Electron Paramagnetic Resonance Properties Research Articles

Radiation effects on TL and EPR of sodalite and application to dosimetry

Sodalite, Na8Al6Si6O24Cl2, a natural silicate mineral, has been investigated concerning its thermoluminescence (TL) and electron paramagnetic resonance (EPR) properties. Samples, both natural and heat treated at 500°C for 30min, present an EPR signal at g=2.01132. The irradiation induces two signals at g=2.0008 and a group of 11 lines due to an O− ion in an intermediate position with respect to two adjacent Al nuclei, that are superimposed on the signal at g=2.0008. An annealing at 500 °C/30min produces a broad signal around g~2.0, probably due to Fe3+ magnetic dipole interactions. The samples were annealed at 500 °C/30min and then irradiated with gamma-ray doses varying from 1 Gy to 20 kGy. All the samples have shown TL peaks at 110, 230 and 270 °C. Similar results were obtained for natural samples. The accumulated dose (AD) was determined to be AD~2.5 kGy and the annual dose of (0.595±0.001) mGy/year was obtained from the concentration of U, Th, and K2O in the sample. Therefore, the age obtained for natural sodalite was (4.2±0.2)106 years.

Selective Synthesis of Zn1 − xMnxSe Nanobelts and Nanotubes from [Zn1 − xMnxSe](DETA)0.5Nanbelts in Solution (x= 0−0.15) and Their EPR and Optical Properties

Zn(1 - x)Mn(x)Se (x = 0-0.15) nanobelts and nanotubes can be synthesized via the removal of diethylenetriamine (DETA) in 1-octadecene (ODE) and ethylene glycol (EG), respectively, using [Zn(1 - x)Mn(x)Se](DETA)(0.5) nanobelts as a template. The as-prepared ZnSe nanobelts are single-crystalline and grown along the [001] direction, and the ZnSe nanotubes consist of nanoparticles assembled along the [001] direction. In addition, Mn(2+)-doped Zn(1 - x)Mn(x)Se (x = 0.05, 0.10, 0.15) nanotubes are prepared for the first time if doped [Zn(1 - x)Mn(x)Se](DETA)(0.5) nanobelts are used as the template. The formation process of Zn(1 - x)Mn(x)Se nanobelts and nanotubes has been studied, and a plausible mechanism is proposed. Photoluminescence (PL) and electron paramagnetic resonance (EPR) spectra of Zn(1 - x)Mn(x)Se nanobelts and Zn(1 - x)Mn(x)Se nanotubes have been investigated.

ESR investigations on doped conjugated polymers diluted in a solid matrix

The electron spin resonance (ESR) properties of a polystyrene (PS) blend of iodine-doped regiorandom (RRa) poly(3-octylthiophene) (P3OT) are investigated to reveal the intrachain spin dynamics of conjugated polymers. The PS blends exhibits broader and anisotropic ESR spectra compared to the isotropic spectrum of the pure RRa-P3OT film. Spectral simulations are performed to reveal the ESR features of the PS blend. It is concluded that intrachain spin motion is less active than interchain motion. In addition, the spin dynamics of the pure film are dominated by intrachain spin motion at low temperature and interchain motion at high temperature.

Variation of the Fermi level in n‐type microcrystalline silicon by electron bombardment and successive annealing: ESR and conductivity studies

AbstractESR and conductivity studies have been preformed on μc‐Si:H exposed to 2 MeV electron bombardment and successive annealing in order to investigate the influence of the defect density on the electronic properties of n‐type μc‐Si:H. With this approach one can vary the defect density in one and the same sample and directly deduce its influence on the electronic properties. The defect density is varied by 2 orders of magnitude with strong influence on the dark conductivity and electron spin resonance (ESR) properties. The relationship of ESR and conductivity data obtained over the whole defect density range is in agreement with the data obtained on the sets of samples deposited with different doping level. The results indicate that the Fermi level position in μc‐Si:H is defined by a balance of defect and donor states densities regardless of which of these quantities is varied. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

