EPR study of Mn2+ ions adsorption on silica gel

The paper deals with the study of the spectra of the electron paramagnetic resonance (EPR) of the Mn²⁺ ions adsorbed on the silica gel. Further on there have been studied the changes in the EPR spectrum of the Mn²⁺ ions adsorbed on the silica gel that are occuring as a result of dehydratation of the introduced samples, as well as the spectra changes of these samples that have arisen owing to the natural adsorption of the water vapour from the air under normal physical conditions.At low concentrations of the Mn²⁺ ions adsorbed on the silica gel, the EPR resonance spectrum contains six hyperfine structure (HFS) components the widths of which enlarge towards higher magnetic fields and do not change with the increase of the Mn²⁺ ions concentration.The spectrum further contains a system of lines that have been identified as the so-called forbidden transitions, when simultaneously with the orientation of the electron spin the orientation of the nuclear spin changes as well. This spectrum corresponds to the Mn²⁺ ions adsorbed in the individual positions considerably far from each other, without mutual dipole-dipole interaction.During further increase of the adsorbed Mn²⁺ concentration, there appears in the EPR spectrum, besides the above-described HFS, a broad symetric line the width of which does not change with increasing concentration, except for high concentrations when it becomes narrower probably due to the exchange interaction. This signal corresponds to the Mn²⁺ ions adsorbed in clusters, probably in the silca gel pores, when the HFS gets smudged as a result of dipole-dipole interaction.It appears that the width of the EPR spectrum broad line is dependent upon the temperature at which the dehydratation process was passing. At the adsorption of water vapour from the air under normal conditions on the dehydrated sample, there arise expressive changes in the EPR spectrum shape. The broad line decomposes in 6 HFS components, the widths of which change with duration time of the water vapour adsorption. Since the HFS of the individually adsorbed Mn²⁺ ions does not change after the dehydratation and the ensuing water vapour adsorption, the EPR spectrum of these samples becomes a superposition of two hyperfine structures coresponding to the two inequivalently bound groups of Mn²⁺ ions.     

A New EPR Data on the MA1 and NIRIM8 (NP1) Phosphorus-Related Centers in Synthetic 1b Diamonds

Microdiamonds grown under high-pressure and high-temperature conditions from a P–C medium at different temperatures have been studied by electron paramagnetic resonance (EPR). Two paramagnetic centers P1 and MA1 were observed in microdiamonds grown at 1,873 K. Analysis of weak lines around these centers suggests that they may be due to ¹³C hyperfine structure (HFS) of the MA1 center. The calculated s/p hybridization parameter for this carbon atom (C₁) was similar to that for the undistorted lattice. At a growth temperature of 1,973 K, in addition to MA1 and P1, a new center, labeled NP1, with HFS from nitrogen and phosphorus atoms and with EPR parameters similar to NIRIM 8 was identified. The NP1 (NIRIM8) centers have an electron spin S = 1/2. We propose that nitrogen–phosphorus defects are created through subsequent migration of nitrogen atoms towards phosphorus atoms upon increasing of the growth.     

New Data on the Nature of the EPR OK1 and N3 Centers in Diamond

The well-known diamond electron paramagnetic resonance (EPR) OK1 and N3 defects are discussed in the context of incorporation of heavy metal, supposedly Ti, in the double semivacancy and substitutional structures, respectively. X-ray phase analysis of inclusions of the growth medium, eclogite, in the diamonds under study shows an atypically high concentration of titanium compounds. To support proposed models, the hyperfine structure (HFS) from ¹³C in the OK1 center has been analyzed more precisely. The data obtained are consistent with attributing additional satellites around the main EPR lines to HFS from ⁴⁷Ti and ⁴⁹Ti. A double semivacancy model describes the directions of g and A tensor components of the OK1 center. The features of infrared and photoluminescence spectra related to the OK1/S1 and N3/440.3 nm centers are presented.     

