Anisotropic Electron Spin Resonance Spectra Research Articles

Study of the effect of ethanol on the properties of poloxamer 338 solutions by rotational viscometry and spin probe method

The aim. Study the properties of 20 % solutions of poloxamer 338 (P338) in water and mixed solvents water-ethanol using rotational viscometry and the spin probe method at various temperatures. Materials and methods. 20 % m/m solutions of P338 in water and water – ethanol mixtures were the objects of research. The solutions were studied by rotational viscometry at various temperatures; the flow behaviour, lower yield stress (t0) and dynamic or apparent viscosity (η) were determined. Spin probes based on fatty acids, which differ in molecular structure, solubility, and radical localization, were added to the solutions. Electron paramagnetic resonance (EPR) spectra were obtained to determine their type and parameters. Results. Depending on the content, ethanol affects the rheological properties of 20 % solution of P338. The solution was demonstrated to be able to thermally induce sol → gel transition at 32 °C when ethanol content is 5 % m/m. The rheological parameters of the gel at 32 °C and 37 °C exhibit an increase (in comparison to the gel without ethanol), accompanied by a reduction in the packing density of polypropylene oxide (PPO) chains within the cores of P338 micelles. At an ethanol content of 10 % m/m, the gel formation temperature rises to 40 °C. At ethanol content of 15 % m/m and above, 20 % P338 solutions do not form gels at temperatures between 25 °C and 40 °C. The values of rotational correlation times (τ) and the order parameter (S) of fatty acid-based spin probes were observed to decrease with increasing ethanol content up to 30 % m/m; in the case of the ammonium salt of 5‑doxylstearic acid (5-DSA NH4 salt), the anisotropic EPR spectra transform, becoming a superposition of two triplets and subsequently a triplet. P338 solutions retain their ability to undergo thermally induced sol ↔ gel transitions as long as the EPR spectra of this probe exhibit anisotropy at temperatures ranging from 25 °C to 37 °C. As the concentration of ethanol in the solution increases, the solvation of the cores of P338 micelles by the dispersion medium of the solution also increases. Conclusions. It was demonstrated that ethanol, when added to the 20 % P338 solution, results in changes to the rheological properties of this solution. However, at the ethanol content of 5-10 % m/m, the ability of P338 to thermally induce sol → gel transition remains unaltered. The rheological properties of the 20 % P338 solution exhibit a correlation with the observed change in EPR spectra types for the 5-DSA NH4 salt. As the ethanol content in the solution increases, the solvation of P338 micelle cores by the dispersion medium increases, accompanied by decreased density and orderliness of the PPO chains packing in the micelle cores

Detection of Electron Paramagnetic Resonance Spectra of Optically Induced Carriers with the Properties of the Effective Mass in the WS2 Transition Metal Dichalcogenide

The spin properties of transition metal dichalcogenides are of interest for applications in spintronics. Anisotropic electron paramagnetic resonance spectra in a WS2 single crystal under optical excitation have been detected. These spectra assumingly belong to localized carriers near the valence band and reflect features of the 5d shell of the crystal. It has been shown that the g-factor for the magnetic field perpendicular to the c‑axis of the crystal (in-plane magnetic field) is larger than that for the magnetic field parallel to the c‑axis (perpendicular to the layer plane), which can provide information on the type of the 5d function. The discussed center is most likely described by the 5d({{z}^{2}} - {{r}^{2}}) wavefunction, which can be associated with the valence band of the crystal.

High-frequency EPR spectroscopy of paramagnetic manganese centers in GaAs : Mn crystals

High-frequency electron paramagnetic resonance (EPR) is used to study the unique properties of manganese centers in a GaAs : Mn crystal in strong magnetic fields at low temperatures. At frequencies of 94 and 130 GHz, EPR transitions were recorded in the MnGa2+-SH complex, which is a manganese ion with spin S=5/2, which replaces gallium (MnGa2+) and an ionized acceptor (A-) associated via an isotropic antiferromagnetic exchange interaction with a shallow hole (SH) with angular momentum J=3/2. A complex system of energy levels of this complex in a magnetic field and the possibility of accurately determining exchange interactions from EPR spectra are analyzed. Another complex was investigated, in which an ionized acceptor MnGa2+ interacts with a localized hole center in the form of a diamagnetic ion O2- replacing As. This complex, MnGa2+-OAs2-, is characterized by axial symmetry along the <111> axis of the cubic GaAs crystal and an anisotropic EPR spectrum. Due to the high Boltzmann factor, in our studies, the order of the fine structure spin levels of this complex was determined. The effect of the Boltzmann populations of the energy levels on the high-frequency EPR spectra was also demonstrated for the MnGa2+-SH complex. Keywords: high-frequency EPR, GaAs crystal, Mn acceptor, shallow hole, exchange interaction.