DFT study of nitroxide radicals: explicit modeling of solvent effects on the structural and electronic characteristics of 4‐amino‐2,2,6,6‐tetramethyl‐piperidine‐N‐oxyl

An explicit DFT modeling of water surroundings on the electron paramagnetic resonance properties of 4-amino-2,2,6,6-tetramethyl-piperidine-N-oxyl (TA) has been performed. A stepwise hydration of TA is accompanied with certain changes in geometrical parameters (bond lengths and angles) and redistribution of partial electric charges in TA. An aqueous cluster of 45 water molecules can be considered as an appropriate model for a complete aqueous shell around TA, although most of the structural and electronic characteristics of TA already converge at about 10 water molecules. Water surroundings induce an increase in electron spin density on the nitrogen atom of the nitroxide fragment due to stabilization of the polar resonance structure > N(+*)-O(-) at the expense of less polar structure > N-O*. The water-induced rise of the isotropic splitting constant a(iso), calculated from the contact term of the hyperfine interaction, comprises Deltaa(iso)(rho(N2)) = 2.2-2.5 G, which is typical of experimental value for TA. There are two contributions to the solvent effect on the a(iso)(rho(N2)) value: the redistribution of spin density in the nitroxide fragment (polarity effect) and water-induced distortions of TA geometry. Microscopic variations in a hydrogen-bonded water network cause noticeable fluctuations of the splitting constant a(iso)(rho(N2)). Calculations of the atomic spin density (sigma(N2)) allowed us to compute the splitting constant from the relationship a(iso)(sigma(N2)) = Qsigma(N2), where Q = 36.2 G. A practical advantage of using this relationship is that it gives 'smoothed' values of the splitting constant, which are sensitive to the environment polarity but remain tolerant to microscopic fluctuations of the hydrogen-bonded water network around a spin-label molecule.

EPR, optical absorption and photoluminescence properties of Cr 3+ ions in lithium borophosphate glasses

Electron paramagnetic resonance (EPR), optical absorption and photoluminescence studies of Cr 3+ ions in lithium borophosphate glasses have been studied. The EPR spectra exhibit two resonance signals with effective g values at g = 4.56 and g = 1.96. These resonance signals are attributed to isolated Cr 3+ ions and exchange coupled Cr 3+ pairs, respectively. The EPR spectra have also been studied at various low temperatures. The population of spin levels ( N) participating at resonance signal g = 1.96 has been calculated as a function of temperature. The spin-population ( N) is found to be increasing with decrease in temperature. The magnetic susceptibility ( χ) was calculated from the EPR data. It is observed that as temperature decreases, χ increases in accordance with Curie–Weiss law. The exchange coupling constant J between Cr 3+ ions is calculated from the relative intensities of g = 1.96 resonance line at various temperatures. The optical absorption spectrum exhibits four bands characteristic of Cr 3+ ions in octahedral symmetry. From the optical absorption spectral data, the crystal field (Dq) and Racah parameters ( B and C) have been evaluated. The luminescence spectrum exhibits a red emission peak, which is assigned to a transition from the upper 4T 2g (F) → 4A 2g (F) ground state of Cr 3+ ions. The optical band gap is evaluated from the ultraviolet absorption edges.

EPR, optical absorption and photoluminescence properties of MnO 2 doped 23B 2O 3–5ZnO–72Bi 2O 3 glasses

