High-temperature VUV spectroscopy of KYF4 crystals doped with Nd3+, Er3+ and Tm3+ ions

Thermal quenching of 5d-4f luminescence from Nd³⁺, Er³⁺ and Tm³⁺ ions doped into KYF₄ crystals has been investigated in the temperature range up to ∼750 K where this luminescence is completely quenched. The obtained temperatures of thermal quenching (T_q) are ∼270, 495, 450 K for Nd³⁺, Er³⁺, Tm³⁺, respectively. At high temperatures, thermal quenching of 5d-4f luminescence from Nd³⁺ and Er³⁺ is accompanied by the appearance of 4f-4f luminescence from the lower-energy 4f levels. It has been shown that the dominating mechanism of thermal quenching for Nd³⁺ and Er³⁺ ions is thermally stimulated non-radiative transitions (intersystem crossing) from the 5d states to lower-energy 4f levels, namely ²G(2)_9/2 and ²F(2)_7/2, respectively, whereas for the Tm³⁺ ion, thermally stimulated ionization of 5d electrons to the conduction band states is responsible for thermal quenching of 5d-4f luminescence. The energy gap between the lowest Tm³⁺ 5d level and the bottom of the KYF₄ conduction band has been estimated to be 0.66 eV.     

Shallow acceptor bound exciton and free exciton states in high-purity zinc telluride

We investigated excitons bound to shallow acceptors in high-purity ZnTe and measured excitation spectra of two-hole luminescence lines at 1.6 K using a tunable dye-laser. The electron-hole coupling in the bound exciton (BE) states appears to be very different for the various acceptors even for almost identical exciton localisation energies. Three different types of BE are reported. For the Li-acceptor BE we observe three sub-components separated by 0.22 and 0.17 meV and interpreted as J = $\text{1}\text{2}$, $\text{3}\text{2}$, $\text{5}\text{2}$ states. The Ag-acceptor BE exhibits a strong ground state and a weak excited state at 1.3 meV higher energy. For the as yet unidentified k-acceptor we observe a single BE level, degenerate with the Ag-acceptor BE ground state. Dips in the excitation spectra due to absorption into free exciton 1S, 2S, and 3S states yield an exciton Rydberg R₀ = 12.8±0.3 meV and a free exciton binding energy FE(1S) = 13.2±0.3 meV.     

长波紫外激发下的三基色发光玻璃

报道了一种新的可用长波紫外有效激发的三基色发光玻璃.这种三基色荧光玻璃样品是在相同硅硼酸盐基质中掺杂Ce³⁺/Mn²⁺, Ce³⁺/Tb³⁺以及Eu²⁺并采用熔融法制备出的.其中Mn²⁺,Tb³⁺,Eu²⁺作为激活离子,Ce³⁺作为敏化剂向激活离子提供能量.由于敏化剂的加入使这种三基色发光玻璃在长波紫外激发下获 关键词： 三基色发光 能量传递 长波紫外 发光玻璃     

铈离子掺杂浓度对氯化镧(LaCl_3:Ce)闪烁晶体发光性能的影响

本文采用垂直Bridgman法,分别生长了未掺杂LaCl3和掺有不同浓度CeCl3的LaCl3闪烁晶体,并对它们的透光性质、发光性质和光衰减特性进行了测试和对比分析.发现未掺杂LaCl3晶体的吸收边位于215 nm附近,本征发射峰为405 nm,衰减时间在1μs以上.该发光属于纯氯化镧晶体的自陷激子发射,但随着CeCl3掺杂浓度的提高,源于自陷激子(STE)的本征发射强度逐渐降低,LaCl3:Ce晶体中的吸收边逐渐红移至300nm,由Ce3+中心所产生的5d→4f发射逐渐增强,其衰减时间加快至～20 ns.这种现象被解释为LaCl3晶格中的自陷激子向Ce3+离子发光中心的能量传递作用所致.     