High-frequency EPR spectroscopy of paramagnetic manganese centers in GaAs : Mn crystals

High-frequency electron paramagnetic resonance (EPR) is used to study the unique properties of manganese centers in a GaAs : Mn crystal in strong magnetic fields at low temperatures. At frequencies of 94 and 130 GHz, EPR transitions were recorded in the MnGa2+-SH complex, which is a manganese ion with spin S=5/2, which replaces gallium (MnGa2+) and an ionized acceptor (A-) associated via an isotropic antiferromagnetic exchange interaction with a shallow hole (SH) with angular momentum J=3/2. A complex system of energy levels of this complex in a magnetic field and the possibility of accurately determining exchange interactions from EPR spectra are analyzed. Another complex was investigated, in which an ionized acceptor MnGa2+ interacts with a localized hole center in the form of a diamagnetic ion O2- replacing As. This complex, MnGa2+-OAs2-, is characterized by axial symmetry along the <111> axis of the cubic GaAs crystal and an anisotropic EPR spectrum. Due to the high Boltzmann factor, in our studies, the order of the fine structure spin levels of this complex was determined. The effect of the Boltzmann populations of the energy levels on the high-frequency EPR spectra was also demonstrated for the MnGa2+-SH complex. Keywords: high-frequency EPR, GaAs crystal, Mn acceptor, shallow hole, exchange interaction.

Высокочастотная ЭПР-спектроскопия парамагнитных центров марганца в кристаллах GaAs : Mn

High-frequency electron paramagnetic resonance (EPR) is used to study the unique properties of manganese centers in a GaAs:Mn crystal in strong magnetic fields at low temperatures. At frequencies of 94 and 130 GHz, EPR transitions were recorded in the MnGa2+ - SH complex, which is a manganese ion with spin S = 5/2, which replaces gallium (MnGa2+) and is an ionized acceptor (A–) associated via an isotropic antiferromagnetic exchange interaction with a shallow hole (SH) with angular momentum J = 3/2. A complex system of energy levels of this complex in a magnetic field and the possibility of accurately determining exchange interactions from EPR spectra are analyzed. Another complex was investigated, in which an ionized acceptor MnGa2+ interacts with a localized hole center in the form of a diamagnetic ion O2– replacing As. This complex, MnGa2+-OAs2-, is characterized by axial symmetry along the <111> axis of the cubic GaAs crystal and an anisotropic EPR spectrum. Due to the high Boltzmann factor, in our studies, the order of the fine structure spin levels of this complex was determined. The effect of the Boltzmann populations of the energy levels on the high-frequency EPR spectra was also demonstrated for the MnGa2+- SH complex.

Comparative study of structural, optical and magnetic properties of Er3+ doped yttrium gallium borates

The paper presents the results of a comprehensive study of yttrium gallium borate (YGa3(BO3)4) crystals low-doped (0.2 wt%) with erbium (Er3+) ions by in-situ X-ray diffraction (XRD), radioluminescence (RL), thermally stimulated luminescence (TSL) and electron paramagnetic resonance (EPR) techniques. Structural and elastic properties of YGa3(BO3)4: Er3+ crystals measured by the XRD method in the temperature range from 300 K to 1073 K have revealed the temperature variation of the volume thermal expansion coefficient. The analysis of RL spectra allowed us to assume the existence of the characteristic Er3+-related emission band located in the near-infrared region. The TSL spectra revealed strong peaks at low temperatures due to shallow traps and much less intense bands located above room temperature. Measurements of heat capacity in a wide temperature range allowed us to determine the Debye temperature θD ≈ 332 K. From the detailed analysis of the strongly anisotropic EPR spectra measured in YGa3(BO3)4:Er3+ crystals, the parameters of the spin Hamiltonian of Er3+ ions were determined: g⊥ = 9.34; g∥ = 1.269; A⊥ = 320.9 MHz; A∥ = 47.7 MHz. The temperature dependence of the spin-lattice relaxation rate of Er3+ ions was described well by the Orbach-Aminov process. It was proposed that Er3+ ions reside yttrium ions in YGa3(BO3)4 crystal lattice and are located in a slightly distorted prism made of six oxygen ions.

