Radiative and non-radiative transitions in ErP5O14 single crystals

The radiative and non-radiative transitions from excited states of ErP₅O₁₄ were studied on the basis of the Judd-Ofelt theory and experimental data. It was shown that the non-radiative relaxations, predominantly multiphonon emission, are very effective in the depopulation of excited states of erbium in erbium pentaphosphate crystals. The concentration quenching of erbium fluorescence is found to be small.     

Photoluminescence and multiphonon resonant Raman scattering in Ni-and Co-doped Zn1−x MnxTe crystals

Low-temperature photoluminescence, exciton reflection, and multiphonon resonant Raman scattering spectra of Ni-and Co-doped Zn_1−x Mn_xTe crystals were investigated. Intense emission occurs in a broad spectral region (1100–17 000 cm⁻¹) in the crystals containing Ni atoms. It is caused by intracenter transitions involving Mn²⁺ ions and transitions between the conduction band and a level of the doubly charged acceptor. The features of the exciton photoluminescence and multiphonon resonant Raman scattering involving longitudinal-optical (LO) phonons at various temperatures are investigated. The insignificant efficiency of the localization of excitons on potential fluctuations in the Zn_1−x Mn_xTe:Co crystals is established. A temperature-induced increase in the intensity of the 5LO multiphonon resonant Raman scattering line due to the approach of the conditions for resonance between this line and the ground exciton state is observed in these crystals. Fiz. Tverd. Tela (St. Petersburg) 40, 616–621 (April 1998)     

晶格弛豫和多声子跃迁理论

近年的发展日益证明,固体中的局域电子态所导致的晶格弛豫(晶格原子位置移动)是局域电子态的一项基本特征,能够引起许多重要的效应。在局域态的吸收和发射光谱中,由于晶格弛豫而引起斯托克斯位移和光谱的多声子结构(多声子的光跃迁)。晶格弛豫又可以在不同电子态之间引起无辐射跃迁,其中的能量变化完全由多声子的发射或吸收来补偿(多声子无辐射跃迁)。本文以线性的电子—声子作用和简谐近似的晶格模型为基础,系统地总结阐明多声子的光跃迁理论和多声子的无辐射跃迁理论。关于光跃迁的部份既介绍了直接计算跃迁几率的方法,也介绍了采用傅里叶变换的理论,并分析了连续谱和多声子结构相互叠加的一个光谱实例。关于无辐射跃迁的部份着重分析和澄清了多年来环绕所谓“康登近似”出现的矛盾,从而把近年来不同的理论发展置于统一的基础之上。介绍了估算跃迁几率的最陡下降法。最后讨论了强耦合和弱耦合的情形。     

Intensity and bandwidth of multiphonon vibronic transitions of rare-earth ions in crystals

The theory of multiphonon vibronic coupling to electronic transitions is applied in analysing fluorescence spectra of Eu²⁺ in BaFCI, which consist of the 4f⁷(⁶P_7/2,) → 4f⁷(⁸S_7/2) and 4f⁶5d → 4f⁷ transitions, and the 4f⁷-4f⁶5d excitation spectrum of Ce³⁺ in YPO⁴. The 4f electrons are weakly coupled to lattice vibration modes so that only weak one- and two-phonon sidebands are observable in the 4f-4f optical transitions, whereas the electron-phonon coupling is significantly stronger for a 5d electron. Accordingly, intensive multiphonon vibronic transitions overwhelmingly dominate the 4f⁶5d → 4f⁷ spectrum. It is shown that the extended Judd-Ofelt theory for weak vibronic coupling in the framework of the M-process is equivalent to the Huang-Rhys theory for the δ-process. In the analysis of experimental data, contributions from local ligand modes and lattice acoustic modes are separated, and the coupling strength is evaluated, in terms of the Huang-Rhys parameter S, for the 4f-4f and 5d-4f vibronic transitions.     

Selection rules for multiphonon transitions at tetrahedral lattice sites

Selection rules for the involvement of phonons of the host crystal in multiphonon transitions associated with substitutional impurities having either a |s> or |p> localized state are derived. It is shown that for P₁ → P₁ transitions only LA and LO phonons are important, and also that a P₄ localized state couples with all phonons. The latter, plus the lack of a selection bar, suggests that such impurity states are more strongly coupled to the lattice that P₁ states.     

