Nonradiative decay from 5d states of rare earths in crystals

Nonradiative decay from 4f^n?1 5d states was investigated for trivalent rare earths in Y₃Al₅O₁₂. The rates of both 5d→4f and 5d→5d transitions were determined from measurements of the lifetimes and intensities of 5d fluorescence from Ce³⁺ and Pr³⁺. Because of the stronger ion-lattice coupling, nonradiative decay rates for transitions involving 5d states are much faster than those between 4f states. Decay rates are dependent upon the temperature and the energy gap to the next-lower level. The temperature dependences of the 5d fluorescence lifetimes from 77 to 700°K are reported.     

Determination of odd-symmetry crystal-field parameters from optical spectra

We have obtained analytical expressions for effective parameters of the crystal field that acts on spin-orbit multiplets of 4f ^N configurations taking into account admixture to them of 4f ^N?15d excited states and ligand-to-metal charge-transfer states. As an example, we analyze splittings of the ground and excited multiplets of Pr³⁺ and Tm²⁺ ions in some crystals without an inversion center. The effect of mixing of states of different configurations is most strongly pronounced for the ¹ G ₄ and ¹ D ₂ excited multiplets. The interconfigurational contribution to splittings is different for different multiplets. This circumstance makes it possible to estimate the values of the parameters of the odd-symmetry crystal field, which causes mixing of the 4f ^N and 4f ^N?15d states, and the covalence parameters of rare-earth ion-ligand bonds.     

X-ray M4,5 resonant Raman scattering from Gd with final 4p hole: calculations with 4p–4d–4f configuration interaction in the final state and comparison with the experiment

We present calculations of resonant Raman scattering (RRS) at the M_4,5 thresholds of Gd in the scattering channel 3d¹⁰4f⁷→3d⁹4f⁸→[4p⁵4f⁸↔4d⁸4f⁹]. We have included in the final state the interaction between the two configurations within the brackets, having one 4p and two 4d holes, respectively. The influence of the configuration interaction on the scattering spectra is shown to be important. The calculations are made within a purely ionic model including only the spectral features dispersing with the incident photon energy and do not account for the M₄ to M₅ Coster-Kronig conversion. The calculations are compared with recent experimental results on Gd metal. The agreement is excellent when choosing the excitation energy in the M₅ region. In the M₄ region the calculations agree with the measurements by assuming that the Coster-Kronig contribution is approximated in shape by the RRS spectrum measured with direct M₅ excitation. The implications of the results are discussed.     

The46Ti(d,τ)45Sc reaction

The⁴⁶Ti(d, τ)⁴⁵Sc reaction has been investigated at an incident energy of 52 MeV. Angular distributions have been taken for 14 τ-groups corresponding to excitation energies below 4 MeV in⁴⁵Sc. Spectroscopic factors were extracted through DWBA calculations. Spins and parities of 5/2⁺ are proposed for states at 1.30, 1.80, 2.91, 3.48, and 3.72 MeV. The hole state spectrum of⁴⁵Sc closely resembles that of⁴⁷Sc. A strong fractionation of the 1d _3/2 strength was not observed in contrast to other recent work. The data provide evidence for (1f _7/2 · 1f _5/2 ¹) components in the ground state wave function of⁴⁶Ti, which demonstrates that such configurations are not restricted to neutron excitations.     

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.     

Excited states dynamics under high pressure in lanthanide-doped solids

Emission related to rare earth ions in solids takes place usually due to 4fⁿ→4fⁿ and 4fⁿ⁻¹5d¹→4fⁿ internal transitions. In the case of band to band excitation the effective energy transfer from the host to optically active impurity is required. Among other processes one of the possibilities is capturing of the electron at the excited state and the hole at the ground state of impurity.The latest results on high pressure investigations of luminescence related to Pr³⁺ and Eu²⁺ in different lattices are briefly reviewed. The influence of pressure on anomalous luminescence and 4fⁿ⁻¹5d¹→4fⁿ luminescence in BaSrF₂:Eu²⁺ and LiBaF₃:Eu²⁺ systems and Pr³⁺ 4fⁿ→4fⁿ emission quenching is presented and discussed. A theoretical model describing the impurity-trapped exciton as a system where a hole is localized at the impurity and an electron is captured by Coulomb potential at Rydberg-like states is developed. The results show the importance of local lattice relaxation for the creation of stable impurity-trapped exciton states. The ligands shifts create a potential barrier that controls the effect of mixing between the Rydberg-like electron and localized electron wave functions.     