Structural and magnetic properties of YAl3(BO3)4 and EuAl3(BO3)4 single crystals doped with Co2+

Structural and magnetic properties of the cobalt-doped YAl3(BO3)4 and EuAl3(BO3)4 crystals were studied using electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), X-ray diffraction analysis, and specific heat methods.It was found that cobalt, when doped to the studied crystals, is in divalent state, substitutes the aluminum ions and forms a system of three magnetically equivalent magnetically nonequivalent centers whose magnetic axes are rotated at 120° relative to one another.We have determined parameters of the spin Hamiltonian of orthorhombic symmetry, describing the anisotropic angular dependence EPR spectra for both types of crystals. Strong broadening of EPR lines is caused by the spin-lattice relaxation.Specific heat of the EuAl3(BO3)4 and YAl3(BO3)4 crystals doped with 0.2% of Co2+ was measured over the temperature range from 3 to 395 K, in zero magnetic field. The Debye temperature was determined to be equal to 355К and 358 K for YAl3(BO3)4 and EuAl3(BO3)4, respectively. Crystallographic structure at ambient conditions as well as variations of unit-cell parameters and thermal expansion coefficients with temperature were studied in detail in the range 302 K < T < 1073 K. NMR study on 27Al and 11B nuclei at room temperature was conducted. It is shown that on changing the rare-earth ion from yttrium to europium, the contribution of hyperfine interactions to the resonance characteristics of the samples increases. It is also shown that there are two nonequivalent environments of boron atoms with an attitude close to 1.1 and 1.2 for the samples containing 0.1 and 0.2% of cobalt ions, respectively.

Structural Studies of Perfluoroaryldiselenadiazolyl Radicals: Insights into Dithiadiazolyl Chemistry.

The synthesis and structural characterization of a series of perfluoroaryldiselenadiazolyls [DSeDA; p-XC6F4CNSeSeN (X = F, Cl, Br, CF3, NO2, and CN for 2a-2f, respectively)] are described. Concentration-dependent solution UV/vis measurements on 2a follow the Beer-Lambert law and the transitions assigned through time-dependent density functional theory (TD-DFT) studies, indicating little propensity for dimerization in solution (10-3-10-4 M). Solution electron paramagnetic resonance (EPR) spectra reveal that these radicals exhibit a broad featureless singlet around g = 2.04 but form well-resolved anisotropic EPR spectra in frozen solution, from which spin densities were determined and found to reflect an increase in the spin density at the chalcogen in relation to the corresponding dithiadiazolyl (DTDA) radicals, p-XC6F4CNSSN. The solid-state structures of 2a and 2d-2f all adopt spin-paired cis-cofacial dimers in which the dimers are held together via multicenter π*-π* "pancake bonding" interactions. Conversely, 2b and 2c exhibit an orthogonal mode of association, which is unique to DSeDA chemistry but which also affords a singlet ground state evidenced by SQUID magnetometry. The more sterically demanding diselenadiazolyl radical 2f was also prepared and exhibits a trans-antarafacial dimerization mode. DFT studies [UPBE0-D3 ccPVTZ-PP(-F)++] on the model radical HCNSeSeN confirm that each dimer is a stable energy minimum on the potential energy surface, reproducing well the experimental geometric parameters with relative stability in the order cis-cofacial > orthogonal > trans-antarafacial. Computational studies reflect stronger dimerization for DSeDA radicals in relation to their sulfur analogues, consistent with the experimental observation: While 2a and 2d are isomorphous with their corresponding DTDA radicals, 2b, 2c, and 2e-2g are all dimeric, in contrast to their DTDA analogues, which are monomeric in the solid-state. A study on 2f reveals that significant geometric strain accumulates in order to support the propensity for both cis dimerization and intermolecular CN···Se interactions. Conversely, p-NCC6F4CNSSN likely forfeits dimerization in the analogous packing motif in order to release strain but retains the favorable intermolecular CN···S interactions.