Defects and impurities in nanodiamonds: EPR, NMR and TEM study

EPR, ¹³C NMR and TEM study of ultradisperse diamond (UDD) samples is reported. The compounds show a high concentration of paramagnetic centers (up to 10²⁰ spin/g), which are due to structural defects (dangling C-C bonds) on the diamond cluster surface. The anomalous reduction in the spin-lattice relaxation time of ¹³C (from several hours in natural diamond to ∼150 ms in UDD clusters) is attributed to the interaction between the unpaired electrons of the paramagnetic centers and nuclear spins. ¹³C NMR line-width reflects the fact that the structure of the UDD surface is distorted in comparison to the ‘bulk’ diamond structure.     

EPR and optical absorption studies of X-irradiated cobalt doped sodium chloride crystals

Sodium chloride crystals containing small concentrations of cobalt (< 10 ppm) do not show any EPR line. A thick block of crystal containing ～25 ppm of Co showed two partially resolved lines, with approximate g-values 2.036 and 2.011. These g-values are not close to those of Co⁺⁺ (4.0 to 4.5) in other crystals. On X-irradiation, pure NaCl crystals show a complex EPR spectrum. X-irradiated Co doped NaCl crystals showed an EPR line superimposed on the complex EPR spectrum. Cobalt doped highly pure crystals, on X-irradiation, showed an EPR line superimposed on the F center EPR line. The g-value of the former is 2.049±0.002 and half width is 62±3 gauss. These results combined with those of dielectric loss and optical studies show that X-irradiation of Co doped crystals produces new centers, labelled as S centers, which produce a dielectric loss peak, a decrease in electrical conductivity, an optical band at 210 nm and the EPR line. Possible models of the S centers are discussed.     

EPR and UV spectrometry investigation of sucrose irradiated with carbon particles

Solid state/EPR (SS/EPR) dosimeters of carbon ions irradiated sucrose are studied with EPR, and their water solutions – with UV spectroscopy. Doses between 20 and 200 Gy are used with linear energy transfer (LET) values for carbon ions of 63, 77, 96 and 230 keV μm^?1. After irradiation all samples show typical for irradiated sucrose EPR and UV spectra. The obtained data are compared with those previously reported for nitrogen particles and gamma rays irradiated sucrose. The identical shape of both the EPR and UV spectra of irradiated with various type radiation samples suggests that generated free radicals are not influenced by the nature of radiation. The lack of difference in the line width of the separate lines or the whole EPR spectrum, obtained for gamma and heavy particles irradiation, suggests negligible spin–spin interaction among the radiation-generated free radicals in the samples. The linear dependence of the EPR response on the absorbed dose radiation is found to be higher when generated by gamma rays, than by the same absorbed dose of heavy particles. In addition, the EPR response for carbon ions is higher than that for nitrogen ions. Water solutions of irradiated sucrose exhibit UV spectrum with absorption maximum at 267 nm, attributed to the recombination products of free radicals. The UV band intensity depends on the absorbed dose radiation. The UV spectra obtained for carbon, nitrogen and gamma rays irradiated sucrose are also compared.     

Optical properties of nanodiamond layers

Thin ultradisperse diamond (UDD) layers deposited from a water suspension are studied by optical and x-ray photoelectron spectroscopy (XPS). The effective band gap determined by the 10⁴-cm^?1 criterion for ozone-cleaned UDD is 3.5 eV. The broad structureless photoluminescence band (380–520 nm) is associated with radiative recombination through a system of continuously distributed energy levels in the band gap of diamond nanoclusters. The optical absorption of the material at 250–1000 nm originates from absorption on the disordered nanocluster surface containing threefold-coordinated carbon. The surface of UDD clusters subjected to acid cleaning contains nitrogen-oxygen complexes adsorbed in the form of NO ₃ ^? nitrate ions. Annealing in a hydrogen atmosphere results in desorption of the nitrate ions from the cluster surface. The evolution of the oxygen (O1s) and nitrogen (N1s) lines in the XPS spectra under annealing of a UDD layer is studied comprehensively.     

