Single Rare Earth Ion Research Articles

Thermoluminescent spectra of rare earth doped MgB4O7 dosemeters.

This paper presents X ray excited TL spectra of magnesium borate doped with either single rare earth ions Dy or Tm, or co-doped with Dy/Tm, Tm/Mn or Dy/Tb. Intrinsic emission from the host material is in the UV/blue region at approximately 375 nm, with a tail extending to 200 nm. The main dosimetric peak is detected at approximately 180 degrees C but slight differences are noted between the glow peak maxima from the different rare earth ions and there were changes following thermal treatments. The results are discussed according to the interaction between trapping and rare earth sites.

Luminescence enhancement in (Nd 3++Ce 3+) doped SiO 2 : Al 2O 3 sol–gel glasses

An enhancement in NIR luminescence from Nd 3+-doped Ce 3+ co-doped SiO 2+Al 2O 3 sol–gel glasses has been observed. The lasing transition ( 4 F 3/2 → 4 I 11/2 ) at 1072 nm from the dual rare-earth Nd 3++Ce 3+-doped glasses has shown an emission strength of about five times that of the single rare-earth ion Nd 3+-doped glass. From the measurement of lifetimes of the transition at 1072 nm, the transfer rate ( W tr), critical distance ( R 0) and energy transfer efficiency ( η) of the neodymium glasses have been calculated.

Probing the binding of Tb(III) and Eu(III) to the hammerhead ribozyme using luminescence spectroscopy.

Divalent metal ions serve as structural as well as catalytic cofactors in the hammerhead ribozyme reaction. The natural cofactor in these reactions is Mg(II), but its spectroscopic silence makes it difficult to study. We previously showed that a single Tb(III) ion inhibits the hammerhead ribozyme by site-specific competition for a Mg(II) ion and therefore can be used as a spectroscopic probe for the Mg(II) it replaces. Lanthanide luminescence spectroscopy was used to study the coordination environment around Tb(III) and Eu(III) ions bound to the structurally well-characterized site on the hammerhead ribozyme. Sensitized emission and direct excitation experiments show that a single lanthanide ion binds to the ribozyme under these conditions and that three waters of hydration are displaced from the Tb(III) upon binding the RNA. Furthermore, we show that these techniques allow the comparison of binding affinities for a series of ions to this site. The binding affinities for ions at the G5 site correlates linearly with the function Z(2)/r of the aqua ion (where Z is the charge and r is the radius of the ion). This study compares the crystallographic nature of the G5 metal-binding site with solution measurements and gives a clearer picture of the coordination environment of this ion. These results provide one of the best characterized metal-binding sites from a ribozyme, so we use this information to compare the RNA site with that of typical metalloproteins.

Theoretical Study of X-Ray Emission Spectroscopy on Magnetic Materials

Theoretical analyses of resonant X-ray emission spectroscopy (RXES) in rare earth compounds and transition metal compounds are given with an impurity Anderson model using the coherent second order optical formula. RXES provides us with important information on the 4f and 3d electronic states in the ground state of these materials, as well as excited states with and without a core hole. For the analysis of RXES of rare earth systems, the impurity Anderson model (including a single rare earth ion) works well, but we need larger clusters in order to explain some aspects of RXES in transition metal compounds.

Dynamical processes involving local modes for rare earth centers in CaF 2

H -, D - and T - ion local modes play a significant role in dynamical processes involving laser excitation of hydrogenic rare-earth ion centers in CaF 2 and SrF 2. The hydrogenic varieties of a particular center are found to have substantially reduced fluorescence lifetimes supportive of models for non-radiative decay to lower energy multiplets through the respective local mode phonons. Likewise, upconversion efficiencies are reduced for the hydrogenic single rare-earth ion centers and are found to vary with inter-center separation consistent with a recent model for single ion upconversion involving a weak coupling between nearby equivalent centers. The efficient channelling of the rare-earth ion excitation into the creation of local mode phonons can induce hydrogenic ion migration to produce re-oriented equivalent centers or new photoproduct centers with different excitation wavelengths.