真空紫外光激发下掺Pr3+铝酸盐的自陷激子对Pr3+发光的影响

研究了SrAl₁₂O₁₉:Pr³⁺ 和Y₃Al₅O₁₂(YAG):Pr³⁺ 在20K, 77K和300K的真空紫外(VUV)光谱，二者除了表现出Pr³⁺特有的发射性质外，还产生了一种特殊的发射现象，即自陷激子(STE)发射,并且其随着稀土离子浓度和温度的降低而增强. 还着重讨论了自陷激子对Pr³⁺发光的影响. 关键词： 自陷激子 真空紫外 3+')" href="#">Pr³⁺ 本征激发     

Low-energy excited states of 3d transition metal ions in zinc selenide

The results of studying the impurity heat capacity of Zn_1?x M _xSe (M = Cr²⁺, Fe²⁺, Ni²⁺, Mn²⁺) solid solutions in the temperature range 1.8–20.0 K are presented. A heat-capacity method is described and applied for the measurement of the intracenter-transition energy in these systems. The role of the Jahn-Teller effect in the formation of low-energy excited states of 3d ions in ZnSe is discussed.     

Luminescence and energy transfer in La2O3–Nb2O5–B2O3:M3+ (M=Bi,Eu, Dy) glasses

The luminescence and energy transfer processes in La₂O₃–Nb₂O₅–B₂O₃:M³⁺ (M=Bi, Eu, Dy) glasses were investigated using luminescence spectroscopy (excitation and emission, down to 4.2 K) and decay time measurements at room temperature. The observation of niobate luminescence implies a considerable degree of short- and intermediate-range order in these glasses. Energy transfer from the niobate groups to the lanthanide ions was observed for Eu³⁺, but not for Dy³⁺, suggesting that the energy transfer process occurs to the charge-transfer state of the Eu³⁺ ion, rather than to its f-levels. Inter-Eu³⁺ energy transfer was negligible in the concentration range investigated (up to 3 mol%). In contrast, cross-relaxation processes between Dy³⁺ ions were active at concentrations as low as 0.5 mol%. In the Bi³⁺ doped glasses the energy transfer was observed from the Bi³⁺ excited levels to the oxygen deficient niobate groups.     

Luminescence of Ca(NbO3)2:Pr3+: Pr3+ and self-trapped exciton emission

Photoluminescence and time resolved photoluminescence spectra of Ca(NbO₃)₂ doped with Pr³⁺, excited under 37,000 cm^?1 (270 nm), obtained at high hydrostatic pressure up to 20 kbar applied in a sapphire anvil cells, are presented. At ambient conditions, the emission spectrum obtained in the time interval 0–1 μs is dominated by spin allowed transitions from the ³P₀ state. The luminescence related to transitions from ¹D₂, characterized by a decay time equal to 33 μs, is observed when one excites directly the Pr³⁺ ion with 30,770 cm^?1 (325 nm) wavelength. The introduction of Pr³⁺ impurities in Ca(NbO₃)₂ does not quench the self-trapped exciton (STE) luminescence. This luminescence, peaking at 20,000 cm^?1 (500 nm), having a decay time of 61 ± 1 μs, still occurs when the crystal is excited with a wavelength of 37,000 cm^?1 (270 nm) or shorter. Under such excitation a fraction of the STE luminescence is reabsorbed by Pr³⁺ ions; in this case the emission lifetime of the ¹D₂ → ³H₄ transition of Pr³⁺ is 64 ± 3 μs. This effect is stable also at high pressure.     

Crystal fields of hexameric rare-earth clusters in fluorites

In solid solutions of alkaline-and rare-earth fluorides with a fluorite structure, ions of most elements of the rare-earth (RE) row form hexameric clusters that assimilate the minor component of the solid solutions (fluorine) and build it into the cubic fluorite lattice without changing its shape. An analysis of the EPR spectra of paramagnetic RE ions (Er³⁺, Tm³⁺, Yb³⁺) in clusters of diamagnetic ions (Lu³⁺, Y³⁺) confirms their hexagonal structure, which was established when studying the superstructures of the compounds under study. In such a cluster, a RE ion is in a nearly tetragonal crystal field, with the parameters of this field differing radically from those of single cubic and tetragonal RE centers in crystals with a fluorite structure. In particular, this field causes high (close to limiting) values of the g_∥ factors of the ground states of the paramagnetic RE ions. Computer simulation is used to determine the atomic structure of a hexameric cluster in MF₂ crystals (M = Ca, Sr, Ba). The crystal field and energy spectrum of Er³⁺, Tm³⁺, and Yb³⁺ ions in such clusters are calculated, and the spectroscopic parameters of the ground states of these ions are determined. The calculations confirm the earlier assumption that the unusual EPR spectra of nonstoichiometric fluorite phases are related to RE ions in hexameric clusters.     

