Motional effects on the epr spectrum of a perturbed H centre in neutron irradiated LiF

The electron paramagnetic resonance (EPR) spectrum of an asymmetric interstitial H centre in LiF has been studied as a function of temperature. The shifts with temperature of the EPR lines have been measured from 7 to 150 K. They are interpreted by a libration process of the molecule ion around its mean orientation. Near 180 K, the EPR lines broaden. The thermally activated broadening arises out from reorientational motions of the molecule ion. The reorientation rates show that there are two distinct jumping motions between 180 and 230 K; one of them implies rotations of the molecule ion around the average 〈111〉 orientation and is assumed to be induced by the same librations modes as those which produce the line-shifts. The other one is the pyramidal jumping motion between the four equivalent 〈111〉 orientations.     

Characterization of ancient and modern papers by CW-EPR spectroscopy

Several old (15th to 18th century), new (1900–1950) and recently produced (after 1990) papers have been investigated by X-band continuous-wave electron paramagnetic resonance (EPR) spectroscopy. Signals from Cu(II), Fe(III), Mn(II) and radicals are apparent. A clear-cut distinction is observed between recently produced papers and the other samples: recent papers show just EPR signals from Mn(II) and sometimes small signals from Fe(III) in rhombic site, while the older samples show usually strong signals from Fe(III), Mn(II), Cu(II) and radicals. Furthermore, Mn(II) EPR signals from recent papers are characterized by small zero-field splitting (ZFS) parameters, indicating Mn(II) in high-symmetry sites, while older samples show broader Mn(II) EPR signals, typical for Mn(II) in low-symmetry sites (increased ZFS parameters).     

玻璃基质中ZnS纳米晶Mn2+的EPR谱

用融熔法制备分散有ZnS:Mn²⁺纳米晶的纳硼硅(Na₂O-B₂O₃-SiO₂)玻璃在不同温度、时间对样品进行退火处理,得到不同尺寸的纳米晶.研究了玻璃基质中ZnS:Mn²⁺纳米晶的EPR谱、微波功率饱和EPR谱及发射光谱,发现Mn²⁺有二种组态,即替位组态和间隙位组态.分析结果表明替位组态Mn²⁺及周围立方晶场的畸变程度直接影响光学发光特性.     

Electron paramagnetic resonance of a nitrogen-related centre in electron irradiated silicon

The observation by electron paramagnetic resonance of a centre related to nitrogen as an impurity in silicon is reported. While all previously reported nitrogen-related centres in silicon were produced by nitrogen implantation, the present centre is observed after electron irridiation of aluminium-doped silicon at low temperatures. We tentatively identify the observed spectrum, labeled Si-NL26, with neutral interstitial nitrogen. Possible models for the incorporation of nitrogen in silicon, before the irradiation, after the irradiation and after thermal anneal, are discussed.     

On Room-Temperature Inversion of EPR Signals of P1 Centre in Synthetic Diamond

Room-temperature inversion of EPR absorption signals of P1 centre in synthetic diamond is studied by the transient nutation technique. Use of the bichromatic field, consisting of a transverse microwave field and longitudinal radio frequency field, allows to investigate the dynamics of P1 centres in the same field configuration as in cw EPR spectrometers. It is shown that the annealing decreases the P1 centre concentration and, respectively, increases the spin-spin relaxation time. As a result, the periodic inversion （nutation） of the P1 centre absorption signal is observed longer. It is assumed that the P1 centre signal inversion, which was previously observed by cw EPR, might be caused by the Bloch-Siegert effect in the bichromatic field.     

New hydrogen associated paramagnetic centre in LiNbO3

A new hydrogen associated paramagnetic centre (g=2.0028±0.0005, A=3±0.5 mT at 77K) was observed in LiNbO₃. This centre is identified as an OH^2? ion, produced as a result of an electron capture by a diamagnetic OH^? ion, substituting the O^2? ion in LiNbO₃. An experimental procedure for detecting the EPR spectrum of the new centre is established. It is based on the observation of the more rapid destruction of O^? and Nb⁴⁺ ions under UV-irradiation at 77 K as compared to the OH^2? ions obtained after γ-irradiation of LiNbO₃ crystals at 77 K.     

