4 T 2→4 A 2 transition in the luminescence spectra of Mn4+ ion in gadolinium gallium garnet at intense laser pumping

The Mn^{4+ 4} T ₂→⁴ A ₂ transition was observed in the luminescence spectrum of Gd₃Ga₅O₁₂:Mn⁴⁺ at intense laser pumping. It is shown that the ⁴ T ₂ → ⁴ A 2 transition becomes more intense than ² E→⁴ A ₂ because of an increase in the role of induced transitions with increasing pump power. This process is most efficient in the region of strongest overlap between the ² E→⁴ A ₂ and ⁴ T ₂→⁴ A ₂ bands, where it leads to strengthening of the zero-phonon line of the latter at 694 nm. It is assumed that GGG:Mn⁴⁺ can be used as an active material in tunable lasers.     

AB initio calculations of the electronic properties of N4− in KN3

The energies of the ground state and low-lying excited states of the N₄^? defect in KN₃ have been calculated using ab initio techniques. A rectangular equilibrium geometry with dimensions X = 2.76 and Z = 2.47 a.u. and ground state symmetry of Γ₄⁺ was determined by calculating N₄^? as a free radical. For this ground state the unpaired electron is in a π orbital which is consistent with the experimental hyperfine tensor only if one edge of the N₄^? radical is parallel to the c axis in KN₃. These results were used to calculate the X²Γ₄⁺ state of N₄^? in the crystal field of KN₃, yielding an energy of ?217.899 Hartrees. The isotropic hyperfine constant was calculated to be a = 2.1 G and the components of the anisotropic hyperfine tensor as B_xx = ?3.4 G, B_yy = 7.0 G and B_zz = ?3.6 G, in good agreement with experimental and INDO results. Several excited states were calculated for the N₄^? defect in KN₃. When an estimate was made of the correlation energy, the transition energy of the X²Γ₄⁺→A²Γ₃^? transition agreed well with the peak energy of the 780 nm absorption band which has been attributed to N₄^?.     

Quadratic crystal field tensors and spin Hamiltonian tensors of Fe3+ in Li2Ge7O15 above and below the phase transition temperature

Dopant Fe³⁺ ions in tetrahedral and octahedral positions of Ge⁴⁺ in the crystal Li₂Ge₇O₁₅ were studied using EPR. Fe³⁺ substitutes for Ge⁴⁺ with a local charge compensation. The octahedral site and the tetrahedral sites significantly differ by the value of the invariant sumS(B ₄) of theB ₄ tensor of the spin Hamiltonian of Fe³⁺. The irreducible tensor products {V ₄ ? V ₄}₄ and {V ₄?V ₄}₂ theV ₄ tensor of the crystal field calculated using the point-charge model for octahedral and tetrahedral complexes provide the predominant contribution of the crystal field to theB ₄ andB ₂ tensors of the spin Hamiltonian of Fe³⁺, respectively. A comparison of the fourth-rank tensorsB ₄ of the spin Hamiltonian and {V ₄?V ₄}₄ of the crystal field determined at 300 K with those determined at 77 K supports the conclusion that the phase transition is accompanied by combined rotation of the [GeO₄] tetrahedra with the [Ge(1)O₆] octahedron almost unaltered. The spectrum lines are narrow and the variety of point defects in the vicinity of the paramagnetic impurity ions Fe³⁺, Cr³⁺ and Cu²⁺ is not detected. These facts are inconsistent with the statistically distributed model for the Li(2) atom. In specific cases at 300 K, when the wings of the two spectrum lines of theM→M+1 and theM+2 →M+3 transitions of Fe³⁺ ions belonging to one system of translationally equivalent positions overlap an extra line appears in the center between these lines. It is suggested that this effect is due to the soft phonon mode above the phase transition temperature.     

