Monte carlo simulation of the electron paramagnetic resonance spectrum displayed by copper ceruloplasmin at 77 K

Summary Human ceruloplasmin contains several intrinsic copper ions, some of them being paramagnetic. The presence of two classes of paramagnetic Cu²⁺ ions (type I and type II) has been suggested, the two types having significantly different spectroscopic features. However, there are conflicting reports both on the ratio of the number of type-I to type-II Cu²⁺ and on their EPR spectroscopic parameters. By using a Monte Carlo method we obtained the best fit of the experimental spectrum with a synthesized composite spectrum generated by two type-I Cu²⁺ (type I _a and type I _b , possessing slightly different spin Hamiltonian parameters) and by one type-II Cu²⁺. Correspondingly, the EPR spectroscopic parameter (viz. theg-tensors, hyperfine couplings, line widths and molar fractions) together with their accuracies were determined.

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Riassunto La ceruloplasmina umana contiene diversi ioni Cu²⁺ alcuni dei quali sono paramagnetici. Di questi ne sono stati ipotizzati due tipi (tipo I e tipo II), ognuno dei quali con caratteristiche spettroscopiche diverse. Tuttavia esistono nei lavori precedenti discordanze sia sul numero di ioni Cu²⁺ che sui loro parametri EPR. Si è usato il metodo Montecarlo per ottenere il best fit dello spettro EPR della ceruloplasmina. Lo spettro simulato è risultato la somma di tre spettri, 2 di tipo I (tipo I _a e tipo I _b ) e 1 di tipo II. Sono stati così determinati i parametri relativi all'hamiltoniana di spin del sistema e le loro deviazioni standard.

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Резюме Человеческий церулоплазмин содержит несколяко собственных ионов меди, некоторые из которых являются парамгнитными. Предполагается наличие двух классов парамгнитных Cu²⁺ ионов (тип I и тип II). Эти два типа имеют существенно различные спектроскопические свойства. Однако имеются противо-речивые публикации об отношении числа ионов Cu²⁺ типа I к числу ионов Cu²⁺ типа II и об их ЭПР спектроскопических параметрах. Исполязуя метод Монте-Карло, мы получаем наилучшее согласие экспериментального спектра с синтесированным составным спектром, образованным спектрами Cu²⁺ типа I (тип I _a и тип I _b имеющие слегка различные параметры спинового Гамилятониана) и одного спектра Cu²⁺ типа II. Определяются ЭПР спектроскопические параметы (а именно:g-тензоры, сверхтонкие связи, ширины линий и молярные доли).

     

The gas-phase electron paramagnetic resonance spectrum of vibrationally excited SO radicals

Gas-phase E.P.R. spectra from the first six vibrational levels in X ³Σ^- SO have been detected and analysed using a Hund's case (b) formalism. The spectra can be fitted using the following molecular constants:

The first and second derivatives of with respect to the normalized internuclear distance ξ were calculated from these constants. A simple analytic formalism is described which enables the variation of with internuclear distance to be separated into contributions from the matrix elements and from the change in energy spacing between interacting states. Both effects are important in the derivatives of , but it is suggested that in SO most of the variation of with rotational and vibrational state is due to the increase in the second-order part of the parameter as the separation between the ground and the dominant perturbing state decreases. This explanation implies that the expression proposed recently by Veseth and Lofthus for the rotational variation of is not accurate, and enables the conflict between the results of their analysis and ab initio calculations to be resolved.     

MgF2:Mn2+光谱、超精细常数和局部结构的关联

基于电子顺磁共振(EPR)超精细常数As确定键长的新方法和半自洽场d轨道理论,对MgF2:Mn2+光谱和EPR超精细常数作出了统一解释.得到室温下MgF2:Mn2+晶体中杂质中心Mn-F的键长为0.2124±0.0010nm.     

Nuclear polarization by electron paramagnetic resonance in paramagnetic crystals

The possibility of the polarization of nuclei in paramagnetic salts by saturation of electron paramagnetic resonance is theoretically analyzed. The proposed method assumes saturation of the forbidden transition of the typeM=±1, m=±1, ±2, for mutually perpendicular external magnetic and high-frequency fields. The analysis is carried out for the case of a large quadrupole moment of the nucleus. The degree of orientation attained is comparable in order of magnitude with Overhauser's method. This method is particularly suitable for the polarization of nuclei of transuranium elements.

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. M=± 1, m==±1, ±2 . . . .

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In conclusion the author would like to thank J. Burget, J. ajko, M. Kolá and M. ott for helping in the laborious solution of system (16).     