A review of spin Hamiltonians applicable to exchange-coupled systems of interest in biomaterials

Dinuclear and tetranuclear complexes of spin-coupled manganese-manganese, manganese-iron, and iron-iron ions occur in enzymes which are of importance in biology and photosynthesis. These multinuclear systems contain higher oxidation states of manganese and iron ions than those of the commonly occurring Mn(II) and Fe(III) mononuclear ions, in which there has been a lot of interest recently with regard to their electron paramagnetic resonance (EPR) spectra. In order to aid the interpretation of EPR spectra, the spin Hamiltonians, and the resulting energy levels, characterizing binuclear and tetranuclear manganese ions will be discussed in this presentation. These will include, in particular, Mn(II)?Mn(III), Mn(III)?Mn(IV), and Mn(IV)?Mn(V) systems for the binuclear situation, and the Mn₄O₂ “butterfly”, cubane, dimer of dimer systems, and diamond structure for the tetramer situation. These coupled systems are characterized by a variety of exchange interactions whose magnitudes and nature, ferromagnetic or antiferromagnetic, affect profoundly the energy levels. In addition, the hyperfine interactions amongst the various⁵⁵Mn nuclei produce a complex EPR spectrum consisting of a large number of hyperfine lines. In order to interpret EPR spectra properly, one needs to simulate them as accurately as possible. Details of a rigorous technique for the simulation of EPR spectra of these systems with matrix diagonalization and homotopy technique will be provided.     

A multifrequency EPR approach to travertine characterisation

The understanding of processes that give rise to travertine deposits is important. This is so because of its widespread use as decorative material, but more so in environmental studies due to the significance, by proxy, of travertine in climatology. In this paper, a multifrequency EPR spectroscopy study of the behaviour of an ubiquitary vicariant of Ca in calcite, Mn(II), is presented. EPR spectra were obtained from a natural sample at 9.5 (X-band), 95, 190, and 285GHz, and interpreted through numerical simulation. An analysis of the distribution of the zero-field splitting interaction revealed the source of some unexpected spectral features in the width of the lines in the X-band. By contrast, the homogeneous broadening plays only a minor role. Moreover, field-dependent anisotropies of the Zeeman and hyperfine tensors were observed at higher frequency. On the basis of results garnered in this study, the ZFS interaction of Mn(II) has been ascribed to the microstructural anomalies of the Mn(II) distribution in calcite. This may be considered as the fingerprint of the physical-chemical conditions at the time of travertine deposition. As a consequence, X-band EPR spectroscopy represents a specific tool to investigate the genesis, and to check the homogeneity of Mn(II) distribution in travertines as well as in other calcite-based materials.     

Diffusion of interstitial Mn in the dilute magnetic semiconductor (Ga,Mn)As: the effect of a charge state

Migration barriers for diffusion of interstitial Mn in the dilute magnetic semiconductor (Ga,Mn)As are studied using first-principles calculations. The diffusion pathway goes through two types of interstitial sites: As coordinated and Ga coordinated. The energy profile along the path is found to depend on the ratio of concentrations between substitutional and interstitial Mn in GaAs. Two regions of distinctly different behavior, corresponding to n-type and p-type (Ga,Mn)As, are identified. The difference in mobility is a reflection of the change in the charge state of Mn interstitials (double donors) that occurs in the presence of substitutional Mn impurities (acceptors). In addition, substitutional Mn impurities are shown to act as traps for interstitial Mn. The effective migration barrier for the positively doubly charged Mn interstitials in p-type (Ga,Mn)As is estimated to vary from 0.55 to about 0.95 eV.     

Defect centre responsible for production of 110 ∘ C TL peak in quartz

110 ^°C thermoluminescence (TL) peak in quartz is well known due to its pre-dose effect, which is used in dating technique. The generally accepted mechanism for the production of this peak is based on Ge impurity contained in quartz. Its role is to substitute for Si in SiO₄ tetrahedron and under irradiation gives rise to [GeO₄/e^?]^? electron centre. Heating for TL read out liberates electron that recombines with hole in [AlO₄/h]^° or [H₃O₄/h]^° centres emitting photon. The investigation, carried out on blue quartz, green quartz, black quartz, pink quartz, red quartz, sulphurous quartz, milky quartz, alpha quartz and synthetic quartz, has shown that the 110 ^°C TL peak in all these varieties of quartz has no correlation with the respective Ge content. Electron paramagnetic resonance (EPR) measurements on any of these varieties of quartz revealed a signal with g₁=2.0004, g₂=1.9986 and g₃=1.974 and this signal does not appear to correspond to any known EPR signals in alpha quartz. Furthermore, isothermal decay measurements are carried out on the above mentioned EPR signal and 110 ^°C TL peak in alpha, blue and green quartz. A close correlation has been observed in the decay behavior. A new mechanism is proposed based on an interstitial O^? centre.     