Selective laser spectroscopy of Mn4+-Mn4+ pair centers in SrTiO3 crystal

Zero-phonon lines of a pair center of Mn⁴⁺ ions are observed in the luminescence and luminescence-excitation spectra of SrTiO₃:Mn crystal. Based on the experimental data, the energy-level structure of the ground state ∣⁴ A _2g ,⁴ A _2g 〉 and excited state ∣⁴ A _2g ,² E _g 〉 of the Mn⁴⁺-Mn⁴⁺ pair center is constructed. It is shown that the exchange interaction in the ground state of the Mn⁴⁺-Mn⁴⁺ pair is antiferromagnetic. Energies of the levels are calculated assuming that the pair is formed by Mn⁴⁺ ions occupying neighboring octahedral positions of Ti⁴⁺ ions along the [110] axis. Experimental values of the exchange integral in the ground state ∣⁴ A _2g ,⁴ A _2g 〉 and energies of spin multiplets in the excited state ∣⁴ A _2g ,² E _g 〉 agree well with calculation of the exchange interaction carried out within the framework of the channel model with the parameters J _ξη = 32 cm^4-1 and J _ζζ = ?45.5 cm^4-1. Experimental data and calculations unambiguously demonstrate that zero-phonon lines in the luminescence and luminescence-excitation spectra have magnetic-dipole nature.     

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.     

Absorption spectrum of the Mn2+ ions in crystals of lanthanum manganese double nitrate and praseodymium manganese double nitrate

The optical absorption spectrum of the Mn²⁺ ions in the two double nitrates La₂Mn₃(NO₃)₁₂ ? 24H₂O and Pr₂Mn₃(NO₃)₁₂ ? 24H₂O were measured up to 30000 cm^?1. Uniaxial stress was applied to the crystals to determine the multiplicity of the observed zero phonon lines belonging to the transitions from the groundstate⁶ A ₁ (⁶ S) to the excited states⁴ E(⁴ G) and⁴ T ₂(⁴ D). The Zeeman effect of the states⁴ A ₁(⁴ G) and⁴ E(⁴ G) was measured with magnetic fields up to 6.4T. The effect of covalency and trigonal field, especially to the states⁴ A ₁ (⁴ G) and⁴ E(⁴ G), is briefly discussed.     

Dispersion relations and low-energy elastic scattering of charged particles from nuclei: (II). Application to N-4He scattering

A “Coulomb-modified” dispersion relation is applied to low-energy p-⁴He forward elastic scattering. To check the validity of the modification, the results are compared with those deduced from n-⁴He elastic scattering. Completely analogous information is obtained for the two processes. The exchange of three bound nucleons is found to contribute strongly to N-⁴He forward scattering. The corresponding ⁴He-³H-p and ⁴He-³He-n coupling constants are evaluated as R_p = 3.8±0.3 and R_n = 3.0±0.3, respectively. These constants are related to the strength of the asymptotic wave function of nucleons in ⁴He and thus to the tail of the nucleon distribution. A comparison of R_p with the empirical proton distribution in ⁴He as deduced from e-⁴He elastic scattering shows excellent agreement. From R_p and R_n the effective ranges of the singlet ³H-p and ³He-n interactions in the ground state of ⁴He are determined to be equal, in accordance with charge symmetry, and to have a value of r_eff = 1.072±0.006 fm.     

Spectroscopic studies of the two-dimensional magnetic insulators chromium trichloride and chromium tribromide—I

The general aspects of the crystal field spectra of the two-dimensional magnetic insulators CrCl₃ and CrBr₃ are considered in detail, with special emphasis on the broad, spin-allowed ⁴T₂ and ⁴T₁ bands. A fit of the point-charge model Hamiltonian to the spectra is presented, with attention to the magnitude of the trigonal field effects. The temperature dependence of the ⁴T₂ and ⁴T₁ dipole strengths in the range 4.2< T < 300°K is analyzed in terms of single-ion (odd-parity trigonal field and odd-parity vibronic) and exchange mechanisms. A moment analysis of the M.C.D. spectra is performed, which (together with dipole strength temperature dependence data) yields a reasonable interpretation of the spin-allowed transition mechanism. The odd-parity crystal field is found to be the most important parity-breaking mechanism in both the ⁴T₂ and ⁴T₁ bands. Both also receive some contribution from the vibronic mechanism. This process is more significant for the ⁴T₂ state than for the ⁴T₁, and may involve a slightly anharmonic low-energy vibration. Evidence for a small exchange contribution is found in the ⁴T₂ band of both systems. Factors influencing the bandshapes are examined quantitatively. The antiresonance formalism of Sturge et al. is applied to the anomalous lineshapes of certain transitions in absorption and M.C.D. and one particularly complex bandshape (that for ⁴A₂ → ⁴T₂ - ²E - ²T₁ in CrCl₃) is analyzed through a computation.     