Multifrequency electron paramagnetic resonance of ultramarine blue

Electron paramagnetic resonance spectra of the S^3/? radical center in ultramarine blue over a factor of about 2500 in frequency (258 MHz to 670 GHz) reveal a substantially Lorentzian shape, without resolution of g anisotropy. Variable temperature measurements found that the line width is independent of temperature, within experimental uncertainty, up to about 90 K at 9.5 GHz and between ca. 5 K and room temperature at 95 and 217 GHz, as expected for an exchange-narrowed signal. Analysis of the increase in the low-temperature line width as a function of frequency above 9 GHz is consistent with an exchange interaction of about 2· 10^?2 K. The line width increases as frequency is decreased from 2.7 GHz to 258 MHz which is attributed to the contribution from nonsecular terms that has been denoted the “10/3” effect.     

Precisiong-factor measurements of electron paramagnetic resonance standards

An attempt is made to measureg-factors of electrons in EPR standard samples with the lowest obtainable uncertainty. An experimental equipment for preciseg-factor measurements is described, working at microwave frequencies in the 9 and 36 GHz region, and at room temperature. The course of the measuring process is mentioned in some detail and the sources of errors, which arise during the measurement, are discussed. As an example, the determination ofg-factors of certain EPR standards (using DPPH single crystals and powdered charred dextrose) is presented. No dispersion ofg-factor value has been observed between 9 and 36 GHz.     

Observation of helicon-excited electron paramagnetic resonance in a high-mobility semiconductor

Helicon waves can be exploited in EPR studies of localized magnetic moments in high-conductivity semiconductors when the free carriers have small effective mass and high mobility. In an electron plasma helicons have precisely the polarization required to elicit EPR, and the plasma background actually enhances the resonance intensity. Microwave transmission experiments on Hg_1-xMn_xTe, a narrow-gap semiconductor with localized Mn⁺⁺ magnetic moments, are described to illustrate this effect.     

Lattice dynamic effects in electron paramagnetic resonance

The electron-phonon interaction for a paramagnetic impurity in an insulating crystal is derived and from that an electron-electron interaction in a higher order is considered. An attempt is made to provide a unified presentation of the effect of phonons on the parameters measured in an electron-paramagnetic resonance experiment. The phonon-induced corrections to the zero-field splitting, the hyperfine field, the superhyperfine splitting and line intensities is reviewed theoretically along with experimental data. Both the point charge and covalent models are discussed. The effect of local and resonance modes is calculated and discussed in terms of isotope effect. Some related problems of the energy levels, dispersion, screening and phonon force are also included in appendices.     

Targeted-ROI imaging in electron paramagnetic resonance imaging

Electron paramagnetic resonance imaging (EPRI) is a technique that has been used for in vivo oxygen imaging of small animals. In continuous wave (CW) EPRI, the measurement can be interpreted as a sampled 4D Radon transform of the image function. The conventional filtered-backprojection (FBP) algorithm has been used widely for reconstructing images from full knowledge of the Radon transform acquired in CW EPRI. In practical applications of CW EPRI, one often is interested in information only in a region of interest (ROI) within the imaged subject. It is desirable to accurately reconstruct an ROI image only from partial knowledge of the Radon transform because acquisition of the partial data set can lead to considerable reduction of imaging time. The conventional FBP algorithm cannot, however, reconstruct accurate ROI images from partial knowledge of the Radon transform of even dimension. In this work, we describe two new algorithms, which are referred to as the backprojection filtration (BPF) and minimum-data filtered-backprojection (MDFBP) algorithms, for accurate ROI-image reconstruction from a partial Radon transform (or, truncated Radon transform) in CW EPRI. We have also performed numerical studies in the context of ROI-image reconstruction of a synthetic 2D image with density similar to that found in a small animal EPRI. This demonstrates both the inadequacy of the conventional FBP algorithm and the success of BPF and MDFBP algorithms in ROI reconstruction. The proposed ROI imaging approach promises a means to substantially reduce image acquisition time in CW EPRI.     

Carbon-based standards for electron paramagnetic resonance spectroscopy

In order to meet the need for a good new EPR intensity andg-value standard whose paramagnetic species are carbon-based radicals, several materials were investigated, including coal, fusinite (a coal maceral), and several carbohydrate chars. Of the prototypical standards prepared, a chemically-treated fusinite is recommended as most suitable because of its chemical stability, spin density, EPR signal line shape and line width, microwave power saturation characteristics, availability, and homogeneity. Effects of dilution with KBr, KCl, and polymer are negligible, although the line width is broadened in the presence of paramagnetic gases. Several model standard compounds have been prepared in a polymer matrix to minimize changes in packing density over time.     

A rigorous evaluation of spin-Hamiltonian parameters and linewidth from a polycrystalline EPR spectrum.

