Superhyperfine Interaction Research Articles

New electron spin resonance spectra from iron–vacancy pair in silicon: II. Hyperfine interactions and isotopic effect

We studied the TU2 and TU3 ESR spectra from an iron–vacancy pair in iron-doped and electron-irradiated float-zone grown silicon samples. Isotopic shifts in the fine structure term and hyperfine and superhyperfine interaction terms of the spin Hamiltonian are considered in detail. We obtained that the change in the spin value of the defect is accompanied by a modification of the iron atom position. The existing model fails to explain observed large anisotropy of the isotopic effect.

X and Q-band endor study of the Fe+ (I) center in chlorinated SrCl2 single crystals

Previous Electron Paramagnetic Resonance (EPR) studies have shown that Fe2+ doped SrCl2 crystals grown in a chlorine atmosphere exhibit new trapped electron Fe+ centers after irradiation with X or γ-rays at 77 K. Two types of such centers, called Fe+ (I) and Fe+ (II), both exhibiting axial ⟨001⟩ symmetry, have been identified. We present here the results of an Electron Nuclear Double Resonance (ENDOR) study in X and Q-band on the Fe+ (I) center, which has been found to be thermally stable up to 700K. The superhyperfine and quadrupole interactions of the paramagnetic Fe+ ion with the nearest Cl− ions have been determined from the ENDOR angular dependence data. The results strongly suggest that the Fe+ (I) center consists of a Fe+ ion in the center of a cube of eight Cl− ions, compressed along an ⟨001⟩ direction.

Theory of NMR in the molecular ring Fe10

The proton nuclear spin–lattice relaxation rate in the molecular ring Fe10 is calculated by linear response theory, starting from the unperturbed eigenvalues and eigenvectors, and introducing dipolar superhyperfine interactions as a perturbation. A strong enhancement of the rate at critical values of the applied magnetic field, recently observed by solid-state NMR at low temperatures, is explained in terms of level crossing between states with |Δ S|=1 and |Δ M|=1.

Superhyperfine interaction in the trigonal center BaF2: Gd3+ and the lattice distortion analysis in the vicinity of the impurity ion

The constants of the superhyperfine interaction of Gd3+ with the 19F nuclear spins in the first four coordination shells were determined from the ENDOR spectra of a trigonal BaF2: Gd3+ center. These data were used for analysis of the crystalline lattice distortions in the vicinity of the impurity ion. It was found that the largest displacements of fluorine nuclei occurred in the vicinity of the ion compensator and the impurity ion. To calculate the anion positions within the first coordinate shell, whose electron-nuclear interaction with Gd3+ depends considerably on chemical bonds in the Gd3+F8− complex, an empirical model is used for the isotropic constants of the superhyperfine interaction of Gd3+ with fluorine nuclei in cubic centers, with allowance made for the impurity ion polarization.

Magnetic resonance of exchange-coupled copper complexes in perovskite-structure crystals: The potassium tantalate and cuprate superconductors

This paper reports on parallel EPR studies of high-temperature superconductors based on the cuprate perovskites RBa2Cu3O6+x (R=Y, Gd, Nd) and of KTaO3: Cu, which also has a perovskite structure. EPR measurements performed on copper-doped KTaO3 crystals revealed Cu2+-Cu2+ copper pair centers. The copper ions making up pairs are assumed to occupy adjacent tantalum sites. The pair centers are chains consisting of two equivalent Cu2+ ions and three oxygen vacancies aligned in the 〈100〉 direction. The crucial point in the model proposed is the presence of an oxygen vacancy sandwiched between two Cu2+ ions, whereas the outer vacancies do not necessarily occupy neighboring sites. In this structure, complete charge compensation is achieved. Ferromagnetic exchange coupling takes place between the two copper ions. An investigation of the exchange and superhyperfine interactions of copper centers in crystalline potassium tantalate has permitted the estimation of the respective interactions in crystals of the cuprate superconductors which exhibit magnetic resonance signals due to exchange-coupled copper clusters in the case of oxygen deficiency.

Adsorbate interactions of Cu(II) in mesoporous Cu-GaMCM-41 studied by electron spin resonance and electron spin echo modulation