Optimal doping control of magnetic semiconductors via subsurfactant epitaxy

"Subsurfactant epitaxy" is established as a conceptually new approach for introducing manganese as a magnetic dopant into germanium. A kinetic pathway is devised in which the subsurface interstitial sites on Ge(100) are first selectively populated with Mn, while lateral diffusion and clustering on or underneath the surface are effectively suppressed. Subsequent Ge deposition as a capping layer produces a novel surfactantlike phenomenon as the interstitial Mn atoms float towards newly defined subsurface sites at the growth front. Furthermore, the Mn atoms that failed to float upwards are uniformly distributed within the Ge capping layer. The resulting doping levels of order 0.25 at. % would normally be considered too low for ferromagnetic ordering, but the Curie temperature exceeds room temperature by a comfortable margin. Subsurfactant epitaxy thus enables superior dopant control in magnetic semiconductors.     

A novel approach to separating EPR lines arising from species with different transition moments

In pulsed EPR, spectral contributions from several species in one sample can be separated based on different EPR transition probabilities. This is usually done by monitoring the Rabi nutations in a 2D experiment. By using long pulses, the FID and echo shapes of species with different transition probabilities differ significantly, including temporal shifts of the observed echo signals in a two-pulse ESE experiment. These shifts can be used to disentangle spectral components in a 1D field-swept ESE experiment by choosing an appropriate detection time. This approach is demonstrated by experiments on a sample containing Mn(2+) and Cr(3+) centers as well as on an exchange-coupled Mn(III)/Mn(IV) system with Mn(2+) contaminations.     

Structural phase transformations in annealed cubic ZnS nanocrystals

The structural changes of cubic ZnS (cZnS) nanocrystals (NCs) doped with 0.2 at.% Mn²⁺ pulse annealed in vacuum and in air, up to 500 °C, were investigated by multifrequency electron paramagnetic resonance (EPR), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The samples, prepared by a surfactant (Tween20)-assisted liquid–liquid reaction at pH = 6, consist of NCs with a tight size distribution around 3 nm and high crystallinity self-assembled into a stable mesoporous structure. The EPR spectra of the as prepared samples contain only the characteristic lines of the substitutional Mn²⁺(I) centers. No spectra from Mn²⁺ ions localized in (hydro)oxidized regions of the NCs surface were observed. The absence of such a surface layer could explain the stability of the cubic (sphalerite) structure observed by XRD and TEM in the investigated cZnS:Mn NCs annealed in vacuum up to 500 °C. The observation of the cubic-hexagonal transformation for the same NCs annealed in air supports the role of such layer in promoting this structural transformation. The narrowing of the EPR spectral lines above 200 °C with the increase in the average size of the cZnS:Mn crystallites was observed. The effect was more pronounced for the sample annealed in air. EPR also revealed the formation of minute amounts of substitutional Mn²⁺-type centers in a hexagonal ZnO structure at T ~ 300 °C, corresponding to the early stages of the thermally induced oxidation of the cZnS:Mn NCs.     

Nonselective excitation of pulsed ELDOR using multi-frequency microwaves

The use of a polychromatic microwave pulse to expand the pumping bandwidth in pulsed electron-electron double resonance (PELDOR) was investigated. The pumping pulse was applied in resonance with the broad (～100 mT) electron paramagnetic resonance (EPR) signal of the manganese cluster of photosystem II in the S2 state. The observation pulses were in resonance with the narrow EPR signal of the tyrosine radical, YD·. It was found that in the case of the polychromatic pumping pulse containing five harmonics with the microwave frequencies between 8.5 and 10.5 GHz the PELDOR effect corresponding to the dipole interaction between the Mn cluster and YD· was about 2.9 times larger than that achieved with a monochromatic pulse. In addition to the dipolar modulation, the nuclear modulation effects were observed. The effects could be suppressed by averaging the PELDOR trace over the time interval between the observation microwave pulses. The polychromatic excitation technique described will be useful for improving the PELDOR sensitivity in the measurements of long distances in biological samples, where the pair consists of a radical with a narrow EPR spectrum and slow phase relaxation, and a metal center that has a broad EPR spectrum and a short phase relaxation time.     