Anomalous luminescence of impurity-bound excitons in lithium borate crystals doped with cerium ions

The spectra and relaxation kinetics of the anomalous (?? < 10 ns) luminescence of Li₆GdB₃O₉:Ce³⁺ crystals have been experimentally detected. The time-resolved vacuum ultraviolet spectroscopy study has shown that optical transitions at 6.2 eV, caused by the transfer of an electron from the 4f ¹ ground state of Ce³⁺ to autoionizing states near the conduction band bottom of a crystal, lead to the formation of an impurity-bound exciton with the hole component localized on the 4f state of Ce³⁺ and the electron localized on states of the conduction band bottom. It has been found that the decay of such an exciton in Li₆GdB₃O₉:Ce³⁺ occurs through radiative recombination, leading to fast luminescence at 4.25 eV. The energy threshold for the formation of the impurity-bound exciton has been determined. The distribution functions of elementary relaxations over the reaction rate constants H(k), which determine the relaxation kinetics and luminescence quenching processes, have been calculated.     

Role of methylene spacer in the excitation energy transfer in europium 1- and 2- naphthylcarboxylates

A series of compounds Ln(RCOO)₃·Phen (Ln=Eu, Gd, Tb; RCOO⁻-1- and 2-naphthoate, 1- and 2-naphthylacetate, 1- and 2-naphthoxyacetate anions, Phen-1,10-phenanthroline) was investigated by methods of optical spectroscopy. Compounds of composition Ln(RCOO)₃·nH₂O with the same carboxylate ligands are also considered. Results of studies of the effects of methylene spacer decoupling the π-π- or p-π-conjugation in the naphthylcarboxylate ligand on the structure of Eu³⁺ coordination centre, on the lifetime of ⁵D₀ (Eu³⁺) state, and on processes of the excitation energy transfer to Eu³⁺ or Tb³⁺ ions are presented. Introduction of the methylene bridge in the ligand weakens the influence of the steric hindrances in forming of a crystal lattice and results in lowering the distortion of the Eu³⁺ luminescence centre, and in elongation of the observed ⁵D₀ lifetime τ_obs. The latter is caused by decrease in contribution of the radiative processes rate 1/τ_r. This is confirmed by the correlation between the lifetimes τ_obs and the quantities “τ_r·const” inversely proportional to the total integral intensities of Eu(RCOO)₃·Phen luminescence spectra. The methylene spacer performs a role of regulator of sensitization of the Ln³⁺ luminescence efficiency by means of an influence on mutual location of lowest triplet states of the ligands, the ligand-metal charge transfer (LMCT) states, and the emitting states of Ln³⁺ ions. The lowest triplet state in lanthanide naphthylcarboxylate adducts with Phen is related to carboxylate anion. A presence of the methylene spacer in naphthylcarboxylate ligand increases the triplet state energy. At the same time, the energy of “carboxylic group-Eu³⁺ ion” charge transfer states falls, which can promote the degradation of excitation energy. In naphthylcarboxylates investigated a range of the carboxylate triplet state energies from 19 150 to 20 600 cm⁻¹ was demonstrated in dependence on the type of the carboxylate anion. The interligand energy transfer from Phen to carboxylate lowest triplet state was revealed in complexes with Phen ligand. The effect of OH-group inserted in 1- or 3-position of 2-naphthoate ligand on the excitation energy transfer is also analyzed.     

Fluorescence enhancement of Eu3+ by Tb3+ in dimethylsulfoxide (DMSO)

Photoexcitation of EuCl₃.6H₂O and TbCl₃.6H₂O mixtures in DMSO at various wavelengths brings about a decrease in the fluorescence intensity of Tb³⁺ and a subsequent enhancement in the fluorescence intensity of Tb³⁺ provided that [Tb³⁺] < [Eu³⁺]. The average value of the electronic excitation energy transfer rate constant k₁₀ which was found to be independent of the excitation wavelength, was determined to be about 1.50 × 10³ M^-1 s^-1. Photoexcitation of Tb³⁺ and subsequent population of high energy excited states is accompanied by rapid nonradiative de-excitation processes to the lowest excited state ⁵D₄, which is the origin of the energy transfer process. A lower limit for the value of the reaction rate constant, associated with the transition ⁵D₃ ? ⁵D₄, namely k₅, is of the order of 10⁵ ?10⁶ s^-1. Excitation at conditions leading to the exclusive population of the ⁵D₄ state of Tb³⁺ gave rise to a value of k₁₀ equal to (2.2 ± 0.4) × 10³ M^-1 s^-1 and a critical separation (R₀)_exp between Tb³⁺ and Eu³⁺ of about 13 Å. A theoretical value of R₀ equal to 14.2 Å was calculated. The energy transfer process does not appear to take place via clear cut dipole-dipole interactions but rather via complex multipole and/or exchange interactions.     