Details are given of a procedure to evaluate the spin-Hamiltonian (SH) parameters and the linewidth from a polycrystalline EPR spectrum by using a least-squares fitting (LSF) technique in conjunction with numerical diagonalization of the SH matrix. The required resonance line positions are computed rather quickly using a homotopy technique, in which the position at an external magnetic field (B) orientation (theta, phi) is used as the initial value in a LSF procedure to estimate the position at an infinitesimally close B-orientation, (theta + deltatheta, phi + deltaphi). The resonance line positions are calculated successively in this procedure for all orientations of B over a grid of (theta, phi) values for the unit sphere. The eigenvectors of the SH matrix are used to calculate the intensities of the EPR lines exactly for each orientation of B. Details are given of how to compute rigorously the first and second derivatives of the chi(2)-function with respect to the SH parameters and the linewidth using the eigenvalues and eigenvectors of the spin-Hamiltonian matrix for a polycrystalline spectrum required in the LSF procedure. It is explained how this technique is generalized to include two or more magnetically inequivalent paramagnetic species, as well as how it is used for the simulation of other EPR-related spectra. The procedure is illustrated by evaluation of the Mn(2+) SH parameters and Lorentzian linewidth from the 249.9-GHz EPR spectrum of Mn(gamma-picoline)(4)I(2).     

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.     

Identification of intra‐grain and grain boundary defects in polycrystalline Si thin films by electron paramagnetic resonance

We investigate the characteristics of intra‐grain and grain boundary defects in polycrystalline Si films, by employing quantitative electron paramagnetic resonance measurements on liquid phase crystallized layers with an average grain size of 200 µm and tailored solid phase crystallized Si layers with similar intra‐grain morphology but systematically varied grain sizes between 0.25 µm and 1 µm. The defect characteristics are found to be composed of two distinctive g ‐values of g = 2.0055 and 2.0032, which are attributed to grain boundary defects and intra‐grain defects, respectively. Additional hydrogenation leads to a reduction of the overall defect concentration, while a rapid thermal annealing process primarily heals intra‐grain defects.

     

Forbidden lines in the hyperfine spectrum of the electron paramagnetic resonance of Mn++ in BaTiO3

The hyperfine spectrum of Mn⁺⁺ in powder BaTiO₃ was studied at 8740 MHz andT=80°K. The measured lines appearing as doublets between the hyperfine lines of Mn⁺⁺ are explained as forbidden transitions of the typeM=±1,m=±1. A theoretical expression for the position of these doublets is given and some of the constants of the spin Hamiltonian are determined:g=2.001±0.001, ¦A¦=86 Oe,D 40 Oe.     

An X-band time domain electron paramagnetic resonance spectrometer

A computer-controlled X-band time domain electron paramagnetic resonance (EPR) spectrometer, with a time resolution of the order of 0.5μsec, has been constructed with many of the crucial microwave components designed and fabricated by the Microwave Engineering Group of TIFR. The spectrometer operates either in a microwave power pulsed mode for determination of spin-lattice relaxation times by the saturation recovery technique or in the kinetic mode for determination of the time dependence of EPR signal after laser excitation. It has an automatic frequency control, an automatic phase control and, most importantly, a field-frequency lock which ensures good stability of the EPR line positions enabling signal averaging for extended periods. The constructional details of the spectrometer and its performance in both the modes are described here by reporting results on certain typical systems.     

The continuous wave electron paramagnetic resonance experiment revisited

When the modulation frequency used in continuous wave electron paramagnetic resonance (cw EPR) spectroscopy exceeds the linewidth, modulation sidebands appear in the spectrum. It is shown theoretically and experimentally that these sidebands are actually multiple photon transitions, sigma(+)+kxpi, where one microwave (mw) sigma(+) photon is absorbed from the mw radiation field and an arbitrary number k of radio frequency (rf) pi photons are absorbed from or emitted to the modulation rf field. Furthermore, it is demonstrated that both the derivative shape of the lines in standard cw EPR spectra and the distortions due to overmodulation are caused by the unresolved sideband pattern of these lines. The single-photon transition does not even give a contribution to the first-harmonic cw EPR signal. Multiple photon transitions are described semiclassically in a toggling frame and their existence is proven using second quantization. With the toggling frame approach and perturbation theory an effective Hamiltonian for an arbitrary sideband transition is derived. Based on the effective Hamiltonians an expression for the steady-state density operator in the singly rotating frame is derived, completely describing all sidebands in all modulation frequency harmonics of the cw EPR signal. The relative intensities of the sidebands are found to depend in a very sensitive way on the actual rf amplitude and the saturation of single sidebands is shown to depend strongly on the effective field amplitude of the multiple photon transitions. By comparison with the analogous solutions for frequency-modulation EPR it is shown that the field-modulation and the frequency-modulation technique are not equivalent. The experimental data fully verify the theoretical predictions with respect to intensities and lineshapes.     

Angular variation of the EPR linewidth of Ni2+ in CaO

We measured the angular variation of the EPR linewidth of Ni²⁺ in CaO. The data is used to estimate the distribution of internal stresses in the sample.