Disorder and defect formation mechanisms in molecular-beam-epitaxy grown silicon epilayers

We investigate the role of disorder, stress and crystallite size in determining the density of defects in disordered and partially ordered silicon thin films deposited at low or moderate temperatures by molecular beam epitaxy. We find that the paramagnetic defect density measured by electron spin resonance (ESR) is strongly dependent on the growth temperature of the films, decreasing from ~2·1019cm−3 at 98°C to ~1·1018cm−3 at 572°C. The physical nature of the defects is strongly dependent on the range of order in the films: ESR spectra consistent with dangling bonds in an amorphous phase are observed at the lowest temperatures, while the ESR signal gradually becomes more anisotropic as medium-range order improves and the stress level (measured both by X-ray diffraction and Raman spectroscopy) is released in more crystalline films. Anisotropic ESR spectra consistent with paramagnetic defects embedded in an epitaxial phase are observed at the highest growth temperature (572°C).

Strong metal–metal coupling in mixed-valent intermediates [Cl(L)Ru(μ-tppz)Ru(L)Cl]+, L = β-diketonato ligands, tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine

Five diruthenium(II) complexes [Cl(L)Ru(μ-tppz)Ru(L)Cl] (1-5) containing differently substituted β-diketonato derivatives (1: L = 2,4-pentanedionato; 2: L = 3,5-heptanedionato; 3: L = 2,2,6,6-tetramethyl-3,5-heptanedionato; 4: L = 3-methyl-2,4-pentanedionato; 5: L = 3-ethyl-2,4-pentanedionato) as ancillary ligands (L) were synthesized and studied by spectroelectrochemistry (UV-Vis-NIR, electron paramagnetic resonance (EPR)). X-ray structural characterisation revealed anti (1, 2, 5) or syn (3) configuration as well as non-planarity of the bis-tridentate tppz bridge and strong dπ(Ru(II)) → π*(pyrazine, tppz) back-bonding. The widely separated one-electron oxidation steps, Ru(II)Ru(II)/Ru(II)Ru(III) and Ru(II)Ru(III)/Ru(III)Ru(III), result in large comproportionation constants (K(c)) of ≥10(10) for the mixed-valent intermediates. The syn-configurated (n) exhibits a particularly high K(c) of 10(12) for n = 1+, accompanied by density functional theory (DFT)-calculated minimum Ru-N bond lengths for this Ru(II)Ru(III) intermediate. The electrogenerated mixed-valent states 1(+)-5(+) exhibit anisotropic EPR spectra at 110 K with average values <g> of 2.304-2.234 and g anisotropies Δg = g(1)-g(3) of 0.82-0.99. Metal-to-metal charge transfer (MMCT) absorptions occur for 1(+)-5(+) in the NIR region at 1660 nm-1750 nm (ε ≈ 2700 dm(3) mol(-1) cm(-1), Δν(1/2) ≈ 1800 cm(-1)). DFT calculations of 1(+) and 3(+) yield comparable Mulliken spin densities of about 0.60 for the metal ions, corresponding to valence-delocalised situations (Ru(2.5))(2). Rather large spin densities of about -0.4 were calculated for the tppz bridges in 1(+) and 3(+). The calculated electronic interaction values (V(AB)) for 1(+)-5(+) are about 3000 cm(-1), comparable to that for the Creutz-Taube ion at 3185 cm(-1). The DFT calculations predict that the Ru(III)Ru(III) forms in 1(2+)-5(2+) prefer a triplet (S = 1) ground state with ΔE (S = 0 - S = 1) ∼5000 cm(-1). One-electron reduction takes place at the tppz bridge which results in species [Cl(L)Ru(II)(μ-tppz˙(-))Ru(II)(L)Cl](-) (1˙(-)-3˙(-), 5˙(-)) which exhibit free radical-type EPR signals and NIR transitions typical of the tppz radical anion. The system 4(n) is distinguished by lability of the Ru-Cl bonds.

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.