The magnetic hyperfine fields of rare earth ions in metals

Data for the magnetic dipole hyperfine interaction of essentially single rare earth ions in metals, measured with different experimental methods, are collected and discussed. Depending on the host, the magnetic hyperfine field of these paramagnetic ions remains undisturbed by the environment, or it is enlarged, or weakened or can even become completely lost. If there are magnetic ions in the neighbourhood, the magnetic interaction can enlarge the hyperfine field of the single ion by a transferred hyperfine field. The reason of the demagnetization effect may be crystal field splitting and hybridization. The core polarization field of the free rare earth ions is redetermined from measurements of the hyperfine interaction in nonmagnetic metals at low magnetic ion concentration.

Crystal field spectroscopy by inelastic neutron scattering

We present a review of inelastic neutron scattering results on very diluted rare earth (RE)-Laves phases. A systematic investigation to study crystal electric fields, experienced by single rare earth ions in a metallic environment, was done on (RE,R)Al2, with RE=Pr, Nd, Tb, Dy, Ho, Er, Tm andR=Sc, Y, La. We show the influence of the 4f-ions on the crystal fields as well as the influence of the host lattices. The rare earth atoms in ScAl2 have been studied for the first time and most of the other alloys, with LaAl2 and YAl2 as hosts, have been studied for the first time with magnetic atom concentrations below 1 at% and/or with good resolution for low energy excitations. Furthermore we studied the dynamics of 4f-moments, which are coupled to the conduction electrons by determining the coupling constants. We present a comprehensive set of crystal field parameters and coupling constants from these systematic studies and discuss them qualitatively within the available theoretical models. The influence of magnetic atom concentration on crystal field spectra will be shown in part II.

Temperature and magnetic field dependence of the elastic properties of the Ho xTb 1− xCo 2 Laves phase compounds

Ultrasonic sound velocity measurements have been carried out in order to determine the adiabatic compressibility and elastic moduli of the pseudobinary Ho x Tb 1− x Co 2 Laves phase compounds. The anomalies, associated with the Curie temperatures and the presence of various spin reorientations, allowed the determination of the spin orientation diagram of the system. The similarity of this spin orientation diagram with that previously established of the Ho x Tb 1− x Fe 2 system strongly supports the single rare earth ion model for the magnetic anisotropy behavior of the compounds studied. Measurements carried out in an external magnetic field revealed the presence of a significant ΔE effect.

Theory of Zeeman effect for rare earth ions in crystal field with D2 symmetry and application to rare earth garnets

Rare earth ions in solids frequently exhibit sharp-line optical spectra. In this paper a part of the theory of the Zeeman effect for the lines is developed, first for a single rare earth ion in D2 symmetry, and secondly for rare earth ions at different inequivalent sites in garnet crystals. The results for the case in which the crystal field and Zeeman splittings are comparable have application for magnetic garnets such as the rare earth iron garnets, while the results for the case where the Zeeman splittings are treated as a perturbation on the crystal field states have application for nonmagnetic garnets such as the rare earth aluminum or gallium garnets. In both cases it is found that when the magnetic field is restricted to the xy plane of the local site coordinate, there exists an antiunitary symmetry operation which simplifies the energy level and transition probability calculations, and which leads to the assignment of a quantum number ζ = ± 1 to the states. There are shown to be two types of states: Type A, for which the assignment of ζ value is not sensitive to the choice of coordinate axes, and Type B states for which it is. It is also shown, in the case of rare earth ions with an odd number of 4f electrons, that the Types of the states are determined by the relative signs of the principal values of the g factors. Furthermore, it is found that the dependence of line intensity on magnetic field direction for A→A and B→B transitions is clearly different from that of A→B and B→A transitions. Finally, and most important, it is shown that studies of the energies and intensities of the Zeeman lines as a function of magnetic field direction in suitable crystal planes can be used to distinguish between spectra arising from different inequivalent rare earth ion sites in garnets. This result has immediate importance for experimental study of site selectivity problems in garnets, and, when combined with the other conclusions of this work, leads to definite suggestions for experimental determinations of at least some of the details of the states.