EPR spectrum of donors in 6H SiC in a broad temperature range

An EPR study of donors in 6H SiC crystals with an uncompensated donor concentration (N _D−N _A) of 2×10¹⁸ to 1×10¹⁶ cm⁻³ performed in the temperature range 4.2 to 160 K at frequencies of 9 and 140 GHz showed that 6H n-SiC samples have two donor states in the gap. One of them originates from nitrogen occupying three inequivalent lattice sites with ionization energies of 150 and 80 meV, and the second is connected with a structural defect lying deeper in the gap than nitrogen. The temperature dependences of donor EPR line intensities have been found to deviate from the Curie law. The observed EPR line-intensity peaks of donors are produced in a temperature-driven successive redistribution of donor electrons between the donor levels. The temperature dependences of EPR line intensities obtained from samples with low donor concentrations were used to determine the valley-orbit splitting of nitrogen in cubic sites. Fiz. Tverd. Tela (St. Petersburg) 40, 1824–1828 (October 1998)     

EPR characterization of organic free radicals in marine diesel

The intensive unresolved central line associated with organic free radicals in electron paramagnetic resonance (EPR) spectra of petroleum is interpreted as resulting from the superposition of signals from different radical species with very close g values. The mobility of a free radical in crude oil is relatively low due to the high oil viscosity. Aimed at characterizing free radicals in the oil byproduct, the present study is concerned with the investigation of marine diesel (bunker), a low-viscosity oil. Marine diesel was studied by EPR spectroscopy at X-band (9 GHz) in the temperature range from 170 to 400 K. Despite the viscosity at room temperature (2.5·10⁻³ kg/m·s) and the tumbling correlation time for free radicals of about 10⁻⁷ s suggesting a high mobility of free radicals in marine diesel, the EPR spectra at room temperature did not exhibit resolved lines, but only an envelope with a poor signal-to-noise ratio. The same behavior was observed from 170 to 373 K. Above 373 K, the EPR spectrum exhibited resolved lines: a superposition of a septet-quartet, a sextet-quartet and a quintet-quartet group of lines was identified with the following parameters: g = 2.003 ± 0.001, proton hyperfine couplings A = 6.41 ± 0.03 G (septet) and A′ = 1.82 ± 0.02 G (quartet); g = 2.003 ± 0.001, A = 6.21 ± 0.03 G (sextet) and A′ = 1.64 ± 0.02 G (quartet); g = 2.003 ± 0.001, A = 6.16 ± 0.03 G (quintet) and A′ = 1.83 ± 0.02 G (quartet), which were attributed to phenalenyl radicals and their derivatives. Below 373 K, phenalenyl dimerization prevented its detection by EPR.     

Low-temperature luminescence of silicon carbide crystals

The high-energy part of the blue fluorescence of-SiC(6H) is found to be dependent on the nitrogen (donor) concentration. The line fluorescence spectrum is found to include a new series of lines due to radiative transitions to A1 acceptor levels.We are indebted to J.G. Pichugin for providing the crystals.     

EPR study of the peculiarities of incorporating transition metal ions into the diamond structure

In this paper previously obtained data is reviewed and new data is discussed about nickel-containing centers in diamonds. These data are used to suggest interpretation of new data about cobalt-containing centers and to understand the influence of iron on the defects in diamonds grown in the iron system. A newly discovered nickel-nitrogen center has three nitrogen atoms in the first neighbor sphere around the double semi-vacancy and looks like the N3 (P2) center. In diamonds grown in the cobalt system two new types of cobalt-containing centers were found (NLO2 and NWO1). Both centers have electron spinS=1/2 and hyperfine structure from one cobalt ion (I=7/2 with natural abundance 100%). A case can be made for a double semi-vacancy structure for these defects. Special growth of diamond in the system enriched in¹⁵N decreased the line width down to 0.6 G, but gave no direct evidence of the existence of nitrogen in the defect structure. Asymmetrical shapes of the lines in the electron paramagnetic resonance (EPR) spectra of cobalt-containing centers with opposite signs in low and high magnetic field parts of spectra are due to very sensitive spin-Hamiltonian parameters of these defects to the lattice distortions. Annealing of cobalt-containing crystals at 2600 K produces the disappearance of all cobalt-containing EPR spectra, probably due to the capture of an additional nitrogen atom and the creation of a 3d⁶ diamagnetic state. In diamonds grown in the iron system with a high content of nitrogen there is evidence of an influence of ferromagnetic inclusion on the exchange interaction between substitutional nitrogen as an additional channel of indirect exchange interaction.     