Gallium-containing mesoporous MCM-41 materials (GaMCM-41) were synthesized by adjusting the gel equilibrium between the source gallosilicate mixture and a GaMCM-41 phase using an organic acid. The location of Cu(II) exchanged into the mesoporous GaMCM-41 and its interactions with various adsorbate molecules were investigated by electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies. The results are compared with those of Cu-AlMCM-41. In a fresh, hydrated Cu-GaMCM-41, Cu(II) is octahedrally coordinated to six water molecules and is located in a cylindrical channel and rotates rapidly at room temperature. Evacuation removes some of these water molecules, leaving the Cu(II) coordinated to fewer water molecules and anchored to oxygens in the channel wall. Dehydration produces one Cu(II) species located on the internal wall of a channel which is accessible to oxygen as evidenced by broadening of its ESR lines by oxygen. Adsorption of water, methanol, ammonia, pyridine, aniline, acetonitrile, benzene and ethylene on dehydrated Cu-GaMCM-41 materials causes changes in the ESR spectra of Cu(II), indicating complex formation with these adsorbates. Cu(II) fully coordinates with a maximum coordination number of polar adsorbates in MCM-41 mesopores. Cu(II) forms a complex with six molecules of methanol as evidenced by an isotropic room temperature ESR signal and ESEM data. Cu(II) forms square planar complexes with four molecules of ammonia, pyridine and aniline based on resolved nitrogen superhyperfine interactions and other ESR parameters. However, Cu(II) forms a complex with six molecules of acetonitrile based on the ESR parameters and ESEM data. Interestingly, Cu(II) does not fully coordinate with nonpolar molecules. Cu(II) interacts directly with only one molecule of benzene, and only part of Cu(II) interacts indirectly with one molecule of ethylene based on ESR and ESEM analyses.

Modelling hydrogen in the group-III nitrides by its pseudo-isotope, muonium

Muon and muonium states in the wide-bandgap semiconductors BN, AlN, and GaN are characterised by various types of μSR measurement on polycrystalline samples. The muonium fractions range from 80% in hexagonal BN to zero in GaN. The hyperfine constants estimated from repolarization curves are 80% of the free muonium value in BN and 95% in AlN, with superhyperfine interactions to the host nuclei is evident. The electronically diamagnetic states show strong level-crossing resonances in AlN and GaN (although none is detectable in BN). These have the signature of cross-relaxation to 14N in AlN and to 69Ga and 71Ga in GaN, suggesting that the diamagnetic states are Mu + and Mu − in these naturally p- and n-type materials, respectively. Mu − diffusion in GaN sets is only above 600 K, with an activation energy of 1 eV.

Investigation of Oxygen-Related Luminescence Centres in AlN Ceramics

The structure of oxygen-related luminescence centres in nominally undoped and Y 2 O 3 doped AlN ceramics was investigated by electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and optically-detected EPR. The photoluminescence-detected EPR lines having g-values of 1.990 and 2.008 were assigned to a recombination between neighbouring donor and acceptor pairs. The two EPR lines at g = 1.987 and 2.003 detected via the recombination luminescence in the afterglow are thought to be due to a recombination between the same, but distant donor and acceptor pairs. The donor was previously speculated to be an electron trapped on an oxygen impurity which substitutes for a nitrogen on a regular lattice site. The defect structure of the acceptor was established by ENDOR to be a hole trapped on an O N -v Al complex (ON oxygen on a regular N site, v Al Al vacancy). The measured superhyperfine interaction is caused by two equivalent 27 Al nuclei both with a = ±29.6 MHz and one 27 Al nucleus with a = ±27.0 MHz.

Molecular and electronic structure of nitridocyanometalates of chromium(V) and manganese(V): a combined experimental and DFT study.

The six-coordinate complexes [M(N)(CN)5]3- (M = Cr, Mn) have been isolated as salts of robust rhodium amine complexes. [Rh(en)3][Mn(N)(CN)5].H2O (1) and [Rh(tn)3] [Cr(N)(CN)5].2H2O (2) have been characterized by single-crystal X-ray crystallography: 1 crystallizes in the hexagonal space group P6(3) with a = b = 15.810(2) A, c = 13.844(3) A, V = 2996.8(8) A3, and Z = 6; 2 crystallizes in the orthorhombic space group Pbcn with a = 9.723(1) A, b = 14.564(2) A, c = 31.498(4) A, V = 4460.3(8) A3, and Z = 8. In 1, all the anions are oriented with their Mn identical to N directions almost coparallel to the crystallographic 3-fold axis. Polarized single-crystal UV-vis spectroscopy of 1 confirms the validity of the Jørgensen-Ballhausen-Gray d-orbital splitting scheme with the lowest energy transition being dxy-->[dyz,dzx]. Single-crystal EPR spectroscopy of [Cr(N)(CN)5]3- diluted into 1 shows the hyperfine (53Cr) and super-hyperfine (14N) tensors to be quite anisotropic with different major axes. For the hyperfine interaction we observe A Parallel > A Perpendicular, whereas, for the super-hyperfine interaction to the terminal nitrido ligand, the reverse ordering is found: A Perpendicular > A Parallel. The complexes [M(N)(CN)5]3-, trans-[M(N)(CN)4(py)]2-, and [M(N)(CN)4]2- (M = Cr, Mn) were investigated by DFT methods. Good reproduction of the molecular structures, vibrational, and UV-vis spectra was obtained. However, pronounced differences between local density and gradient corrected functionals were observed in the description of the weak bonding to the ligands trans to the nitrido ligand. For the five-coordinate [M(N)(CN)4]2- complexes the LUMO is predicted to be a strongly admixed dz2(M)-pz(M) hybrid.

