VUV 5<Emphasis Type="Italic">d</Emphasis>-4<Emphasis Type="Italic">f</Emphasis> luminescence of Gd<Superscript>3+</Superscript> and Lu<Superscript>3+</Superscript> ions in the CaF<Subscript>2</Subscript> host

The first observation and characterization of Lu³⁺ 4f ¹³5d-4f ¹⁴ luminescence from the CaF₂: Lu³⁺ crystal are reported, and the multisite structure in the spectra of Ce³⁺, Gd³⁺, and Lu³⁺ ions in the CaF₂ host is analyzed with the high-resolution VUV spectroscopy technique using synchrotron radiation. It is shown that vibronic structure in the emission and excitation spectra of interconfigurational transitions in Gd³⁺ and Lu³⁺ ions doped into CaF₂ differs from that observed for Ce³⁺ ions entering mainly at the tetragonal (C _4v ) sites. However, the exact types of sites in which the Gd³⁺ and Lu³⁺ ions reside in a CaF₂ lattice cannot be identified using only the obtained experimental spectroscopid data. The text was submitted by the authors in English.     

Broadening of the lines due to the 4f n -4f n − 15d transitions of Ce3+, Pr3+, and Tb3+ ions in the absorption spectra of CdF2 crystals

The structure of the absorption spectra of Ce³⁺, Pr³⁺, and Tb³⁺ ions in the vicinity of 4f-5d transitions has been investigated. At low temperatures the absorption spectra exhibit a weakly pronounced fine structure, in contrast to narrow-line spectra in crystals of Ca, Sr, and Ba fluorides. The spectra of Ce³⁺, Pr³⁺, and Tb³⁺ ions in CdF₂ can be considered as the absorption spectra of these ions in alkali-earth fluorides, broadened by 60–75 cm^?1. The broadening is related to the autoionization of electron from the local 5d(e _g ) level to the energy-degenerate states of the conduction band of CdF₂ crystal.     

中子探测晶体Li6Gd(BO3)3:Ce的光谱特性研究

文章用提拉法生长出Li₆Gd(BO₃)₃:Ce晶体,并对其光谱性能与发光过程进行了探索. 借助于真空紫外-紫外透过光谱测试,发现晶体的透过光谱中存在Ce³⁺离子和Gd³⁺的特征吸收峰,同时还存在与Ce⁴⁺离子相关的电荷迁移带. 对晶体的真空紫外-紫外激发发射光谱进行研究发现,在晶体存在着Ce³⁺离子的5d→4f辐射跃迁发光与Gd³⁺离子的4f→4f辐射跃迁发光,而且存在着Gd³⁺→Ce³⁺之间的能量传递. 对Li₆Gd(BO₃)₃:Ce晶体的X射线与γ射线激发发射光谱研究可知,晶体在高能射线激发下的闪烁光主要是Ce³⁺离子的发光. 关键词： 6Gd(BO₃)₃:Ce晶体')" href="#">Li₆Gd(BO₃)₃:Ce晶体 真空紫外-紫外透过光谱 真空紫外-紫外激发发射光谱 能量传递     

The laser-diode-excited 5d-4f luminescence of Ce3+ and Pr3+ ions embedded into a BaR2F8 matrix

We show the possibility of obtaining UV luminescence from 5d-4f transitions of rare-earth ions in the BaY₂F₈: (Yb³⁺, Pr³⁺, Ce³⁺) crystal under upconversion excitation by standard laser diodes with lasing wavelengths of 960, 808, and 840 nm. Various upconversion mechanisms of pumping for populating the higher-lying energy levels of the active ions, as well as methods of adaptation of the active medium BaY₂F₈: (Yb³⁺, Pr³⁺, Ce³⁺) to these mechanisms, are considered.     