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.     

Optical,magneto-optical and mössbauer spectroscopy on Co3+ substituted cobalt ferrite Co2+Fe2−xCo3+xO4(0 ⩽ x ⩽ 2)

The magneto-optical spectra of Co_1+xFe_2?xO₄ show with increasing Co³⁺ content an increasing intensity of the 4A₂ ? 4T₁(F) and 4A₂ ? 4T₁(P) transition of Co²⁺ at 0.8 and 2.0 eV. A decrease in the Co²⁺-Fe³⁺ charge transfer transitions on octahedral sites is found. In the optical spectra a strong increase in optical absorption is found with dominant transitions at 0.8, 1.6 and 2.6 eV due to Co³⁺ crystal field transitions on octahedral sites and a Co²⁺-Co³⁺ charge transfer. Conversion Electron Mössbauer Spectroscopy has been used to determine the cation distribution in the surface layer of the samples. The results indicate a shift of Co²⁺ from octahedral to tetrahedral sites when Co³⁺ is substituted in CoFe₂O₄. This results in enhanced optical absorption, enhanced magneto-optical effects and a lower Curie temperature.     

Paramagnetic defects in manganese-doped lead tungstate

In manganese-doped PbWO₄ crystals, low-intensity signals of triclinic clusters Mn⁴⁺-V _O and Fe³⁺-V _Pb have been revealed in addition to signals of Mn²⁺ tetragonal centers. The Mn⁴⁺-V _O cluster is formed by a Mn⁴⁺ ion in the W⁶⁺ position, which is associated with a vacancy of the nearest neighbor O^2?ion, and the Fe³⁺-V _Pb cluster consists of a Fe³⁺ ion substituting for Pb²⁺ with a local compensation of by a lead vacancy. It has been shown that, in PbWO₄: Mn, there is also a small amount of Mn⁴⁺ tetragonal centers located in the Pb²⁺ position with a nonlocal compensation of an excess charge.     

Broadband infrared emission of erbium–thulium-codoped calcium boroaluminate glasses

The near-infrared emission from calcium boroaluminate (CABAL) glasses codoped with Er³⁺ and Tm³⁺ has been investigated. It is shown that by controlling the [Tm]/[Er] concentration ratio a fairly flat emission with a bandwidth of 370 nm can be achieved in the wavelength range from 1.4 to 2.0 μm. The broadband emission is formed by three bands centered at 1.4, 1.5 and 1.8 μm, which are related to the emission from the Tm³⁺: ³ H ₄→³ F ₄, Er³⁺: ⁴ I _13/2→⁴ I _15/2 and Tm³⁺: ³ F ₄→³ H ₆ transitions, respectively. Compared to Er-only doping and Tm-only doping at the same concentration, codoping with both ions leads to a reduction of the intensity and lifetime of the Er-related band at 1.53 μm and to an intensity enhancement of the two Tm-related emission bands at 1.46 and 1.80 μm. This is a result of energy transfer (ET) processes between Er³⁺ and Tm³⁺ ions that are relevant when determining the emission spectrum shape. Two dominant ET processes are identified, both consisting of transferring the energy of the Er³⁺ first excited level (⁴ I _13/2) in the first case to the Tm³⁺ first excited level (³ F ₄), which is excited to the third excited level (³ H ₄), and in the second case to the Tm³⁺ ground state (³ H ₆) which is excited to the first excited level (³ F ₄).     

Exciton-exciton annihilation in TbPO4

A pulsed tuneable dye laser has been used to create high densities of excitons in TbPO₄ by resonant optical pumping into the ⁵D₄ Manifold. Non-exponential decay of the fluorescent radiation from ⁵D₄ is ascribed to exciton-exciton annihilation whose rate γn, is proportional to the exciton density n, with γ = 5 × 10^-15 cm³sec^-1. Anti-Stokes' radiation which is emitted by the annihilation products is also observed.     