The Role of Metal Ions in the Study of Ancient Paper by Electron Paramagnetic Resonance

Samples from covers and endleaves of sixteenth to eighteenth century books in good conservation state from the Biblioteca Nazionale Marciana in Venice have been investigated by means of electron paramagnetic resonance (EPR) spectroscopy and optical microscopy to characterize the embedded paramagnetic species and to test differences and similarities of the materials in the two types of book components. We detected the Mn(II), Fe(III), and Cu(II) paramagnetic ions, and analyzed their EPR signals by comparing them with previous results in literature of EPR studies on paper. Interestingly, the Mn(II) EPR spectrum profile appears as a fingerprint for samples coming from the same book, either from cover or endleaf, and it looks different for samples extracted from different books. We discuss the role of EPR as a spectroscopic tool for characterizing the interaction of the ions with the paper components and as possible agents of degradation.     

光合作用水裂解机理与EPR应用研究

在绿色植物放氧光合作用中,光系统II（PSII）催化着自然界利用光能将水裂解并释放出氧气的化学反应. 在研究PSII的生物催化机理中,电子顺磁共振（EPR）波谱学发挥着非常重要的作用. 该文综述了与水裂解有关的EPR应用研究,如锰簇Mn₄O₅Ca的不同氧化中间态,以及这些不同氧化态的锰簇与酪氨酸Y-Z自由基的磁性相互作用等,这些进展揭示了水裂解过程是一个逐步的与质子耦联的电子传递过程.     

EPR study on the La2/3Ca1/3Mn1−x Cu x O y system

The interaction between Mn and Cu ions is studied by measuring the resistance and electron paramagnetic resonance (EPR) at different temperatures of the Cu-doped compound La_2/3Ca_1/3Mn_1−xCu_xO_y. A new transition inR-T curve and substantial enhancement in magnetoresistance are induced by the substitution of Mn ions by Cu ions. The EPR measurement shows that two resonance signals appear at temperature lower than the spin-ordering temperature of Mn ions. A tentative interpretation for the observed phenomena is proposed by considering the interaction between the Cu/Mn ions besides the Mn³⁺/Mn⁴⁺ ions.     

常温下YGaG:Fe3+体系的EPR谱研究

本文采用对角化三角晶场中d5组态离子的完全能量矩阵的方法,研究了YGaG:Fe3 体系的EPR谱与局域晶格畸变的关系,利用EPR谱的低对称参量D和(a-F)的实验值确定出Fe3 离子在YGaG:Fe3 体系中的键长R=0.2005 nm,键角θ=51.498°,以及晶格畸变角Δθ=0.954°。     

Density-functional theory study of half-metallic heterostructures: interstitial Mn in Si

Using density-functional theory within the generalized gradient approximation, we show that Si-based heterostructures with 1/4 layer delta doping of interstitial Mn (Mn(int)) are half-metallic. For Mn(int) concentrations of 1/2 or 1 layer, the states induced in the band gap of delta-doped heterostructures still display high spin polarization, about 85% and 60%, respectively. The proposed heterostructures are more stable than previously assumed delta layers of substitutional Mn. Contrary to widespread belief, the present study demonstrates that interstitial Mn can be utilized to tune the magnetic properties of Si, and thus provides a new clue for Si-based spintronics materials.     

YAG晶体色心的EPR谱

本文描述了8种YAG样品的室温和77K的EPR谱.从这些谱的参数与光谱的实验数据对照,认为晶体中有三种顺磁性的色心,其中g～2.00和g～1.98两条顺磁谱相应于光谱2×10~4cm~(-1)～3.4×10~4cm~(-1)的吸收带,为YAG基质晶体中的缺陷俘获一个电子构成的电子型缺陷中心;而 g～30.60的顺磁谱线与氧空位和掺杂的金属离子Mg~(2+) 、Cr~(3+)有关,推测为氧空位团或氧空位与金属离子构成的S>1/2的复合顺磁中心缺陷.YAG晶体随着在大气氛中高温退火,氧的进入和金属离子的掺入,造成电子和空间体积的不平衡,从而使晶体色心浓度加大.