Chemical Shift of the <Emphasis Type="Italic">K</Emphasis><Subscript>α1</Subscript> and <Emphasis Type="Italic">K</Emphasis><Subscript>α2</Subscript> Lines of the X-ray Emission Spectrum of Yb(II)/Yb(III) Fluorides: a Quantum-Chemical Investigation

Chemical shifts of the K_α1 and K_α2 lines (the 2p_3/2 → 1s_1/2 and 2p_1/2 → 1s_1/2 transitions, respectively) of the X-ray emission spectrum of the Yb atom in fluorides have been calculated by ab initio modeling the electronic structure. The valence transition Yb(II) → Yb(III) has been analyzed by examples of YbF₂, YbF₃, and Yb₂F₄ molecules and YbF₂⁺ cation. The relativistic pseudopotential and basis sets corresponding to it have been constructed for the ytterbium atom. They were used in calculations by the two-component noncollinear version of the density functional theory (DFT) with the PBE0 exchange-correlation functional. Results for the three-coordinated Yb(II) in the FYbF₂YbF dimer demonstrated a very weak dependence of the chemical shift on the coordination number of the Yb atom and on the molecular association of ytterbium difluoride. Chemical shifts of the X-ray emission spectrum for the ytterbium compound are related mainly to the change in the occupation of the 4 f shell.     

Non-radiative relaxation processes of the Pr3+ ion

Conclusions The³P₀→¹D₂ non-radiative decay rates A_NR of Pr³⁺ and the transition probabilities A_VIB for the vibronic³H₄→³P₀ excitation transitions of Pr³⁺ were measured at 4.2 K in several host lattices. Besides the well-known dependence on the number of phonons involved in the radiationless transition, the multiphonon relaxation rates are observed to be greatly enhanced by increasing covalency and/or decreasing Pr-ligand distances. The enhanced multiphonon relaxation is ascribed to a stronger electron-phonon coupling. The increase in the electron-phonon coupling strength is confirmed by the increase of the vibronic transition probabilities A_VIB in the same sequence as the A_NR rates. The observed host lattice dependence of A_NR can be qualitatively accounted for by a non-linear multiphonon relaxation mechanism. Under certain conditions (viz. low-lying 4f5d states, and a force constant stronger in the 4f5d than in the 4f² states) fast³P₀→¹D₂ non-radiative relaxation via intersystem crossing through the 4f5d state becomes the dominant relaxation mechanism, even at 4.2 K. Debye Institute, Utrecht University P. O. Box 80000, 3508 TA Utrecht, The Netherlands. Published in Zhurnal Prikladnoi Spektroskopii, Vol. 62, No. 4, pp. 63–71, July–August, 1995.     

Nuclear excitation and prompt fission in muonic238U

A study of muonic²³⁸U has been performed in a combined (μ ^?,γ f) and (μ ^?,γγ) coincidence experiment to investigate the role of non-radiative transitions and their fission probabilities. An augmentation of the outer fission barrier ofΔE _b =(0.6±0.1) MeV due to the presence of the muon is deduced. A significant contribution to the prompt fission yield not only results from the (2p→1s) and (3d→1s) non-radiative transitions, but also from other radiationless transitions. Specifically, the measured fission probabilities of the transitions (2p→1s), (3d→1s), and (3p→1s) are (1.5±0.4)%, (5.7±1.7)%, and (5.3±1.9)%, respectively.     

Studies of the red anti-stokes emission of erbium ions in the Y2O2S: Er,Yb phosphor system

Excitation of the anti-Stokes red emission radiation from erbium ions by 1000 nm radiation in the Y₂O₂S: Er, Yb system has been investigated using a range of samples with widely varying erbium but fixed ytterbium content. A square law intensity dependence is found favouring the models of Sommerdijk and of Hewes and Sarver. Comparison of Stokes and anti-Stokes excitations of both green and red erbium emissions then eliminates the Hewes and Sarver model. Studies using twin-beam excitation as by Kuroda et al. (1000 nm plus 1500 nm) and over a range of temperatures give evidence that the previous assumption of the non-radiative, multiphonon relaxation between the ⁴I_{$\text{11}\text{2}$} and ⁴I_{$\text{13}\text{2}$} states following the first 1000 nm excitation is not satisfactory and suggest that this transition is radiative (2500 nm radiation).     

The electronic absorption spectra of [(CH3)3NH] MnCl3.2H2O single crystals

The electronic absorption spectra of [(CH₃)₃NH] MnCl₃.2H₂O single crystals are reported in the 15,000–45,000 cm^-1 region. In addition to the normally studied sextet → quartet transitions, special attention has been paid to the sextet → doublet transitions. Crystal field parameters evaluated (including the Trees' correction factor) to fit the observed spectra are B = 800, C = 2900, Dq = 680, and α = 76 cm^-1.     