Collective bands of the positive parity states inf 7/2 shell nuclei

Collective bands of the positive parity states in odd-A f _7/2 shell nuclei are described in the framework of deformed Hartree-Fock theory by projecting states from lowest energy intrinsic states with (d ₃ ^2/−1 f ₇ ^2/n+1 ) one hole configurations. In the calculation empirical (d _3/2 −f _7/2)² effective matrix elements have been used to test the tacit assumptions of the Bansal and French model.     

Luminescence study of the 4f5d configuration of Nd in LiYF4, LiLuF4 and BaY2F8 crystals

Ultraviolet fluorescence of Nd³⁺ ions induced by triphotonic excitation process was studied in Nd-doped LiYF₄, LiLuF₄ and BaY₂F₈ crystals using a technique of time-resolved spectroscopy. The observed ultraviolet luminescence was due to transitions between the bottom of 4f²5d configuration and 4f³ states of Nd³⁺ ions. Narrow emission lines superposed to the broadband emissions were observed. A detailed analysis of luminescence spectrum revealed that the narrow emissions are due to parity and spin allowed radiative transitions from the Stark levels of ⁴K_11/2(5d) state created by the electrostatic interaction between the 5d electron and the two electrons of the 4f² configuration. The narrow emissions are related to the high spin state (S=3/2) which gives f-f characteristics to the f-d broadband emissions. The narrow emissions superposed to the wide emission correspond to 18%, 34% and 43% of the integrated broadband emission at 262 nm observed in LiYF₄, LiLuF₄ and BaY₂F₈ crystals, respectively. Although the 5d-4f² interaction is observed to be weaker than 5d-crystal field interaction, it is stronger enough to select only the radiative transitions from 4f²5d configuration to 4f³ states that preserves the total spin S=3/2.     

The 4d auger,coster-kronig,and recombination spectra of the lanthanides

A complete set of electron-excited 4d-basedAuger spectra of the lanthanide metals from lanthanum to lutetium (except the unstable promethium) is presented, in both differential and integral forms. It is believed that the set is more representative of clean surfaces of the lanthanides than any published hitherto. With the help of binding energy and electron loss measurements made in this laboratory and elsewhere, values of the various possible Auger, Coster—Kronig, and direct recombination transition energies are calculated, and for each transition a “centre of gravity” is derived based on relative intensities of final state multiplets, electron occupation of core levels, etc. By using arguments based on trends in the spectra across the series, on theoretically and experimentally derived values of U_eff for the difference 4fⁿ⁺¹→4f^n?1, and on plausibility, values of U_eff for 4fⁿ→ 4f^n?2 as well as for the other final state hole pair configurations are allocated. The relaxed transition energies so calculated are then compared with the experimental energies, from which it is possible in most cases to make assignment of the spectral features to the various transitions. As a result it is found that there are some significant disagreements with the theoretical rates of McGuire for lanthanide free atoms. The reasons for these disagreements are discussed, and an empirical model based on effective 4f and 5d populations and on the restrictions imposed by spin alignment is used to resolve the differences qualitatively.     

Radiative constants in the spectra of ions of the erbium isoelectronic sequence

The probabilities of the 4f ¹³6p→4f ¹³5d lectric dipole transitions and the lifetimes of levels of the 4f ¹³6p and 4f ¹³5d configurations are calculated for spectra of ions Yb III, Lu IV, Hf V, and Ta VI of the erbium isoelectronic sequence. The wave functions of the intermediate coupling scheme, necessary for calculating the relative line strengths, are obtained semiempirically from experimentally measured energy intervals between fine-structure levels. To pass to the absolute values, the radial integrals of transitions, evaluated with the Hartree-Fock functions, are used.     