Optical properties of BiTeI semiconductor with a strong Rashba spin-orbit interaction

We have modeled the 4f ¹-5d ¹ absorption spectrum of a LiYF₄:Ce³⁺ crystal at zero temperature using a microscopic model of the electron-phonon interaction and the real spectrum of LiYF₄ lattice vibrations. Effects caused by mixing of the wave functions of different states of the 5d ¹ excited configuration of the Ce³⁺ ion, which is induced by the electron-phonon interaction, are considered based on the calculations of the second-, third-, and fourth-order exact moments of curvature of the spectrum envelope. We have shown that the large value of the splitting between the maxima of the bands in the absorption spectrum that correspond to transitions to the third and fourth 5d ¹ levels is a result of the nonadiabatic interaction of 5d electrons with lattice vibrations.     

Spectral characteristics of solid solutions LiY1−x Lu x F4 doped by Ce3+ ions

During crystallization, the LiF-YF₃-LuF₃ compounds form a continuous series of solid solutions having a scheelite structure at any proportion of the YF₃ and LuF₃ components. It is established that the crystals conform to the Vegard law and the Rutgers rule. The spectral characteristics of the 5d–4f transitions of Ce³⁺ ions and color centers induced by ultraviolet radiation in these crystals were studied as a function of the Lu³⁺ concentration. It is shown that the active media based on LiF-YF₃-LuF₃: Ce³⁺ solid solutions are more efficient as compared to LiYF₄: Ce³⁺ and LiLuF₄: Ce³⁺ crystals in possible directional changes in the spectral-kinetic characteristics of impurity ions and the parameters of the losses induced by pumping radiation and in enhancement of energetic and spectral characteristics of lasers on their basis.     

A novel scheme to acquire enhanced up-conversion emissions of Ho3+ and Yb3+ co-doped Sc2O3

A detailed investigation about the effect of Sc₂O₃: 1 mol%Ho³⁺/5 mol%Yb³⁺ co-doped with Ce⁴⁺ ions prepared by sol-gel methods was performed systematically. Under the excitation of 980 nm laser diode, both green emission (553 nm, ⁵F₄/⁵S₂→⁵I₈) and red emission (672 nm, ⁵F₅→⁵I₈) were both observed in the emission spectra of the samples, which were found to be two-photon process and sensitized by Yb³⁺ ions. With the increasing of Ce⁴⁺ ions, the up-conversion green emission intensity are increased by 6.52, 8.69, 10.85, 13.92 and 16.66 fold, corresponding to the Ce⁴⁺ ions concentrations from 5 mol% to 13 mol%, respectively. The number of photons are necessary to populate the upper emitting state decreases to 2 and the infrared absorption coefficient is reduced, when the Ce⁴⁺ ions concentration increase to 13 mol%. Ce⁴⁺ ions play an important role in tailoring the local crystal field around Ho³⁺ ions, lowering the highest phonon cut-off energy of matrix and reducing the infrared absorption coefficient, thus hindering the non-radiative processes, which contribute to the increased emission intensity. The excellent enhancement makes it a promising multifunctional optical luminescence material.     

Vacuum ultraviolet spectroscopy of pure and Ce3+-doped Na0.4Lu0.6F2.2

The spectroscopic properties of wide-band fluoride Na_0.4Lu_0.6F_2.2 crystals activated by Ce³⁺ were investigated. The absorption edge for the matrix was found to be at about 9.5 eV. In the 4- to 8-eV region of the absorption spectrum of Na_0.4Lu_0.6F_2.2: Ce³⁺, all 4f-5d transitions of the Ce³⁺ ion are observed. In Na_0.4Lu_0.6F_2.2: Ce³⁺ crystals, ultraviolet/visible emission, reflection and time-resolved vacuum ultraviolet/ultraviolet excitation spectra were recorded at liquid helium and room temperatures.     