Synthesis and luminescent properties of Y6W2O15:Eu phosphors

In this paper, a novel phosphor, Y₆W₂O₁₅:Eu³⁺ was synthesized by thermal decomposition and phase transition of its decatungstate gel precursor. With stepwise increase of temperature to 750 °C, a crystalline phase of Y₆W₂O₁₅:Eu³⁺forms that gives intense red emission when excited at 466 nm, the emission is attributed to the Eu³⁺ ions transitions from ⁵D₀ excited states to ⁷F_J (J=0-4) ground states. The long excitation wavelength proves the Eu³⁺ transition follows the photoexcitation of the oxygen-metal (O→W lmct) charge transfer bands in yttrium tungstate. Some structural information regarding Y₆W₂O₁₅ provided by luminescence is in accord with that characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The long-wavelength excitation properties of this material may find application in the production of red phosphors for white light-emitting diodes (LEDs).     

Luminescence properties of NaGd(PO3)4:Eu3+ and energy transfer from Gd3+ to Eu3+

The luminescence properties of polyphosphates NaEu _x Gd_(1?x)(PO₃)₄ (x = 0–1.00) and the energy transfer from Gd³⁺ to Eu³⁺ were studied. In undoped NaGd(PO₃)₄ sample, the photon cascade emission of Gd³⁺ was observed under ⁸S_7/2 → ⁶G_J excitation (201 nm) in which the emission of a red photon due to ⁶G_J → ⁶P_J transition is followed by an ultraviolet photon emission due to ⁶P_J → ⁸S_7/2 transition. When part of Gd³⁺ ions in the host NaGd(PO₃)₄ were substituted by Eu³⁺ ions, the NaGd(PO₃)₄:Eu³⁺ sample showed intensive red emission under 172-nm vacuum-ultraviolet (VUV) excitation which is suitable for mercury-free fluorescent lamps and plasma display panel applications. Based on the VUV–visible spectroscopic characteristics and the luminescence decay properties of NaGd(PO₃)₄:Eu³⁺, it was found that the quantum cutting by a two-step energy transfer from Gd³⁺ to Eu³⁺ can improve the red emission of Eu³⁺ ions under VUV excitation but only a part of the excitation energy in the excited ⁶P_J states within Gd³⁺ ions can be transferred to Eu³⁺ ions for its red emission, and the nonradiative energy transfer efficiencies from the excited ⁶P_J states within Gd³⁺ to Eu³⁺ were calculated.     

Enhanced luminescence of Dy in Y3Al5O12 by Bi co-doping

In the present paper, phosphors with the composition Y_3−x−yAl₅O₁₂:Bi³⁺_x, Dy³⁺_y were synthesized with solid state reactions. The luminescence properties of Bi³⁺ and Dy³⁺ in Y₃Al₅O₁₂(YAG) and the energy transfer from Bi³⁺ to Dy³⁺ were investigated in detail. Bi³⁺ in YAG emits one broad band peaking at 304 nm which can be ascribed to the transition from excited states ³P_{0, 1} to ground state ¹S₀. Dy³⁺ in YAG emits two groups of peaks around 484 and 583 nm, respectively, which can be ascribed to the transitions from excited state ⁴F_9/2 to ground states ⁶H_15/2 and ⁶H_13/2. The co-doping of Bi³⁺ enhances the luminescent intensity of Dy³⁺ by ∼7 times because Bi³⁺ can transfer the absorbed energy to Dy³⁺ efficiently. The mechanism of energy transfer was also discussed.     

Luminescence self-quenching in Tm3+: YLF crystals: II. The luminescence decay and macrorates of energy transfer

Microparameters of migration and self-quenching of luminescence from the (³ P ₀, ¹ I ₆) levels of Tm ³⁺ ions are calculated and the luminescence decay from the ³ H ₄, ¹ G ₄, and ¹ _{D 2} levels are analyzed in the concentration series of LiY_1?x Tm_xF₄(x=0.5–10 at. %) crystals upon weak selective excitation. It is shown that the luminescence decay of a thulium ion concentration higher than 0.5 at. % cannot be described by the energy transfer that takes into account only the dipole-dipole interaction. The comparison of experimental kinetic curves with theoretical curves calculated within the framework of the known energy transfer models showed that multipole-multipole interaction is responsible for the self-quenching of the thulium levels. Contributions of higher multipole interactions to the self-quenching are estimated. The decay of luminescence from the ³ H ₄, ¹ G ₄, and ¹ D ₂ levels of Tm³⁺:YLF crystals with different concentrations of thulium ions is described analytically.     