Electron paramagnetic resonance (EPR), transmission electron microscopy (TEM), optical absorption and photoluminescence (PL) spectroscopic measurements are performed on Mn 2+ doped high bismuth containing zinc–bismuth–borate glasses. TEM images reveal homogeneously dispersed Bi o nanoparticles (NPs) of spherical shape with size about 5 nm. EPR spectra exhibit predominant signals at g≈2.0 and 4.3 with a sextet hyperfine structure. The resonance signal at g≈2.0 is due to Mn 2+ ions in an environment close to octahedral symmetry, where as the resonance at g≈4.3 is attributed to the rhombic surrounding of the Mn 2+ ions. The hyperfine splitting constant ( A) indicates that Mn 2+ ions in these glasses are moderately covalent in nature. The zero-field splitting parameter D has been calculated from the allowed hyperfine lines. The optical absorption spectrum exhibits a single broad band centered at 518 nm (19,305 cm −1) is assigned to the 6A 1g(S)→ 4T 1g(G) transition of Mn 2+ ions. The visible and near infrared (NIR) luminescence bands at 548, 652 and 804 nm have been observed when excited at 400 and 530 nm, respectively. These luminescence centers are supposed to be caused by the lower valence state of bismuth, such as Bi 2+ and Bi + ions, generated during melting process.

Pulsed EPR investigations of the Mo(V) centers of the R55Q and R55M variants of sulfite dehydrogenase from Starkeya novella

Continuous-wave and pulsed electron paramagnetic resonance (EPR) spectroscopy have been used to characterize two variants of bacterial sulfite dehydrogenase (SDH) from Starkeya novella in which the conserved active-site arginine residue (R55) is replaced by a neutral amino acid residue. Substitution by the hydrophobic methionine residue (SDH(R55M)) has essentially no effect on the pH dependence of the EPR properties of the Mo(V) center, even though the X-ray structure of this variant shows that the methionine residue is rotated away from the Mo center and a sulfate anion is present in the active-site pocket (Bailey et al. in J Biol Chem 284:2053-2063, 2009). For SDH(R55M) only the high-pH form is observed, and samples prepared in H(2)(17)O-enriched buffer show essentially the same (17)O hyperfine interaction and nuclear quadrupole interaction parameters as SDH(WT) enzyme. However, the pH dependence of the EPR spectra of SDH(R55Q), in which the positively charged arginine is replaced by the neutral hydrophilic glutamine, differs significantly from that of SDH(WT). For SDH(R55Q) the blocked form with bound sulfate is generated at low pH, as verified by (33)S couplings observed upon reduction with (33)S-labeled sulfite. This observation of bound sulfate for SDH(R55Q) supports our previous hypothesis that sulfite-oxidizing enzymes can exhibit multiple pathways for electron transfer and product release (Emesh et al. in Biochemistry 48:2156-2163, 2009). At pH > or = 8 the high-pH form dominates for SDH(R55Q).

G-tensor and hyperfine splitting of , , defect centers in KCl from DFT calculations

We have studied the electron paramagnetic resonance (EPR) properties of a classical group of paramagnetic centers in solid KCl, the H center (a trapped species) and the analogous and centers. The calculations have been performed at the density functional theory level using pure and hybrid exchange-correlation functionals in combination with cluster models embedded in point charges. The attention has been focused on the reproduction of key EPR parameters such as the g- and the A-tensors. The A-tensor is satisfactorily reproduced for all three species. The g-tensor is well described in the case of the H center, while large deviations are found for the and trapped radicals. The different behavior is rationalized in terms of different symmetries of the singly occupied molecular orbitals, σ in and π in and .

Observation of a paramagnetic defect at the epitaxial Ge 3N 4/(111)Ge interface by electron spin resonance

A multifrequency electron spin resonance (ESR) study of Ge 3N 4/(111)Ge entities with nm thick epitaxial Ge 3N 4 layers, of subcritical thickness for mismatch relaxation, grown in a nitrogen plasma reveals the presence of an anisotropic paramagnetic interface center of trigonal symmetry. Building on the analysis of its specific ESR properties, including magnetic field angular mapping data and in the light of previous insight, the signal is tentatively ascribed to the interfacial •Ge N 3 center. The defect occurrence is discussed within the specific interface matching, without misfit dislocations. As an interfacial Ge dangling bond defect, it may operate as an inherent electrically detrimental trap.