Nature of optical properties of GdFe3(BO3)4 and GdFe2.1Ga0.9(BO3)4 crystals and other 3d5 antiferromagnets

Influence of the partial substitution of paramagnetic Fe³⁺ ions by diamagnetic Ga³⁺ ions in the trigonal crystal GdFe₃ (BO₃)₄ on its optical and magnetic properties is studied and discussed in connection with problems common for all antiferromagnets containing 3d ⁵ ions. Polarized optical absorption spectra and linear birefringence of GdFe₃ (BO₃)₄ and GdFe_2.1Ga_0.9 (BO₃)₄ single crystals have been measured in the temperature range 85–293 K. Specific heat temperature dependence (2–300 K) and structure of GdFe_2.1Ga_0.9 (BO₃)₄ crystal have been also studied. As a result of substitution of 30% Fe to Ga the Neel temperature diminishes from 38 till 16 K, the strong absorption band edge shifts on 860 cm^-1 (0.11 eV) to higher energy and the d-d transitions intensity decreases substantially larger than the Fe concentration does. Strong absorption band edge is shown to be due to Mott-Hubbard transitions. Correlation between position of the strong absorption band edge and the Neel temperature of antiferromagnets has been revealed. Properties of the doubly forbidden d-d transitions in the studied crystals and in other antiferromagnets are explained within the framework of the model of the exchange-vibronic pair absorption, which is theoretically analyzed in detail. The model permitted us to determine the connection between parameters of d-d absorption bands (intensity, width and their temperature dependences), on the one hand, and the exchange, spin-orbit and electron-lattice interactions, on the other hand.     

Intrinsic and impurity luminescence of rare earth ions doped KYF4 nanophosphors

The KYF₄ nanopowders, non-doped and doped with Ce³⁺ or Tb³⁺, having well-crystallized, unaggregated, monodisperse (±15%) nanoparticles with the cubic (the size in the range from ∼15 to ∼30 nm) or hexagonal (from ∼30 to ∼50 nm) crystal structure have been successfully synthesized by microwave-hydrothermal treatment of as-precipitated gels. In KYF₄ hexagonal nanopowders an intense STE-type luminescence at ∼4.4 eV was observed which is not quenched at room temperature. In contrast to single crystals or cubic nanopowders, in KYF₄ hexagonal nanopowders doped with Ce³⁺ or Tb³⁺, a rather efficient energy transfer is observed from the host to Ce³⁺ or Tb³⁺ ions, respectively, because of overlapping the emission spectrum of STE-type luminescence and the spectrum of efficient absorption on 4f-5d transitions in Ce³⁺ or Tb³⁺.     

Inversion of the relative probabilities of the f-f and f-d transitions in the Ln3+ lanthanide ions at their radio- and sonoexcitation as compared to photoexcitation

The results of measuring the efficiencies of the formation of electronically excited states of the Ln³⁺ lanthanide ions in aqueous solutions in the processes of radioluminescence and multibubble sonoluminescence are analyzed. In both cases, electronic excitation occurs due to inelastic collisions of Ln³⁺ ions with (for radioluminescence) charged ionizing particles in liquid and (for multibubble sonoluminescence) high-energy particles, primarily electrons, in the gas phase of cavitation bubbles. In both processes, the efficiencies of exciting ions whose luminescence states appear in the 4f-5d transitions (Ce³⁺ and Pr³⁺) are significantly lower (by an order of magnitude or larger) than the efficiencies of exciting ions whose luminescence states appear in the 4f-4f transitions (Gd³⁺ and Tb³⁺). Therefore, the probability of the f-d transitions is lower than the probability of the f-f transitions in lanthanide ions excited by collisions with the charged particles and the relative probabilities of these transitions are inverted in these processes as compared to photoexcitation. Original Russian Text ? G.L. Sharipov, 2007, published in Pis’ma v Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2007, Vol. 85, No. 9, pp. 559–562.     

Sc2O3:Ce nanocrystals: Fluorescence and its dependence on the surrounding gas pressure

The fluorescence spectra of nanocrystalline powder samples of Sc₂O₃:Ce³⁺ prepared by the sol-gel process were studied. The strong shift of the fluorescence spectra with the excitation wavelength was observed. The effect of the surrounding gaseous medium on 4f⁰5d¹→4f¹ fluorescence of Ce³⁺ ions in nanoparticles, discovered earlier in YAG:Ce³⁺, was also observed and studied in Sc₂O₃:Ce³⁺. The fluorescence intensity and the excitation power dependence of fluorescence dramatically depend on the pressure of the gaseous media.     

