Optical spectra of octahedral cubic and trigonal Yb3+ impurity centers in KMgF3 and KZnF3 crystals

Groups of lines corresponding to octahedral cubic and trigonal impurity centers have been isolated in complex many-center luminescence and excitation spectra of Yb³⁺-doped KMgF₃ and KZnF₃ crystals. The crystal-field potentials derived from the spectra are in good agreement with those of similar centers in CsCaF₃:Yb³⁺ crystal studied earlier. Fiz. Tverd. Tela (St. Petersburg) 40, 2029–2034 (November 1998)     

EPR and optical spectroscopy of neodymium ions in KMgF3 and KZnF3 crystals

KMgF₃ and KZnF₃ crystals doped with Nd³⁺ ions were studied using EPR and optical spectroscopy methods. Several types of paramagnetic centers of Nd³⁺: KMgF₃ — two of tetragonal and one of rhombic symmetry; KZnF₃ — one of tetragonal and one of trigonal symmetry were found. Parameters of the corresponding spin Hamiltonians were determined. Using optical spectroscopy paramagnetic centers Nd²⁺ and Nd⁴⁺ in KMgF₃ were found.     

The first and second nearest neighbour Cr3+ pairs in ruby

The irreducible representations of the states of the first and second nearest neighbour Cr³⁺ pairs in ruby were deduced from the known states of the single ions by the group theoretical method of induction. The summetry selection rules and polarizations for electric dipole radiation are discussed.Supported by the Deutsche Forschungsgemeinschaft within the Sonderforschungsbereich 65 Frankfurt-Darmstadt.     

The first and second nearest neighbour Cr3+ pairs in ruby

The optical spectrum of the exchange coupled first nearest neighbour Cr³⁺ pairs in ruby was studied by absorption, fluorescence and excitation spectroscopy. It was analysed by means of the group theoretical considerations given in part I [1]. The analysis could be verified by measuring the Zeeman effect in fields up to 150 KG. The Zeeman splitting deviates considerably from that of pure spin states even for the orbital singulet of the pair ground state. The exchange integrals of the ground state are found to beJ=–115 cm^–1 andj=–0.7 cm^–1.This work was supported by the Deutsche Forschungsgemeinschaft, SFB 65     

Radiation effects in KMgF 3 crystals

Effects of x, g and UV irradiation on Tb 3+ and Eu 2+ doped KMgF 3 crystals were studied. Thermoluminescence (TL) excited by VUV radiation was compared to that induced by x and g rays. TL excitation spectra showed maxima at 125, 140, 155 and 180 v nm. The main TL peaks appeared after VUV irradiation at about the same temperatures and with the same thermal activation energies as after x or g irradiation, indicating that these peaks are due to the same trapping levels. In samples, which had previously been exposed to ionizing radiation, TL could also be excited with longer wavelengths that could not excite any TL in un-irradiated crystals; this is attributed to a process of phototransfer (PTTL). The x and UV excited luminescence was measured at LNT and RT and showed essentially the same main emission bands that appeared in the TL and PTTL peaks of these crystals. The dose dependence of the 730 v K TL peak in of KMgF 3 :Eu 2+ was linear up to about 2000 v Gy. The TL sensitivity of KMgF 3 :Eu 2+ was by an order of magnitude higher than that of KMgF 3 :Tb 3+ , which was by a factor of two greater than that of the known TLD-l00 phosphor.     

A chiral axial next nearest neighbour xy-model for antiferroelectric liquid crystals

We describe a chiral axial next nearest neighbour xy-model to account for the various subphases exhibited by antiferroelectric liquid crystals made of chiral rod-like molecules. The assumed form of the interlayer interactions is based on physical processes which are discussed. Using a discrete model, the predicted sequence of transitions is SmA-SmC-SmC-FI_H-FI_I-FI_L-SmC ^* A, where FI stands for a ferriphase, as seen in many compounds. The ferri and SmC phases are characterized by relatively large angles between the c-vectors of successive layers and occur only when the compounds have high optical purity. The calculated field induced structures exhibit a plateau of the apparent tilt angle at , where is the tilt angle of the molecules in the ferriphase. The conoscopic figures in the presence of a field and ellipsometric parameters in the absence of a field have also been generated, which agree extremely well with the experimental results. Recent anomalous X-ray scattering studies prove the xy-character of the configurations, though the commensurate structures that are found in the ferriphase require an extension of the model to include lock-in terms. Received 23 August 1999     

Spin chains with nearest and next nearest neighbour interactions

The 1/n-expansion is applied to a classical one dimensionaln-vector model in an external field with nearest and next nearest neighbour exchange interactions of both signs. Various effects caused by helical spin structures are discussed for the Heisenberg magnet (n=3).—For the spherical model with multi-neighbour interactions Lifshitz points of higher order are found.     

Critical temperature for a system with Heisenberg-like nearest neighbour and Ising-like next nearest neighbour interactions

A ferromagnetic system with $\text{S =}\text{1}\text{2}$, where the interactions between nearest neighbours are isotropic (having Heisenberg character) and those between the next nearest neighbours anisotropic (Ising-like), is investigated. Using the effective hamiltonian approach we find the change in the critical temperature due to the inclusion of next nearest neighbour interactions of different character. The change is greatest in two dimensions where the critical temperature is shifted up from its zero value for a system with the Ising-like interactions switched off. We also calculate the T_c for a system with both nearest and next nearest neighbours interactions of the Ising-type. The results for the two models are compared.     