Effect of annealing on the spin density of dangling bonds and the structures of amorphous germanium

Electron paramagnetic resonance was used to study the dependence of the volume spin density of dangling bonds ina-Ge films produced by cathode sputtering in argon on annealing temperature. The structure of the films is determined by x-ray diffraction analysis. Two EPR lines with g=2.019 and g=2.003 are observed whose intensities change nonmonotonically with annealing temperature. The g=2.019 line is characteristic only of the amorphous state of germanium, while the g=2.003 line persists even after crystallization of the films. When the results are compared with structure data, the conclusion may be drawn that the observed lines in the EPR spectrum are related to the dangling bonds in the peripheral regions of two types of clusters. The g=2.003 line is due to dangling bonds in the peripheral region of clusters with the usual cubic packing of atoms, and the g=2.019 line is due to clusters of the hexagonal type, which is not characteristic of the normal structure of crystalline germanium.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 51–57, July, 1989.     

EPR spectra of a GdMnO3 thin film on a SrTiO3 substrate

Electron paramagnetic resonance (EPR) spectra of a GdMnO₃/SrTiO₃ thin film in the X band have been measured in the temperature interval from 200 to 450 K. Signals from two types of paramagnetic centers have been observed in the spectra. The first paramagnetic center is a subsystem of Gd³⁺ ions, in the EPR spectrum of which the fine structure lines are resolved below 350 K. The second paramagnetic center is a system of manganese and gadolinium ions, in the EPR spectrum of which an exchange-narrowed line is observed with the width ΔH several times less than the width ΔH of an exchange-narrowed line observed in the GdMnO₃ single crystal. Unusual magnetic properties are due to the mismatch of the lattice parameters of the GdMnO₃ thin film and the SrTiO₃ substrate.     

Influence of annealing treatment on as-grown Ib-type diamond crystal at a high temperature and high pressure

In this paper,we report on the influence of annealing treatment on as-grown Ib-type diamond crystal under high pressure and high temperature in a china-type cubic anvil high-pressure apparatus.Experiments are carried out at a pressure of 7.0 GPa and temperatures ranging from 1700 C to 1900 C for 1 h.Annealing treatment of the diamond crystal shows that the aggregation rate constant of nitrogen atoms in the as-grown Ib-type diamond crystal strongly depends on diamond morphology and annealing temperature.The aggregation rate constant of nitrogen remarkably increases with the increase of annealing temperature and its value in octahedral diamond is much higher than that in cubic diamond annealed at the same temperature.The colour of octahedral diamond crystal is obviously reduced from yellow to nearly colorless after annealing treatment for 1 h at 1900 C,which is induced by nitrogen aggregation in a diamond lattice.The extent of nitrogen aggregation in an annealed diamond could approach approximately 98% indicated from the infrared absorption spectra.The micro-Raman spectrum reveals that the annealing treatment can improve the crystalline quality of Ib-type diamond characterized by a half width at full maximum at first order Raman peak,and therefore the annealed diamond crystals exhibit nearly the same properties as the natural IaA-type diamond stones of high quality in the Raman measurements.     

Structure and energy level of native defects in as-grown and electron-irradiated zinc germanium diphosphide studied by EPR and photo-EPR

The properties of defects in as-grown p-type zinc germanium disphosphide (ZnGeP₂) and the influence of electron irradiation and annealing on the defect behavior were studied by means of electron paramagnetic resonance (EPR) and photo-EPR. Besides the well-known three native defects (V_Zn, V_P, Ge_Zn), an S=1/2 EPR spectrum with an isotropic g=2.0123 and resolved hyperfine splitting from four equivalent I=1/2 neighbors is observed in electron-irradiated ZnGeP₂. This spectrum is tentatively assigned to the isolated Ge vacancy. Photo-EPR and annealing treatments show that the high-energy electron irradiation-induced changes in the EPR intensities of the zinc and phosphorus vacancies are caused by the Fermi level shift towards the conduction band. Annealing of the electron-irradiated samples induces a shift of the Fermi level back to its original position, accompanied by an increase of the EPR signal associated with the V_Zn⁻ and a proportional increase of the EPR signal assigned to the V_P⁰ under illumination (λ<1 eV) as well as generation of a new defect. The results indicate that the EPR spectra originally assigned to the isolated V_Zn⁻ and V_P⁰ are in fact associated defects and the new defect is probably the isolated phosphorus vacancy V_Pi.     

