer of excitations to the rare-earth activator from silver ions and Ag _m ⁿ⁺ oligomer clusters located on the surface of silver nanoparticles.     

Spectral-luminescent properties of Eu<Superscript>3+</Superscript> ions in inorganic D<Subscript>2</Subscript>O and POCl<Subscript>3</Subscript>-SnCl<Subscript>4</Subscript> solvents

Absorption and luminescence spectra of Eu³⁺ ions in D₂O and POCl₃-SnCl₄ inorganic solvents were measured. Oscillator strengths and spontaneous radiation transition probabilities for the most intense electron transitions in Eu³⁺ were calculated. Judd-Ofelt parameters, the radiative lifetime of the ⁵ D ₀ metastable level of Eu³⁺ in D₂O-Eu³⁺ and POCl₃-SnCl₄-Eu³⁺ solutions, and matrix elements for radiative transitions from the ⁵ D ₀ level were determined. The luminescence lifetime of the ⁵ D ₀ level of Eu³⁺ in these solutions was measured. The photoluminescence quantum yield for transitions from the ⁵ D ₀ level of Eu³⁺ in ⁵ D ₀ and POCl₃-SnCl₄-Eu³⁺ solutions was found to be 0.32 and 0.88, respectively.