Spectral and luminescent characteristics of trivalent lanthanide ions in a POCl3-SnCl4 inorganic solvent

We have measured the absorption and luminescence spectra of trivalent lanthanide ions (Pr³⁺, Sm³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, and Yb³⁺) in the visible and near-IR wavelength ranges in a POCl₃-SnCl₄ inorganic solvent. In terms of the Judd-Ofelt model, the oscillator strengths of absorption bands, the probabilities of radiative transitions, the luminescence branching coefficients, the lifetimes of excited states, and the luminescence quantum yields have been calculated. Possibilities of creating new laser media have been evaluated. A conclusion is drawn regarding the symmetry of the environment of trivalent lanthanide ions in the POCl₃-SnCl₄ solvent.     

VUV excited luminescence of MGdF4:Eu (M=Na, K, NH4)

Eu³⁺-doped NaGdF₄, KGdF₄ and NH₄GdF₄ phosphors with little oxygen contamination have been synthesized by hydrothermal technique. The emission spectra show that the doped Eu³⁺ ions are located in noncentrosymmetric sites in the three compounds. The two-photon emission has been observed in NaGdF₄:Eu³⁺ and KGdF₄:Eu³⁺ compounds under VUV excitation from the ground states to higher ⁶G_J excited states of Gd³⁺ ions, while in Eu³⁺-doped NH₄GdF₄, emissions from ⁵D_1,2,3 excited states of Eu³⁺ cannot be detected in the luminescence spectra.     

Europium luminescence in fluorite upon high-energy excitation

The reflection and luminescence excitation spectra of CaF₂ crystals containing europium ions in divalent (Eu²⁺) and trivalent (Eu³⁺) states were measured in the range from 4 to 16 eV. It was established that, in CaF₂ : Eu³⁺ crystals, luminescence of Eu³⁺ ions (the f-f transitions) is effectively excited both in the charge-transfer band (at ～8 eV) and in the region of the 4f–5d transitions (at ～10 eV) but is virtually not excited in the fundamental region of the crystal (at an energy higher than 10.5 eV). Luminescence of Eu²⁺ ions (the 427-nm band) in CaF₂ : Eu³⁺ is effectively excited in the fundamental region of the crystal; i.e., luminescence of divalent europium ions occurs through the trapping mechanism. Emission of Eu²⁺ ions in CaF₂ : Eu²⁺ crystals is characterized by the excitation band at an energy of 5.6 eV (the 4f → 5d,t _2g transitions), as well as by the exciton and interband luminescence excitations. The results obtained and data available in the literature are used to construct the energy level diagram with the basic electron transitions in the CaF₂ : Eu crystals.     

A study of energy transfer phenomenon leading to photon up-conversion in Ho3+:Yb3+:CaF2 crystalline powders and its temperature sensing properties

When Ho³⁺:Yb³⁺:CaF₂ crystalline powders prepared by combustion synthesis were exposed to near-infrared (λ ～ 975 nm) radiation, intense photon up-conversion (UC) was observed at the visible with emission bands peaked at ～ 545, ～650 and ～750 nm identified as 4f-4f transitions from higher levels (⁵F₄, ⁵S₂) and ⁵F₅ to lower levels ⁵I₈ and ⁵I₇ of Ho³⁺. The emission bands at the green and red, in particular, have been demonstrated to be useful for temperature sensing based on luminescence intensity ratio technique. However, no model is available in literature to explain the change of the electronic populations of states (⁵F₄, ⁵S₂) and ⁵F₅ with temperature. The UC phenomenon was studied from both theoretical and experimental points of view. A rate equation model with temperature dependent parameters for Ho³⁺ and Yb³⁺ electronic populations considering a high sensitization of Ho³⁺ ions by Yb³⁺ ions was used. High Yb³⁺ → Ho³⁺ energy transfer efficiency was found (～88% at room temperature). The change with temperature predicted by the model for the luminescence intensity ratio of the UC green and red emission lines agrees well with the experimental data.