The contribution of 5d electrons to the saturation magnetic moments and magnetic hyperfine fields in the heavy rare earth metals

The main results of a model for 5d electrons in the heavy rare earth metals are presented. The model involves the use of wave functions based on published analyses for 4fⁿ5d6s² atomic configurations, and the spreading of each of these energy levels uniformly over a band of width W in the metals. Excess saturation magnetic moments above those of the tripositive ions can be explained by the model with W in the range 0.84±0.16eV in the five metals Gd, Tb, Dy, Er and Tm. The magnetic hyperfine fields in the metals include negative contributions from the 5d electrons which have been shown to amount to about ?250koe in Gd, Er and Tm.     

Influence of excited configurations on the intensities of electric-dipole transitions of rare-earth ions

The theory of induced electric-dipole transitions of rare-earth ions in crystals and glasses is improved by taking into account the third-order effects of perturbation theory with respect to the energies of virtual excitations of 4f electrons to the 5d states. Since the energy regions of excited 4f ^{N ? 1}5d states are usually superimposed with the charge-transfer bands, the effects caused by a virtual transfer of an electron from the outer shells of ions of the surroundings (ligands) to the unfilled 4f ^N shells are also considered. The Pr³⁺, Sm³⁺, and Eu³⁺ ions are considered as examples. It is found that some difficulties inherent in the Judd-Ofelt calculation scheme are successfully overcome. The agreement of the calculated results with the experimental data improves.     

The study of autoionizing states of the samarium atom

The investigation of the samarium (Sm) atom autoionizing states, belonging to the 4f⁶ 6pnl and 4f⁵ 5d6snl (l=0, 2) configurations, has been carried out by using a three-step isolated-core-excitation (ICE) scheme. Fifty-seven new autoionizing states of the Sm atom have been observed in the energy region between 60,700 and 66,500 cm^?1. These autoionizing states are classified into five autoionization series in the light of different ionization limits. In most cases, the transition profiles of these autoionizing states are nearly symmetric, from which the peak positions and widths can be easily obtained by Lorentzian-profile-fitting procedure. Some autoionizing states have multi-peak profiles whose spectroscopic properties are analyzed in order to understand the complicated structures.     

Particle-hole states in 42K studied by the 44Ca(d, α)42K reaction

The ⁴⁴Ca(d, α)⁴²K reaction was studied at a bombarding energy of 11 MeV using a broadrange magnetic spectrograph and solid-state detectors. About thirty energy levels were identified in ⁴²K, up to an excitation energy of 3.5 MeV. Excitation functions corresponding to some of the low-lying energy states of ⁴²K were measured in the incident energy range from 10.4 to 11.6 MeV. Angular distributions of the α-particle groups corresponding to the excitation of about twenty energy levels in ⁴²K have been measured. DWBA calculations were performed and L-values for most of the above transitions were deduced. The results were compared with previous information and possible spin and configuration assignments are proposed for some of the states. Centroids of states interpreted as belonging to the (1d_{$\text{3}\text{2}$}^?1)_p (1f_{$\text{7}\text{2}$}³)_n, (2s_{$\text{1}\text{2}$}^?1)_p (1f_{$\text{7}\text{2}$}³)_n and the (1d_{$\text{3}\text{2}$}^?1)_P (1d_{$\text{3}\text{2}$}^?1 1f_{$\text{7}\text{2}$}⁴)_n configurations were found to be in good agreement with calculations based on the weak-coupling model.     

Spectroscopic investigations and configuration-interaction-assisted crystal field analysis of Pr in YPO4 single crystal

An optical study of a Y_0.99Pr_0.01PO₄ single crystal is presented. Measurements of optical absorption, excitation, and emission by selective excitation into ¹D₂, ³P₀ and ³P₁, at different temperatures between 20 and 295 K, are described. A detailed account of the line assignments is given for absorption in the 4300-23 000 cm⁻¹ spectral range, and for emission in the 6400-23 000 cm⁻¹ range. The lifetimes of the emitting levels are determined. Vibronic sidebands accompanying absorption, emission and excitation spectra are reported. The decay processes of the ³P₁, ³P₀ and ¹D₂ levels are discussed. The aim of this study is a test of the configuration-interaction-assisted crystal field analysis as well as the accurate experimental determination of the energy level scheme. It was reported previously that the introduction of configuration interaction between the ground 4f² configuration with the excited 4f6p configuration always resulted in a decrease (≈50-60%) in the standard deviations between the observed and calculated energy levels. In the present work the 4f5d configuration is included as well. The crystal field is analysed in the theoretical D_2d site symmetry with and without configuration interaction. The results with 4f², 4f²+4f5d, 4f²+4f6p and 4f²+4f5d+4f6p are given. The calculation on the basis of the 315 (4f²+4f5d+4f6p) levels gives the best overall standard deviation lowering it by 75% with regard to the calculation on the 91 4f² levels only.     