Optical and electron paramagnetic resonance properties of native Cr 2O 3 surface over CrO 2

Chromium dioxides of a tetragonal rutile structure were synthesized by the thermal decomposition of precursors CrO 3 powder at 673 K under the pressure of 1.5 bars in the autoclave reactor. The phase analysis reveals the major phases of chromium dioxide (CrO 2) along with a single impurity phase of Cr 2O 3. The CrO 2 phase has lattice parameters a=0.4423 nm and c=0.2918 nm, which yields a unit cell volume V 0=0.0571 nm 3. The dilution of the surface of the CrO 2 is confirmed by the XPS after the observation of the Cr 2p 3/2 peaks of the Cr 4+ state at binding energy ( E b)~576.4 eV and the Cr 3+ states at E b~576.8 eV. At room temperature, the spontaneous magnetization of ~62 Am 2kg −1 along with T C ~390 K is observed. A broad band of Cr 4+ ions in EPR spectrum is observed with an effective g-value of ~1.98 and Δ H~131 mT. Optical absorption in the Cr 4+ oxidation state shows the strong band at 265, 387 and 553 nm along with a single low-intensity band of Cr 3+ ions at 335 and 653 nm. The intense absorption band of Cr 4+ions at 275 nm is associated with the transition from 3A 2 to 3T 1, while the weak absorption at 553 nm is caused by the interaction of 1E and 3T 2 levels.

Photochemical and electrochemical properties of porphyrin dimers containing an anhydride spacer

A study on thermogravimetric analyses, surface photovoltage (SPV), electron paramagnetic resonance and electrochemical properties comparing a porphyrin dimer with that of corresponding monomer was presented. The SPV intensity is weaker than that corresponding monomer, and with field-induced surface photovoltage spectroscopy (FISPS) reveal that all the compounds are p-type semiconductors. The spectral bands of all the porphyrins corresponded to π → π* transitions. Electrons (or holes) can be trapped on the porphyrins by applying both light and negative (or positive) electric field. The electrochemistry and EPR of the porphyrins show clearly that a π–π interaction existed between the two macrocycles of the dimer.

In-depth investigation of EPR spectra ofMn2+ ions in ZnS single crystals with pure cubic structure

The X (9.8 GHz)-band electron paramagnetic resonance (EPR) properties of substitutionalMn2+ ions in high quality cubic ZnS single crystals grown fromPbCl2 flux have been thoroughly investigated. Accurate spin Hamiltonian (SH) parameters:g = 2.002 25 ± 0.000 06;a = (7.987 ± 0.008) × 10−4 cm−1 and A = −(63.88 ± 0.02) × 10−4 cm−1 were obtained by simulation and fitting to the experimentallyallowed transitions recorded for the magnetic field aligned within ± 0.25° along the main crystal axes. The normally forbidden hyperfine , Δm = ± 1 transitions were also observed. Their position was found to be in agreement, within the experimentalaccuracy of ΔH = ± 0.01 mT, with calculations using the same SH parameters. The angular variation of the ratios ofthe intensities of the central forbidden to the allowed transitions could be accounted foronly by including an additional constant contribution. The observed line broadening of the and fine structure transitions and their line width variation in a (110) plane have beenquantitatively described by considering a random distribution of lattice strains at theMn2+ impurityions. The influence of the forbidden transitions and line broadening on the EPR spectra line shape ofthe Mn2+ ions in cubic ZnS crystalline powders is also examined.

Electron spin resonance across the charge-ordering transition inYBaMn2O6

We investigated the metal-ordered manganite system ${\text{YBaMn}}_{2}{\text{O}}_{6}$ using electron spin resonance (ESR) in the paramagnetic regime across the charge ordering and structural phase transition at ${T}_{\text{CO}}=480\text{ }\text{K}$ and ${T}_{t}=520\text{ }\text{K}$, respectively. All ESR parameters exhibit jumplike changes at ${T}_{t}$ while the charge ordering at ${T}_{\text{CO}}$ manifests itself only as a weak and broad anomaly. Above ${T}_{t}$ the ESR spin susceptibility is reduced with respect to the dc susceptibility, indicating that only the ${t}_{2g}$-core spins of Mn ions contribute to the resonance absorption. The contribution of the ${e}_{g}$ spins is suppressed by the time scale of the polaronic hopping process of the ${e}_{g}$ electrons. The linewidth in this regime is reminiscent of a Korringa-type relaxation behavior. In this picture the ESR properties below ${T}_{t}$ are dominated by the slowing down of the polaronic hopping process. The charge fluctuations persist down to the temperature ${T}^{\ensuremath{\ast}}\ensuremath{\approx}410\text{ }\text{K}$, below which the system can be described as a charge-ordered assembly of ${\text{Mn}}^{3+}$ and ${\text{Mn}}^{4+}$ spins.