Optical study of single crystals grown by the Czochralski method from Yb3+-doped (Gd1−x Y x )2SiO5 solid solution

Yb³⁺-doped crystals were grown by the Czochralski method from (Gd_0.5Y_0.5)₂SiO₅ melt employing seeds cut out from single crystals of Yb:Gd₂SiO₅ and Yb:Y₂SiO₅. XRD analysis revealed that the structure of the solid solution crystals grown is consistent with the structure of the seed, namely C2/c for the Yb:Y₂SiO₅ seed and P2₁/c for the Yb:Gd₂SiO₅ seed. Optical absorption spectra, emission spectra and luminescence decay curves were recorded at 10 K and 300 K. Analysis of gathered spectroscopic data made it possible to evaluate radiative transition rates for the ²F_5/2 multiplet of Yb³⁺ in the two structures and to assess crystal field splitting of multiplets involved in radiative transitions. It has been concluded that transition intensities and relaxation dynamics of Yb³⁺ in the systems studied are similar but the intensity distribution of emission and absorption bands depends significantly on the system structure. This feature, combined with inhomogeneous broadening of spectral lines, may be advantageous for the tailoring of lasers employing ytterbium-doped crystals for specific applications.     

Unified model of energy transfer for arbitrary Franck-Condon offset and temperature

The Huang-Rhys-Pekar W_p(T, S) function is known to give the thermal-Franck-Condon (FC) weight of single-center radiative and non-radiative transitions for arbitrary energy mismatch p?ω₀, temperature T, and FC offset S. The thermal-FC weight for energy transfer is shown also to be W_p(T, S), with S enlarged to be the sum of the S values of the energy donor and energy acceptor. This result generalizes an earlier result of Miyakawa and Dexter for low-temperature Poisson functions. The W_p function with enlarged S is equivalent to the overlap of the donor emission and acceptor absorption bands in the Förster-Dexter theory. Experimental verification is good except for the p dependence of non-radiative and energy-transfer rates for S small (e.g. 4f → 4f transitions). Here, if the smooth, exponential p dependences (exponential energy-gap laws) that have been drawn through many 4f → 4f transition rates in certain hosts (Y₂O₃, LaF₃) are fitted with a W_p(T, S) function with a single S value, this S value is too large to describe 4f → 4f transitions.     

Optical and EPR study of BaY2F8 single crystals doped with Yb

Optical and electron paramagnetic resonance study have been carried out on BaY₂F₈ single crystals doped with Yb ions at 0.5 and 10 mol%. The crystals have been obtained using the Czochralski method modified for fluoride crystal growth. Optical transmission measurements in the range of 190-3200 nm and photoluminescence measurements were carried out at room temperature. Absorption spectra of BaY₂F₈ single crystals doped with Yb due to the ²F_7/2→²F_5/2 transitions have been observed in the 930-980 nm range. To analyze the possible presence of Yb²⁺ ions in the investigated crystals, irradiation with γ-quanta with a dose of 10⁵ Gy have been performed. The observed photoluminescence bands show usual emission in IR and other one in VIS, being an effect of cooperative emission of Yb³⁺ ions and energy up-conversion transitions of photons from IR to UV-vis(visible) due to hoping process between energy levels of paired Yb³⁺ and Er³⁺, where Er³⁺ ions are unintentional dopants. The EPR spectra of BaY₂F₈:Yb 10 mol% consist of many overlapping lines. They have been analyzed in terms of spin monomers, pairs, and clusters. The angular dependence of the resonance lines positions have been studied also to find the location of coupled ytterbium ions in the crystal structure.     

A study on the structural phase transitions of ferroelastic [N(CH3)4]2CuCl4 from microhardness measurements

Measurements of Vickers microhardness (MHv) as a function of temperature has been carried out on [N(CH₃)₄]₂CuCl₄ single crystals. The suitability of this technique for the determination of transition temperatures in the paraelastic-incommensurate→ferroelastic (commensurate) phase transitions sequence is shown.     

Optical transitions of Eu2+ IONS in RbMgF3 crystals

Because of the need for tunable solid state lasers in the blue region of the spectrum, a number of recent studies have been on the optical properties of Eu²⁺ in various crystals lattices. The absorption and emission from Eu²⁺ ions in fluoride hosts arise from both 5d→4f and 4f→4f transitions. The absorption is in the ultra-violet and the emission is in the blue. The 4f-4f transitions are very sharp and are located around 360 nm. The 5d→4f emission bands at 405 nm and 525 nm are broad and host lattice dependent. We have studied the temperature dependence of the optical properties of RbMgF₃:Eu²⁺ including absorption, emission, and excitation. In the case of the emission studies both temperature dependence and the lifetimes of the transitions have been measured. These transitions are discussed in detail.     