Analysis of the spectra of In XII–XIV and Sn XIII–XV in the far-VUV region

The 4p ⁶4d?(4p ⁶5p + 4p ⁶4f + 4p ⁵4d ²) transitions in the spectra of In XIII-Sn XIV and the 4p ⁶4d ²?(4p ⁶4d5p + 4p ⁶4d4f + 4p ⁵4d ³) transitions in the spectra of In XII-Sn XIII are investigated. About 150 spectral lines in the wavelength range 120–160 Å, which is important for projection vacuum UV lithography, have been classified for the first time. The fine structure intervals of the ground state 4p ⁶4d ² D and the energies of eight levels of the 4p ⁶5p, 4p ⁶4f, and 4p ⁵4d ² configurations are measured in each spectrum of In XIII and Sn XIV. The energies of all levels of the ground configuration 4p ⁶4d ² and the energies of 35 levels of the interacting configurations (4p ⁶5p + 4p ⁶4f + 4p ⁵4d ³) are determined in the spectra of In XII and Sn XIII. The resonant transitions 4p ^{6 1} S ₀?4p ⁵4d ¹ P ₁ in the spectra of In XIV and Sn XV are also identified. The semiempirical parameters of the configurations under study determined by mean-square fitting of measured level energies are in agreement along both isoelectronic and isonuclear sequences of the spectra.     

Analysis of the 4p 64d 7-(4p 64d 64f + 4p 54d 8) transitions in the Sn VIII spectrum

The spectrum of the 4p ⁶4d ⁷-(4p ⁶4d ⁶4f + 4p ⁵4d ⁸) transitions in Sn VIII occurring in the wavelength range 140–170 Å and excited in a vacuum spark plasma was investigated. For the first time, 113 spectral lines of these transitions were classified and the energies of 38 levels of the 4d ⁶4f and 4p ⁵4d ⁸ configurations were experimentally determined. The semiempirical energy parameters determined by least-squares fitting of the experimental energies of Sn VIII are in good agreement with the parameters of the corresponding configurations obtained previously in the analysis of the Sn VI and Sn VII spectra.     

Recombination processes related to Eu2+ energy level position in ionic CaxCd1-xF2 crystals

Abstract

We study efficiency of autoionization and recombination transitions for europium in Ca_xCd_1-xF₂ host lattice depending on the energy level position of Eu²⁺ 4f⁶5d¹(e) excited states in respect to the conduction band (CB) states. From photoluminescence (PL) and absorption measurements we conclude that for x in the range of 1 - 0.65 thermalization within the 4f⁶5d¹(e) multiplet is faster than the appropriate autoionization transition. For x > 0.65, when all the excited 4f⁶5d¹ states are degenerate with the CB autoionization is immediate, Eu²⁺ intra-ion emission is quenched and a new Eu-related emission appears, which we tentatively assign to a radiative capture emission of hot nonequilibrium electrons.     

Ultraviolet luminescence of Li6Gd(BO3)3: Ce crystals under selective excitation in the region of 4d → 4f transitions

The subnanosecond time-resolved ultraviolet luminescence of Li₆Gd(BO₃)₃: Ce crystals under selective excitation by ultrasoft X-rays in the region of the 4d??4f core transitions at temperatures of 7 and 293 K has been investigated for the first time. The performed investigation has revealed the following features: an intense fast component of the luminescence decay kinetics in the subnanosecond range due to the high local density of electronic excitations and the processes of Auger relaxation of the core hole; the modulation of the luminescence excitation spectrum by the ??giant resonance?? absorption band of the 4d-4f photoionization in the energy range 135?C160 eV; and a new broad luminescence band at an energy of 4.44 eV due to the direct radiative recombination between the genetically related electron in the states of the conduction band bottom and hole in the 4f ground state of the Ce³⁺ ion.