Thermoluminescence, electron paramagnetic resonance and optical absorption in natural and synthetic rhodonite crystals

Thermoluminescence, electron paramagnetic resonance and optical absorption properties of rhodonite, a natural silicate mineral, have been investigated and compared to those of synthetic crystal, pure and doped. The TL peaks grow linearly for radiation dose up to 4 kGy, and then saturate. In all the synthetic samples, 140 and 340°C TL peaks are observed; the difference occurs in their relative intensities, but only 340°C peak grows strongly for high doses. Al2O3 and Al2O3 + CaO-doped synthetic samples presented several decades intenser TL compared to that of synthetic samples doped with other impurities. A heating rate of 4°C/s has been used in all the TL readings. The EPR spectrum of natural rhodonite mineral has only one huge signal around g = 2.0 with width extending from 1,000 to 6,000 G. This is due to Mn dipolar interaction, a fact proved by numerical calculation based on Van Vleck dipolar broadening expression. The optical absorption spectrum is rich in absorption bands in near-UV, visible and near-IR intervals. Several bands in the region from 540 to 340 nm are interpreted as being due to Mn3+ in distorted octahedral environment. A broad and intense band around 1,040 nm is due to Fe2+. It decays under heating up to 900°C. At this temperature it is reduced by 80% of its original intensity. The pink, natural rhodonite, heated in air starts becoming black at approximately 600°C.

Mössbauer and magnetic studies on nanocrystalline NiFe2O4 particles prepared by ethylene glycol route

NiFe2O4 nanoparticles have been synthesized by co-precipitation method at 145°C in N2 atmosphere using ethylene glycol as solvent and capping agent. This gives the promising synthesis route for nanoparticles at low temperature. The as-synthesized NiFe2O4 is subsequently heated at 400°C, 500°C, 700°C and 800°C. Crystallite size increases with the heat treatment temperature. The heat treatment temperature has direct effect on the electron paramagnetic resonance and intrinsic magnetic properties. The room temperature Mossbauer spectrum of the 800°C heated sample shows the two sextets pattern indicating that the sample is ferrimagnetic and Fe3 + ions occupy both tetrahedral and octahedral sites of spinel structure.

Critical Role of Arginine 160 of the EutB Protein Subunit for Active Site Structure and Radical Catalysis in Coenzyme B12-Dependent Ethanolamine Ammonia-lyase