Luminescence of YAG doped with Eu,Yb, and Mn ions under VUV excitation

Hydrothermal synthesis has been successfully used to obtain fine-crystalline powders of yttrium aluminum garnet (YAG) doped with manganese ions and codoped with cerium and manganese ions. Using the method of high-temperature solid-state synthesis, ceramic specimens of YAG that contain europium and ytterbium ions have been obtained. In synthesized YAG:Eu and YAG:Yb ceramics, no luminescence that can be attributed to 5d-4f transitions in Eu²⁺ or Yb²⁺ ions has been detected, even though the scheme of energy levels of these ions constructed with respect to YAG energy bands indicates that there is a potential possibility of the occurrence of 5d-4f luminescence for Eu²⁺ ions in YAG. At room temperature, the luminescence spectrum of hydrothermally synthesized YAG doped with manganese ions consists of two broad bands with maxima at ～600 and ～750 nm and does not contain any narrow bands in the red or IR range. Therefore, the spectrum contradicts to the properties of the luminescence of Mn²⁺, Mn³⁺, or Mn⁴⁺ ions in YAG described in the literature, even though the obtained hydrothermal specimens can contain noticeable concentrations only of Mn³⁺ ions.     

Time-resolved luminescence spectroscopy of pure and doped with Ce3+ ions SrAlF5 crystals

The results of a study of time-resolved photoluminescence (PL) and energy transfer in both pure and doped with Ce³⁺ ions SrAlF₅ (SAF) single crystals are presented. The time-resolved and steady-state PL spectra in the energy range of 1.5–6.0 eV, the PL excitation spectra and the reflectivity in the energy range of 3.7–21 eV, as well as the PL decay kinetics were measured at 8.8 and 295 K. The lattice defects were revealed in the low temperature PL spectra (emission bands at 2.9 and 4.5 eV) in the undoped SAF crystals. The luminescence spectra of the doped Ce³⁺:SAF crystals demonstrate a new selective emission bands in the range of 3.7–4.5 eV with the exponential decay kinetics (τ ≈ 60 ns at X-ray excitation). These bands correspond to the d-f transitions in Ce³⁺ ions, which occupy nonequivalent sites in the crystal lattice.     

luminescence of BaGa<Subscript>2</Subscript>Se<Subscript>4</Subscript> crystals activated by Eu<Superscript>2+</Superscript> and Ce<Superscript>3+</Superscript> ions

We have studied the effect of doping with Eu²⁺ and Ce³⁺ ions on the photoluminescence (PL) of BaGa₂Se₄ crystals in the temperature range 77–300 K. We have established that the broad bands with maxima at wavelengths 456 nm and 506 nm observed in the photoluminescence spectra of BaGa₂Se₄:Ce³⁺ crystals are due to intracenter transitions 5d → ²F_7/2 and 5d →²F_5/2 of the Ce³⁺ ions, while the broad photoluminescence band with maximum at 521 nm in the spectrum of BaGa₂Se₄:Eu²⁺ is associated with 4f⁶ 5d → 4f⁷ (⁸S_7/2) transitions of the Eu²⁺ ion. We show that in BaGa₂Se₄:Eu²⁺,Ce³⁺ crystals, excitation energy is transferred from the Ce³⁺ ions to the Eu²⁺ ions.     

Investigation of gain characteristics in mixed crystals LiMeF4 (Me = Y,Lu, Yb) doped by Ce3+ ions

Differential gain spectra in the range 295–335 nm were measured in crystals of scheelite structure LiY_{1 ? x} Lu _x F₄ (x = 0–1), doped by Ce³⁺ ions. It is shown that variation of Lu³⁺ and Y³⁺ ions relative content in LiY_{1 ? x} Lu _x F₄ crystals allows to manipulate the spectral width of the amplification band. Cross-sections of excited-state absorption at the wavelengths of Ce³⁺ luminescence, probability ratios of formation and thermal destruction of color centers depending on the Y³⁺ ions content in LiY_{1 ? x} Lu _x F₄ crystals were estimated. Even better gain characteristics have been demonstrated by LiLuF₄:Ce³⁺, doped by Yb³⁺ ions. The highest optical gain coefficient with a wide amplification band among studied samples was observed in LiLuF₄:Ce³⁺ crystal, codoped by Yb³⁺ ions.     