Pb2+ Defects in perovskite-like KMgF3 crystals

Abstract

The physical properties of Pb²⁺ impurities in the perovskite-like lattice of KMgF₃ are very similar to those of (ns)² ions in alkali halides. The impurity inhomogeneous distribution leads to high dopant levels in the bottom region of the crystals, with a segregation coefficient equal to 0. 020. The optical features of the 336 nm emission support its attribution to luminescence of Pb²⁺ ions from the relaxed excited B state.     

Optical spectra of Eu3+ ion pairs in Y2O3

All the optical transitions of Eu³⁺ ions substituted on Y³⁺ lattice sites of point symmetryC ₂ orC _3i in Y₂O₃ show weak extra lines (satellites) extending over a range of few wavenumbers on both sides of the main line. It is proved by a study of the concentration dependence of the intensities of these satellites that they arise from pairs of two identical Eu³⁺ ions, interacting with each other when their separation is less then ～9 Å. By means of absorption and fluorescence excitation spectra of the transitions of Eu³⁺ at lattice sites of point symmetryC _3i , first, second and third nearestC _3i -C _3i neighbour pairs could be identified. The mean pair interaction is of the order of 5 cm^?1 and is assumed to be due to superexchange via the oxygen ions.     

ESR of a Mn-S nearest neighbour complex in GaP

ESR measurements on a manganese-sulphur nearest neighbour complex having axial symmetry along 〈111〉 axes are reported. The spin Hamiltonian parameters are found to be g = 2.002 ± 0.001, A = (-57 ±1) × 10^-4 cm^-1, D = (-306 ± 2) × 10^-4 cm^-1 and (a − F) = (+12 ±2) × 10^-4 cm^-1. The formation of these complexes is discussed. briefly.     

EPR,ENDOR and optical spectroscopy of Yb3+ ion in KZnF3 single crystals

The paramagnetic center of tetragonal symmetry formed by the Yb³⁺ ion in the KZnF₃ crystal has been studied using methods of EPR, ENDOR and optical spectroscopy. The location of the impurity ion and the structural model of the complex differing from the model of the Yb³⁺ center in KMgF₃ have been established. The empirical scheme of the energy levels of the Yb³⁺ ion has been found. The parameters of its interaction with the crystal electrostatic field and the hyperfine interaction with ligands of the nearest environment have been determined. The parameters of the crystal field were used for the analysis of the distortions of the crystal lattice in the vicinity of Yb³⁺. The parameters of the transferred hyperfine interaction have been calculated for the distances between Yb³⁺ and F⁻ ions of the nearest environment obtained taking into account the found distortions. They are in good agreement with the experimental values.     

Optical spectra of one-dimensional defect photonic crystals

The effect exerted by defects that represent a local disturbance of the structural periodicity on the band spectrum of a one-dimensional photonic crystal has been investigated. A classification of single defects has been proposed. It has been demonstrated that the position of defect minibands in the band gap of the spectrum of the crystal can be controlled by varying the type of a defect and its location in the crystal structure. The presence of two defects in the structure leads to the formation of two minibands, so that the spacing between the minibands and their intensity depend on the type of defects and on the distance between them.     

Optical spectra and electronic transitions in gallium phosphide crystals

Analysis of the long-wave edge of the self-absorption spectrum of gallium phosphide crystals not distorted by line structure at low temperatures enabled us to determine the characteristic parameters of indirect transitions. In GaP crystals of a different type the appearance of a discrete structure in the long-wave absorption edge is related to the localization of excitons near impurities of an unknown nature. This is confirmed by the appearance in these crystals of an intense green electroluminescence band at low temperatures.The authors are grateful to I. Ryzhikov, A. Kruchinin, and Yu. Il'in for providing the crystals used in the experiments.     

Optical spectra of rare-earth ions in magnesium-aluminum spinel crystals

Journal of Applied Spectroscopy -     

Optical absorption spectra of I3 - molecular ions in KI crystals and aqueous solutions

A semiempirical molecular orbital method including spin-orbit interaction is used to study optical absorption spectra of I₃ ^- molecular ions in (KI + I₂) aqueous solutions and in KI crystals coloured additively or electrolytically. The splitting of two strong absorption bands observed for these specimens is considered to be due to the spin-orbit interaction in the iodine atoms. The values of the spin-orbit coupling constants for I₃ ^- in aqueous solutions and coloured KI crystals are estimated to be 0·56 and about 0·47 eV, respectively. The oscillator strengths of the two strong bands for these specimens is also discussed.     

Optical phonon spectra of PbF2 single crystals

The spectra of dipole-active optical phonons are measured for the cubic and orthorhombic phases of PbF₂ single crystals. The frequencies and eigenvectors of normal modes in the Pnma orthorhombic phase are calculated. It is found that the spectrum of the cubic phase exhibits excess vibrational modes of the PbF₂ orthorhombic phase.