The protein chemical, kinetic, and electron paramagnetic resonance (EPR) and electron spin-echo envelope modulation (ESEEM) spectroscopic properties of ethanolamine ammonia-lyase (EAL) from Salmonella typhimurium with site-directed mutations in a conserved arginine residue (R160) of the active site containing EutB protein subunit have been characterized. R160 was predicted by a comparative model of EutB to play a critical role in protein structure and catalysis [Sun, L., and Warncke, K. (2006) Proteins: Struct., Funct., Bioinf. 64, 308-319]. R160I and R160E mutants fail to assemble into an EAL oligomer that can be isolated by the standard enzyme purification procedure. The R160K and R160A mutants assemble, but R160A EAL is catalytically inactive and reacts with substrates to form magnetically isolated Co(II) and unidentified radical species. R160A EAL activity is resurrected by externally added guanidinium to 2.3% of wild-type EAL. R160K EAL displays catalytic turnover of aminoethanol, with a 180-fold lower value of k(cat)/ K(M) relative to wild-type enzyme. R160K EAL also forms Co(II)-substrate radical pair intermediate states during turnover on aminoethanol and (S)-2-aminopropanol substrates. Simulations of the X-band EPR spectra show that the Co(II)-substrate radical pair separation distances are increased by 2.1 +/- 1.0 A in R160K EAL relative to wild-type EAL, which corresponds to the predicted 1.6 A change in arginine versus lysine side chain length. 14N ESEEM from a hyperfine-coupled protein nitrogen in wild type is absent in R160K EAL, which indicates that a guanidinium 14N of R160 interacts directly with the substrate radical through a hydrogen bond. ESEEM of the 2H-labeled substrate radical states in wild-type and R160K EAL shows that the native separation distances among the substrate C1 and C2, and coenzyme C5' reactant centers, are conserved in the mutant protein. The EPR and ESEEM measurements evince a protein-mediated force on the C5'-methyl center that is directed toward the reacting substrate species during the hydrogen atom transfer and radical rearrangement reactions. The results indicate that the positive charge at the residue 160 side chain terminus is required for proper folding of EutB, assembly of a stable EAL oligomer, and catalysis in the assembled oligomer.

EPR Evidence for Premonitory Charge-Ordering Fluctuations in Nanomanganite Pr0.57Ca0.41Ba0.02MnO3

Electron paramagnetic resonance (EPR) and magnetic properties of nanocrystalline $Pr_{0.57}Ca_{0.41}Ba_{0.02}MnO_3$ are studied and compared with those of the bulk material. The nanoscale samples prepared by the sol–gel method were annealed at different temperatures to obtain particles of different sizes. The crystallite and particle sizes were estimated by X-ray diffraction and transmission electron microscopy and found to be approximately equal to 30, 60 and 100 nm. Magnetization studies on the 60 and 100 nm particles show charge-ordering at 230 K similar to the bulk sample prepared by solid-state reaction, while in the 30 nm particles the charge-ordering signature is absent. At low temperatures all the three nanosamples show ferromagnetic transitions. This result is confirmed by the EPR intensity behavior as well. However, the EPR line width, which is reflective of the spin dynamics, shows a shallow minimum for the 30 nm particles at the temperature of 230 K, corresponding to the charge-ordering transition. We interpret this result as an indication of the presence of charge-ordering fluctuations in the nanoparticles of 30 nm size even though the static charge order is absent, thus heralding the occurrence of charge order in the larger particles.

Electron paramagnetic resonance investigation of metalloendofullerene derived carbon nanotube peapods

Single Wall Carbon Nanotubes (SWCNT) prepared by the "super growth" method and arc-grown material were used as templates for peapod preparation with La@C(82). A qualitative change of the electron paramagnetic resonance (EPR) properties of La@C(82) is observed after incorporation into SWNT. The loss of lanthanum hyperfine interaction in combination with the observed increase of EPR susceptibility by two orders of magnitude after peapod preparation when comparing with signals from "empty" tubes is indicative for the generation of itinerant spins by charge and spin transfer from La@C(82) to the tubes. This interpretation is supported by the observation of fast spin dephasing, detected with pulsed EPR techniques.

Thermoluminescence of natural and synthetic diopside

Diopside, a natural silicate mineral of formula CaMgSi 2O 6, has been investigated concerning its thermoluminescence (TL) and electron paramagnetic resonance (EPR) properties. Glow curves and TL vs. γ-dose were obtained irradiating natural samples to additional dose varying from 50 to 10,000 Gy. Except for a 410 °C peak found in the Al-doped artificial diopside, all the other peaks grow linearly with radiation dose, but saturate beyond ∼1 kGy. To investigate high-temperature effect before irradiation, measurements of TL intensity in samples annealed at 500–900 °C and then irradiated to 1 kGy γ-dose were carried out. Also the TL emission spectrum has been obtained. To compare with natural diopside, a synthetic pure polycrystal was produced and further those doped with iron, aluminum and manganese were also produced.