Analysis of the ν2 + ν3 band of 12CH4 and 13CH4

The ν₂ + ν₃ bands of ¹²CH₄ and ¹³CH₄ occurring in the region 4400–4650 cm^?1 have been studied from spectra recorded with a high-resolution Fourier transform spectrometer (resolution better than 0.01 cm^?1). Champion's Hamiltonian expansion, Canad. J. Phys.55, 1802 (1977), is applied to the problem of the two interacting F₁ and F₂ vibrational sublevels of this type of a band. As the P branch of ν₂ + ν₃ is strongly overlapped by neighboring bands, a combination-difference method, adapted to tetrahedral XY₄ molecules has been developed to help assignments of lines. A fit of 700 transitions has been performed using 13 new effective constants in the case of ¹²CH₄. In the case of ¹³CH₄, 532 transitions have been fit to 18 constants. The known parameters, relative to the vibrational ground state and the ν₃ state for both methanes, and the ν₂ state for ¹²CH₄ were fixed throughout. Most of the perturbed levels, up to J′ = 12, are well reproduced and the general agreement between experimental and calculated transitions is satisfactory with standard deviations of 0.047 cm^?1 (¹²CH₄) and 0.041 cm^?1 (¹³CH₄). The results (order of magnitude of obtained (ν₂ + ν₃) parameters and comparison of observed and computed intensities) indicate that the ν₂ + ν₃ band is perturbed by many other bands.     

Investigation by EPR and ENDOR spectroscopy of the novel 4Fe ferredoxin fromPyrococcus furiosus

The hyperthermophilic archaeonPyrococcus furiosus contains a four-Fe ferredoxin (Pf- Fd) that differs from most other 4Fe-Fd’s in that its [Fe₄S₄] cluster is anchored to protein by only three cysteinyl residues.Pf- Fd also is of interest because in its reduced form, [Fe₄S₄]⁺, the cluster exhibits bothS = 1/2 andS = 3/2 spin states. Addition of excess cyanide ion converts the cluster exclusively to anS = 1/2 state (g₁ = 2.09, g₂ = 1.95, g₃ = 1.92), however dialysis restores the EPR signal of native reduced protein indicating that the cluster is not irreversibly altered by cyanide. Both the native protein and protein in the presence of excess cyanide ion (Pf- Fd 4Fe-CN) were investigated here using the techniques of electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy. In particular,Pf- Fd 4Fe-CN was investigated using¹³CN^? and C¹⁵N^? ligands.¹³C and¹⁵N ENDOR indicated that a single cyanide ion bound directly, with the cluster showing an unusually small contact interaction (a_iso(¹³C)～ ?3 MHz, a_iso(¹⁵N) ～ 0). This is in contrast to cyanide bound to monomeric low-spin Fe(III)-containing proteins such as transferrin and myoglobin, for which the¹³C hyperfine coupling has a large isotropic component (a_iso(¹³C) ≈ ?30 MHz). This small contact interaction is not due to low spin density of Fe, as⁵⁷Fe ENDOR of the singly and triply labeledPf- Fd 4FeCN isotopologs, [⁵⁷FeFe₃S₄]⁺ and [Fe⁵⁷Fe₃S₄]⁺, show hyperfine coupling characteristic for [Fe₄S₄]⁺ clusters, particularly for the Fe to which cyanide binds. Thus, the low spin density on¹³C is not due to low spin density on the Fe ion to which it binds. Further theoretical work is needed to explain the contrast between the strong electronic effect of cyanide ion binding with the low spin density on the ligand.     

Isotropic and anisotropic exchange interactions in the Orbitally degenerate excited (4 A 2×2 T 2) state of a Cr3+ ion pair possessing D 3h symmetry

Experimentally observed optical absorption spectra of isolated pairs of chromium ions in Cs₃Cr₂Br₉ corresponding to transitions of pairs from the ground (⁴ A ₂ײ A ₂) state to a singly excited (⁴ A ₂ײ T ₂) state are theoretically interpreted. It is shown that the position of the exchange multiplets in the excited (⁴ A ₂ײ T ₂) state is determined mainly by the resonance exchange interactions removing the degeneracy with respect to the exchange by an excitation within the pair. It is established that the observed fine structure of the pair in the (⁴ A ₂ײ T ₂) state is due to a superposition of the contributions from the one-and two-center spin-orbit interaction and resonance antisymmetric exchange.     