Threshold spectra of SBS at a finite frequency mismatch of the interacting waves

A problem of scattered radiation spectrum line width of stimulated Brillouin scattering (SBS) in plasma with two species of ions is considered. Threshold conditions, when SBS shifted frequency lines, which characterize the scattered radiation spectrum, are comparatively wide and are comparatively narrow, are defined. A possibility of existence of the scattered radiation spectrum with the frequency line width comparable to the ion-acoustic frequency is revealed.     

EPR Study of 5-Chlorosalicylate-Cu(II)-3-Pyridylmethanol Ternary Complex Systems in Frozen Water–Methanol Solutions

Two copper(II) ternary complex systems containing 5-chlorosalicylic acid (5-ClsalH) and different copper(II) salts with varying 3-pyridylmethanol (ron = ronicol) concentration, system I [CuSO₄ (aq) +2(5-ClsalH(solv)) + xron(l)] and system II [Cu(ac)₂(aq) + 2(5-ClsalH(solv)) + xron(l)], where x = 0, 2, 4, 6 and 8, were prepared and studied by electron paramagnetic resonance (EPR) spectroscopy in frozen water–methanol solutions to observe the effects of different copper(II) salts and varying neutral ligand concentration on the formation of resulting complexes in solution. The trend in g-values (g _|| > g _⊥ > 2.0023) indicates that the unpaired electron on the copper ion is localized in the d_{x² - y²} d_{{x}^{\rm{2}} - {y}^{\rm{2}}} orbital. The detailed analysis of the second-derivative Cu(II) EPR spectra has shown well-resolved ¹⁴N superhyperfine splitting in the perpendicular part of the axially symmetric spectra. The resolution of nitrogen superhyperfine multiplet patterns increased with increase in the ronicol concentration (ligand-to-metal ratio x). The number of superhyperfine lines was found to be constant (nonet) when x > 4 for system I and x ≥ 4 for system II. This fact indicates that for these x-values four equivalent nitrogen atoms could be coordinated to the central copper atom in the equatorial plane of both systems.     

Paramagnetic frenkel defects in irradiated alkali-haloid crystals

In the EPR spectra of γ-irradiated NaF,⁶LiF, and LiF crystals with natural content of isotopes (independent of the impurity composition), the hyperfine structure (HFS) is observed against the background of a broad band. Absorption saturation in the system of defects responsible for the HFS and the broadband occurs at widely different power levels of microwave radiation, and broad band suppression takes place at registration in quadrature. The experimentally measured intensity distribution and the number of EPR lines in the⁶LiF crystal correlate with the calculated data when the spin interaction of an unpaired electron with 14 equaivalent fluorine nuclei is taken into account. A model of major radiation-induced paramagnetic defects in the form of Frenkel pairs, in which one component (the negatively charged quasi-molecule consisting of two halogen atoms) can be responsible for the HFS and the other component (F-center) for the broad band in the EPR spectrum, is proposed.     

Lithium naphthalocyanine as a new molecular radical probe for electron paramagnetic resonance oximetry

A new lithium naphthalocyanine dye aggregate [Li₂Nc][LiNc] is reported as a potential electron paramagnetic resonance (EPR) oximetry probe for accurate measurement of oxygen concentration in biological systems. The Li₂Nc is diamagnetic; however, the LiNc molecule has an unpaired electron and hence is paramagnetic. The aggregate shows a strong and single line EPR signal that is non-saturating at normal EPR power levels. An oxygen-dependent peak-to-peak EPR spectral width ranging from 0.51 G (at pO₂: 0 mmHg) to 26.2 G (at pO₂: 760 mmHg) has been observed. The application of this probe has been demonstrated in the measurement of arterial and venous oxygen tensions in a rat.     

用EPR方法研究人造金刚石中杂质氮

 本文用电子顺磁共振（EPR）方法，研究人造金刚石中氮的电子结构。提出用自旋浓度来表征氮含量的方法，并对不同方法制得的金刚石进行测定。