Multiphonon relaxation of excited rare-earth ions in ionic matrices

A theory of multiphonon relaxation for rare-earth ions in ionic crystals and glasses is presented. The theory is based on the static coupling scheme. The electron-vibration coupling is assumed to be linear with respect to the relative ligand rare-earth displacements. The multiphonon transitions are induced by higher-order lattice anharmonicity of the ionic motions. The probability of multiphonon transitions could be related to the IR multiphonon spectrum. A simplified form of the general expression is proposed. The electronic part of the expression obtained has exactly the same form as the corresponding expression in the well-known Judd-Ofelt theory for optical transitions between multiplets of rare-earth ions in solids.     

Multiphonon relaxation in fluoride and ternary sulfide laser crystals with neodymium ions

It is shown that, in ternary sulfide crystal matrices, in contrast to fluoride matrices with similar phonon spectra, the multiphonon relaxation rate as a function of the number p _eff of “effective phonons” (the energy gap ΔE _min between the J′ and J levels of the Nd³⁺ ion) has a steep slope, which does not decrease with increasing p _eff. This indicates that, compared to fluoride crystals, the multiphonon relaxation rate of mid-IR transitions (in the range 4–5 μm) in ternary sulfide matrices sharply decreases, by three to four orders of magnitude.     

Room-temperature diode-pumped Yb<Superscript>3+</Superscript>-doped LiYF<Subscript>4</Subscript> and KYF<Subscript>4</Subscript> lasers

We present a comparative analysis on the growth, the spectroscopic features, and the cw laser action of room-temperature Yb(5%):LiYF₄ (YLF) and Yb(10%):KYF₄ (KYF) crystals. Optical slope efficiencies of 33% and 52% have been demonstrated for Yb:YLF and Yb:KYF crystals, respectively. A remarkable wide wavelength tunability from 1.01 to 1.07 μm has been obtained for both laser crystals.     

Additive colouring of CaF2:Yb crystals: determination of Yb2+ concentration in CaF2:Yb crystals and ceramics

When growing CaF₂ crystal doped with rare-earth ions, most of these ions are present in a trivalent state. However, due to contact with graphite crucible, a small proportion of a number of ions (Eu, Sm, Yb and Tm) are reduced to a bivalent state. A similar situation takes place during fabrication of CaF₂ ceramics doped with rare-earth metals. This fact is of particular importance for laser CaF₂:Yb crystals (ceramics), a promising material for short-pulse, high-power, high-energy diode-pumped solid state lasers since the presence of bivalent Yb ions can be a source of thermal losses. To date, there has been no technique to determine Yb²⁺ concentration in as-grown crystals. The proposed technique is based on a total reduction of Yb³⁺ ions via the heating of as-grown CaF₂ crystals with known concentration of Yb in the reducing atmosphere of metal vapour and determining the cross section of absorption bands of Yb²⁺ ions. The knowledge of these parameters allows estimation of the Yb²⁺ content in CaF₂:Yb crystals or ceramics by analysing their absorption spectra. Examples of using this technique are given. The technology of CdF₂ crystals reduction (an “additive colouring”) and features of colouring of crystals doped with rare-earth ions are considered.     

Influence of borate content on the radiative properties of Nd ions in fluorophosphate glasses

Spectroscopic properties of Nd³⁺ in barium fluoroborophosphate glassy matrix have been analyzed by fitting the experimental data with the standard Judd-Ofelt theory. Various spectroscopic parameters viz. radiative transition probabilities, radiative decay time, fluorescence branching ratios, electric dipole line strengths, stimulated emission cross-sections and optical gain of the principal fluorescence transitions from the ⁴F_3/2 metastable level are obtained. Results show that addition of borate content to the fluorophosphate matrix will reduce the fluorescence spectral properties of Nd³⁺. However, the radiative properties of the present fluoroborophosphate glassy matrix are found to be well improved over that of pure borate and phosphate matrix and is attributed to the influence of fluorine content in the glassy matrix. The changes in the position and the Judd-Ofelt intensity parameters are correlated with the structural changes in the host glass matrix. The shift of the hypersensitive band shows that the covalency of the rare earth to oxygen bond increases with the increase of Na₂O content. This covalency effect and the formation of the BO₄ groups with the addition of Na₂O content are responsible for the increase in the radiative properties of the present system. Quantitative estimation of the non-radiative processes such as multiphonon relaxation and quenching by water content was carried out and the results show that both are below the critical level for optimum laser performance.