Fourth-order spin Hamiltonian terms for Fe2+ in rutile structure antiferromagnet FeF2

The paper deals with the spectroscopic and magnetic properties of Fe²⁺ ions in FeF₂. The microscopic spin Hamiltonian theory for Fe²⁺ in crystalline environments with second-kind orthorhombic symmetry is considered. Explicit formulas for the parameters B₀⁽²⁾(D),B₂⁽²⁾(E), g_x, g_y, g_z and, for the first time in the literature, the fourth-order parameters B₀⁽⁴⁾, B₂⁽⁴⁾ and B₄⁽⁴⁾, are derived. Using semi-empirical data for the ⁵D-term energy levels of Fe²⁺ ion in FeF₂, the pressure dependence of the parameters B_q^(k) in the region from 0 to 133 kbar is discussed. The relative role of the fourth-order parameters with respect to the second-order ones is found to increase strongly with pressure (e.g. in the region studied, D increases only by a factor of 3, whereas B₀⁽⁴⁾ increases by a factor of nearly 20). The magnetocrystalline anisotropy of FeF₂; is considered in the strong anisotropy model taking into account the fourth-order spin Hamiltonian terms. The uniaxial anisotropy constants K₁ and K₂ are derived theoretically and their pressure dependence is discussed quantitatively. The theory and numerical results of this paper are useful with regard to Fe²⁺ in other isomorphic fluorides, namely: MgF₂, ZnF₂, VF₂ and MnF₂. It is found that the fourth-order spin Hamiltonian parameters are accessible to experimental detection from spectroscopic studies on Fe²⁺ in non-magnetic fluorides and magnetic studies on Fe²⁺ : MnF₂ and FeF₂, preferably under high pressure.     

Temperature and concentration dependence of spectroscopic properties of Nd-doped yttrium aluminum garnet crystal

The optical absorption spectra of yttrium aluminum garnet (YAG) crystals doped with Nd³⁺ ions with different concentrations (0.6 at%, 1.0 at%, 1.2 at%) at the temperature range from 300 K to 500 K have been measured. The calculated Judd–Ofelt (JO) parameters Ω_t (t=2, 4, 6) based on the spectra have been used to predict the radiative transition probabilities, branching ratios and radiative lifetimes of the transitions from ⁴F_3/2 level to the lower levels (⁴I_13/2, ⁴I_11/2, ⁴I_9/2) at each concentration and temperature. The three JO parameters Ω_t (t=2, 4, 6), according to the calculation, decrease with the increasing doped concentration at each temperature. The JO parameters Ω₂ and Ω₄ increase, while the parameter Ω₆ decreases with the increasing temperature at each concentration. The branching ratios and radiative transition probabilities of the transitions from the ⁴F_3/2 level to ⁴I_13/2 and ⁴I_11/2 levels decrease, while the transition from the ⁴F_3/2 level to ⁴I_9/2 level increases with the increasing temperature. The obtained results at each concentration and temperature are discussed.     

The analysis of spin Hamiltonian and crystal field tensors for Fe3+ in crystals of LiCaAlF6 and LiSrAlF6

In single crystals of LiCaAlF₆ and LiSrAlF₆ doped with Fe³⁺ the trigonal EPR spectra with multiplicityK _M = 1 were observed due to Fe³⁺ substituted for Al³⁺. The spin Hamiltonian parameters describing the fine structure and the superhyperfine structure were determined. It is argued that the tensorsB ₂ andB ₄ of the spin Hamiltonian for Fe³⁺ ions are essentially determined by the quadratic contributions of the crystal field at the substitution site. The signs and the relative values of the elements in the spin Hamiltonian tensorB ₄ of rankL = 4 for Fe³⁺ are determined by the irreducible tensor product [V ₄ ?V ₄]₂ of the crystal field tensorV ₄ of rankL = 4 at the substitution sites. The ratio between the invariant sum of the spin Hamiltonian tensorB ₄ for Fe³⁺ in oxygen octahedra [FeO₆] and that in fluorine octahedra [FeF₆] is directly proportional to the fourth power of the ratio between the effective charges of surrounding ions. The sign of the spin Hamiltonian parameterB ₂₀ corresponds to the sign of the element [V ₄ ?V ₄]₂₀ in the irreducible tensor product [V ₄ ?V ₄]₂ of rankL = 2.