Growth-Induced Noncubic Anisotropy Arising from the Tetrahedral Sites in Garnets

We have observed noncubic growth-induced anisotropy in garnets prepared to preclude the mechanism based on rare-earth ions, such as Eu3Fe5−xGaxO12 with a single rare-earth ion, or such as Y3−2xCa2xVxFe5−x O12 with only diamagnetic ions on the dodecahedral sites; all samples contain a mixture of constituents on tetrahedral sites. We show that the noncubic anisotropy can be explained by distortion of the local symmetry around tetrahedral ions caused by preferential site ordering among their tetrahedral neighbors. A similar distortion of tetrahedral symmetry by ordering of dodecahedral ions should contribute in mixed rare-earth garnets.

ENHANCEMENT IN A Ho3+–Yb3+ QUANTUM COUNTER BY ENERGY TRANSFER

In all the infrared quantum counter systems reported to date, a single rare-earth ion dopant was used as the activator. This limits the optimum concentration to about 1%, since above this value nonradiative processes begin to dominate. In order to increase the number of ions which can absorb the infrared signal Yb3+ was used as a sensitizer. By making use of the energy transfer from Yb3+ to Ho3+ the green quantum counter fluorescence was increased by more than two orders of magnitude.

Far-Infrared Spectra of Holmium, Samarium, and Gadolinium Iron Garnets

Far-infrared (10–100 cm−1) spectroscopic transmission measurements have been made on sintered powder samples of ferrimagnetic holmium, samarium, and gadolinium iron garnets. Both single-ion transitions and also the Kaplan—Kittel exchange resonances have been observed in HoIG and SmIG. A breakdown of selection rules permits the observation of the single-ion transitions, in which the effective angular momentum of a single rare-earth ion changes in the presence of the crystalline electric and iron exchange fields. Because no magnetic absorption is observed in GdIG, single-ion transition probabilities appear to be governed by the effect of the local crystalline electric field on the rare-earth ion, as expected theoretically. Experimentally, one can distinguish the collective modes because their resonant frequencies are more temperature dependent than those of the single-ion transitions.

Far Infrared Spectra of Magnetic Materials

The far infrared spectral region (∼10−100 cm−1) corresponds to kT for T=15−150°K or βH for H=105−106 oe. Materials with characteristic temperatures or fields of these orders of magnitude may have interesting far infrared spectra. We have studied far infrared resonance spectra in antiferromagnetic and in ferromagnetic rare-earth iron garnets. Antiferromagnets have a resonance frequency which depends on the exchange and anisotropy fields, HE and HA. If HE is found from χ⊥, HA can be found from ω0, and compared with theory. We have done this with FeF2, MnO, and NiO, which have resonances for T≈0 at frequencies of 52.7 cm−1, 27.5 cm−1, and 36.6 cm−1, respectively. As the temperature is raised, these frequencies fall, reaching zero at TN. When a magnetic field is applied along the easy axis of FeF2, there is a first-order Zeeman splitting, from which a g value of 2.25 was determined. In MnO and NiO, however, there is an easy (111) plane. In this case the resonant mode is nondegenerate and hence the Zeeman effect is second-order and unobservably small. The exchange coupling between the rare earth and iron ions in garnets is typically of order 20 cm−1 (30°K), while the iron ions are coupled strongly together, corresponding to their Curie temperature of about 550°K. Because anisotropy breaks down selection rules, transitions in which a single rare-earth ion spin flips in the exchange field of the iron may be observed at far infrared frequencies. We also observe Kaplan-Kittel exchange resonances, in which the entire sublattice of rare-earth ions precess together and induce a corresponding precession of the iron sublattice. Since the frequency of such a resonance depends on the rare earth sublattice magnetization, it is quite temperature dependent, in contrast to the single ion absorptions. To obtain quantitative agreement with experiment, the Kaplan-Kittel theory must be generalized to take account of anisotropy energy. In YbIG at 2°K, with the magnetization along the [111] easy direction, we find single ion resonances at 23.4 and 26.4 cm−1 and an exchange resonance at 14.1 cm−1. The latter rises in frequency as the temperature is raised, whereas the former frequencies are nearly constant up to 60°K, where the intensity becomes too low for observation. The corresponding frequencies at low temperatures in ErIG are 18.2, 21.6, and 10.0 cm−1, whereas in SmIG the exchange resonance occurs at 33.5 cm−1 and decreases with increasing temperature. The small temperature dependence of the single ion exchange splittings suggests a rare earth-rare earth coupling (perhaps via spin waves in the iron sublattice) of magnitude ∼4% of the iron-rare earth coupling.