Photostimulated luminescence process in the x-ray storage phosphor CsBr:Ga+

CsBr doped with Ga+ has previously proved to be an efficient X-ray storage phosphor with a figure of merit comparable to that of the commercially used BaFBr:Eu2+. By measuring the magnetic circular dichroism of the optical absorption (MCDA), the MCDA-detected electron paramagnetic resonance and the photostimulated luminescence (PSL), the paramagnetic defect centers generated upon x-irradiation involved in the storage and read-out process were investigated and found to be particularly simple: The electron traps are F centers, the hole trap centers are Ga2+ centers with a hyperfine interaction with Ga69 of A69=6.5 GHz. No superhyperfine interaction was resolved. It is proposed that the PSL-active Ga2+ center is an isolated Ga2+ on a Cs+ site.

OH-D2 (OD-H2) isotopic exchange at the surface of CaO monitored by electron paramagnetic resonance

Electron paramagnetic resonance was employed to follow the isotopic exchange reaction between gaseous hydrogen and surface deuteroxyl groups at the surface of CaO. This was possible because of the superhyperfine interaction between the H (D) nucleus of surface OH (OD) group and a nearby paramagnetic centre constituted, in one case, by a surface trapped single electron and, in a second case, by a surface-adsorbed superoxide ion. Copyright © 2000 John Wiley & Sons, Ltd.

Site of the Eu2+ Activator in the X-Ray Storage Phosphor SrFCl

With electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) the surrounding of the activator Eu 2+ was investigated in SrFCl single crystals. The analysis of the Eu 2+ superhyperfine interaction with the nuclei of the next fluorine shells yielded that Eu 2+ substitutes for Sr 2+ and is not shifted from a regular lattice site. This result settles previous conflicting conclusions about such a shift.

Effective mass donors in silicon carbide — a study with electron nuclear double resonance

With electron nuclear double resonance the superhyperfine interactions of numerous 29Si and 13C lattice neighbours of 14N donors in 3C and 4H–SiC have been measured. It was attempted to explain them using the effective mass theory (EMT). The interactions of N donors on cubic sites in 3C–SiC can be well explained with EMT assuming N to be on C sites, but not on Si sites. N in 3C–SiC represents probably the only true EMT donor, since for shallow donors in Si EMT is not sufficient as shown previously. The interactions of N on the hexagonal sites in 4H–SiC are still reasonably well explained, while the EMT approach fails to explain N on the quasi-cubic sites. Comparison between 3C–SiC and 4H–SiC shows that the “central cell corrections” due to the chemical nature of N introduced into EMT previously for silicon is very small. The major contribution comes from inter-valley interactions.

ODMR of bound excitons in Mg-doped GaN

GaN bulk crystals compensated with Mg exhibit a strong yellow photoluminescence (PL) band. However, optically detected magnetic resonance (ODMR) experiments show that the microscopic origin of this luminescence is completely different from the one observed in n-type GaN. The ODMR spectra of GaN:Mg bulk crystals are dominated by a fine-structure-split pair of lines. At magnetic field orientations between 45 and 60° from the c-axis, additional substructure on these peaks is resolved, characterized by a splitting of 14mT, and tentatively assigned to a superhyperfine interaction. Additionally a half-field resonance split by 50 up to 70mT is detected. The spectra are described by an electron–hole pair located at a close donor–acceptor pair. Axial donor and acceptor g-values of ge||=ge⊥=1.83, gh||=2.18, and gh⊥=2.22 are found. The exchange interaction can be described by an axial fine-structure tensor with D||=0.18cm−1 and D⊥=0.09cm−1.

Magnetic resonance investigations on oxygen-contaminated BaFBr

After room temperature X-irradiation of normally-grown stoichiometric BaFBr single crystals two new oxygen hole centres were discovered and investigated by electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR). One of these two new centres is an oxygen ion (O−) on a bromide site, however, with different super-hyperfine interactions compared to the one investigated previously. The second one is an oxygen molecule (O3- 2) with one O atom on a fluoride site and one on the neighbouring bromide site, both sharing one hole. In total, there are now four different paramagnetic oxygen hole centres known is stoichiometric BaFBr single crystals.