Thermodynamics stability,electronic structures and spectroscopic properties of defects and Ce3+ ions in Y2O3

Thermodynamic properties, electronic structures and spectroscopic properties of defects and Ce³⁺ in Y₂O₃ are studied by using the hybrid density functional theory associated with multi-reference configuration interaction ab-initio calculations. Thermodynamic transition energy levels of the easily generated oxygen vacancies in the host are analyzed according to HSE06-calculated formation energies, which may be conducive to interpretations of the persistent luminescence (PersL) of Y₂O₃-based phosphors. Besides, the locations of impurity states (caused by V_O and Ce³⁺) in energy bands are obtained from derived density of states. Moreover, energies and oscillator strengths of 4f₁ → 5d₁₋₅ transitions of Ce³⁺ ions (at Y1 and Y2 sites) calculated from the CASSCF/CASPT2/RASSI−SO method agree reasonably well with experimental excitation spectra of Y₂O₃: Ce³⁺ phosphors, achieving the assignment of excitation spectra. The presented calculations can be applied to identify luminescent centers in Ce³⁺-doped phosphors and reveals possible native defects and their roles in the PersL of phosphors.     

Photoconductivity and photodielectric effect in LiY1 − x Lu x F4 crystals doped with Ce3+ and Yb3+ ions

Time and spectral dependences of the dielectric permittivity of the LiY_{1 ? x} Lu _x F₄ (x = 0, 0.5, and 1) crystals doped with Ce³⁺ and co-doped with Yb³⁺ ions under UV laser excitation were studied by the 8-mm microwave resonant technique at room temperature. The obtained photoconductivity spectrum in 240–310 nm spectral range was interpreted as a stepwise photoionization spectrum of the Ce³⁺ ions due to sequential 4f–5d and 5d–6s transitions. Average lifetimes of free and defect trapped (color centers) charge carriers were estimated.     

Luminescence spectroscopy of Ce3+-doped ABaPO4 (A=Li, Na, K) phosphors

Ce³⁺ doped ABaPO₄ (A=Li, Na, K) phosphors were prepared by conventional high temperature solid-state reaction. The phosphors were investigated by XRD, photoluminescence excitation and emission spectra, and luminescence decay curves. The five 5d levels corresponding to the 4f¹→4f⁰5d¹ transition of Ce³⁺ ions were identified. The spectroscopic parameters, e.g., the 5d barycenter, the crystal-field splitting, and the Stokes shift, were discussed. LiBaPO₄:Ce³⁺ phosphor could be efficiently excited by the near-UV lights (330–420 nm) and showed a broad emission band in the range of 430–620 nm with the maximum wavelength at 468 nm. In contrast, Ce³⁺-doped NaBaPO₄ and KBaPO₄ showed only excitation bands in a limited UV region (230–370 nm) and have blue emission at 385 nm and 416 nm, respectively. The temperature quenching of luminescence and the chromaticity coordinates were reported. The luminescence properties were discussed by analyzing the crystal structure and the local surroundings of Ce³⁺ ions on the Ba²⁺ sites.     

Growth and optical properties of Yb and Tb codoped BaB2O4 crystals

Optical absorption and luminescence spectra of ytterbium and terbium codoped BaB₂O₄ (β-BBO and α-BBO) crystals grown in different conditions have been studied. Low-temperature absorption peaks were observed in all samples. Features related to rare earth ions were observed in absorption and luminescence spectra. Absorption and emission in the range 860-1000 nm are caused by ²F_5/2↔²F_7/2 transitions in Yb³⁺ ions. Emission peaks at 500, 550, 590 and 630 nm correspond to ⁵D₄→⁷F₆, ⁷F₅, ⁷F₄, and ⁷F₃ transitions of Tb³⁺ ions, respectively. The probable reasons of variations in spectroscopic features related to Yb in BBO host are discussed. It has been shown that the replacement of Ва²⁺ by Yb³⁺ in the lattice of ВаВ₂О₄ results in the decrease in the symmetry of oxygen surrounding of Yb³⁺.