Covalent bonding in NH 4Br:Cu 2+

The paramagnetic centres of Cu 2+ in ammonium bromide crystals were studied at 300 and 77 K with X-band and Q-band EPR. We determined the spin Hamiltonian parameters, contact interaction and covalent bonding parameters of the paramagnetic centres of the NH 4Br:Cu 2+. The superhyperfine interaction to the Cu paramagnetic centres in NH 4Br is much stronger than in NH 4Cl. The results obtained for the contact interaction (with a new assignment that we gave to the EPR spectra) are in agreement with those found for other copper complexes. The comparison of the results of the covalent bonding parameters with those of other compounds showed a strong covalency of NH 4Br:Cu 2+, suggesting that orbital admixtures in this complex are large and apparently much bigger than estimates based on overlap effects alone with no covalent bonding. The Cu 2+ wave function of the NH 4Br written as an admixture of |3 z 2− r 2〉 and | x 2− y 2〉 orbitals is not suitable to describe the system. A real model must take into account the admixtures with the ligand orbitals. The d–s mixing is high in the CuBr 4(NH 3) 2 2− centre, indicating strong bonding between the Cu 2+ ion and the two NH 3 molecules. In the case of the CuBr 4(NH 3)(H 2O) 2− and CuBr 4(H 2O) 2 2− centres the binding between the Cu 2+ ion and the H 2O molecules is much weaker.

Simultaneous Binding of Fluoride and NO to the Nonheme Iron of Photosystem II: Quantitative EPR Evidence for a Weak Exchange Interaction between the Semiquinone QA-and the Iron-Nitrosyl Complex

The effects of NO and fluoride on the iron-quinone complex of the acceptor side of Photosystem II (PSII) are examined by X- and Q-band EPR spectroscopy. It is found that the EPR signal of the iron-nitrosyl complex changes upon addition of F-. The change is determined to be due to a superhyperfine interaction between the electronic spin (S = 3/2) and the fluorine nuclear spin (I = 1/2), indicating that both F- and NO are bound to the same nonheme iron. To the best of our knowledge, this is the first report of simultaneous binding of both F- and NO to the same iron of a mononuclear nonheme protein. On the basis of an analysis of the hyperfine interaction, a cis F−Fe−NO coordination is indicated. Upon illumination of (NO, Cl-)- or (NO, F-)-treated PSII membranes at 200 K, new X- and Q-band EPR signals are observed in B1||B and B1⊥B modes. Quantitative simulations of these signals provide an unambiguous assignment to the iron-quinone complex, QA-{FeNO},7 of the acceptor site of PSII. The exchange interaction between the iron-nitrosyl complex (S = 3/2) and the semiquinone QA- radical (S = 1/2) is determined to be J = +0.5 cm-1 (F-) and +1.3 cm-1 (Cl-) for Hex = JS1S2. A distribution in the exchange coupling is required to satisfactorily simulate the EPR spectra. This distribution is correlated to small structural variations of the iron-quinone acceptor side of PSII.

EPR of gamma irradiated ammonium and caesium formates

In this study, gamma irradiated NH 4HCOO single crystal and CsHCOO powders were investigated at room temperature. The species, produced by gamma irradiation was found to be ĊO − 2, in NH 4HCOO, and the principal values of g, and the hyperfine coupling tensors of the unpaired electron with the 13C nucleus were obtained. In CsHCOO powders, the species was attributed to the ĊO − 3 radical, and a superhyperfine interaction with the 55Cs nucleus, of nuclear spin I=7/2, has been observed, and this has been explained by the interaction of the unpaired electron with a nearby 55Cs nucleus. The superhyperfine constant was found to be ≈1 mT.

Linewidth Analysis of Spin Labels in Liquids: I. Theory and Data Analysis

We present a method of simulating the EPR spectra of spin labels in liquids using direct convolution of hyperfine splitting with Lorentzian linewidths. The aim is to simulate the experimental lineshape by considering all spectrometer characteristics as well as inhomogeneous and homogenous linewidth effects. A major advance in this method is the correction for the broadening produced by Zeeman modulation commonly used to obtain EPR signals; this allows experimenters much more freedom to optimize their experimental conditions for the best signal-to-noise ratio. Microwave power broadening (saturation) effects on the EPR lines are significant even at very low observer levels. Successful simulation requires that all contributions from unresolved hyperfine splittings be explicitly included. Inhomogeneous broadening is dealt with by including all spins that interact with the electron (as a set of superhyperfine interactions); there is no “effective Gaussian” to substitute for the correct superhyperfine interactions. The effects of spin exchange on the linewidth and lineshape can be observed and must be taken into account in order to extract the fundamental linewidths.

ENDOR Investigation of the Eu2+ Activator in the X-Ray Storage Phosphor BaFBr

Electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) have been used to investigate the surrounding of the activator ion Eu 2 in single crystals of the X-ray storage phosphor BaFBr. An analysis of the Eu 2+ superhyperfine interaction with the next fluorine neighbours showed that Eu 2+ substitutes for Ba 2+ and is not shifted from a regular position within experimental error, in spite of the large difference in ionic radii. EPR and ENDOR investigations on BaFBr:Eu 2+ powders established that different methods of producing these materials have no influence on the local environment of Eu 2+ .