Electron spin resonance observation of field-induced charge carriers in ultrathin-film transistors of regioregular poly(3-hexylthiophene) with controlled in-plane chain orientation

Ultrathin-film polymeric transistors with controlled in-plane chain orientation have been fabricated based on Langmuir–Blodgett technique by cospreading liquid crystal molecule with regioregular poly(3-hexylthiophene). The mobilities parallel to the chain direction reached 0.001–0.01 cm2/V s for 1 to 5 monolayer thick transistors. Mobility ratio was about 2 for the parallel and perpendicular directions. Electron spin resonance (ESR) signals of the field-induced polarons exhibited clear in-plane anisotropies due to unpaired π-electrons. The anisotropic ESR spectra together with the optical dichroism determine the detailed molecular orientations in the channel of such ultrathin transistors.

Electron Spin Resonance of Charge Carriers in Organic Field-Effect Devices

We have developed a field-induced electron spin resonance (ESR) technique to detect charge carriers with spins that are generated at the interface of organic field-effect devices. ESR signals have been successfully observed in the devices of high-mobility organic materials, such as regioregular poly(3-alkylthiophene) (RR-P3AT), in the case of polymer, and pentacene, in the case of small organic molecules. ESR signals of these devices provide valuable information concerning the spin–charge relationship, the spatial extent of charge carriers and molecular orientations at the device interfaces. Instead of Al2O3 gate insulators initially used in devices for ESR, more conventional SiO2 gate insulators on the relatively low-doped Si substrates have been introduced in obtaining ESR signals without lowering the Q-value of the ESR cavity. We present gate-bias dependence of ESR signals of RR-P3AT devices with different gate insulators, where the spins tend to saturate above the charge concentration of about 0.2%, regardless of gate insulating materials. This fact indicates that the charge carriers of the polymers change from polarons with spins to spinless bipolarons or polaron pairs at higher doping levels. Anisotropic ESR spectra of unpaired π-electrons in the devices with SiO2 gate insulators have revealed preferential orientation of the molecular plane, consistent with the formation of self-organized lamellar structures of RR-P3AT as in the case of Al2O3 gate insulators. Motional narrowing of ESR spectra has been observed, depending on the alkyl-chain lengths of RR-P3AT, which could be related to the reported alkyl-chain dependence of carrier mobility.

Polaron Dynamics of Heavily Doped Regioregular and Regiorandom Poly(3-alkylthiophenes) Revealed by Electron Spin Resonance Spectroscopy

Electron spin resonance (ESR) features in heavily doped conjugated polymers are investigated through the comparison of temperature dependences of ESR spectra between head-to-tail coupled regioregular (RR) and regiorandom (RRa) poly(3-octylthiophenes) (P3OTs). RR-P3OT, used as a model of having crystalline grains in the solid film, is found to exhibit anisotropic ESR spectra, whereas RRa-P3OT gives almost isotropic ESR spectra similar to those of usual heavily doped conjugated polymers. This difference in the degree of spectral anisotropy primarily arises from a difference in their film morphology. Spectral simulations show the anisotropy observed in RR-P3OT to be caused by g-anisotropy. The presence of the g-anisotropy in RR-P3OT indicates that its polarons spend most of the time within a single crystalline grain that has some domains with a common direction of the g-tensor. The g-anisotropy turns out to decrease with increasing temperature. This result is explained by thermally activated hopping motions between crystalline grains. We emphasize that the decrease in the g-anisotropy with temperature should be associated with its activated type of temperature dependence of conductivity. In RRa-P3OT, its isotropic ESR spectra are suggested to be caused by the interchain motion as well as the intrachain one.

Electron paramagnetic resonance in transition metal-doped ZnO nanowires

The wide-band-gap zinc oxide-based diluted magnetic semiconductors currently attract considerable attention due to their possible use in spintronic devices. In this work, we studied ZnO nanowire samples synthesized on 10×10 mm2 a-plane sapphire substrates by high-pressure pulsed laser deposition. The samples were characterized by scanning electron microscopy (SEM) and electron paramagnetic resonance (EPR) in the X-band (≃9.3 GHz) from T=4 to 300 K. According to the SEM pictures, the nanowires exhibit a length of about 1 μm and are aligned perpendicular to the substrate surface. The structures have a hexagonal cross section and their diameter ranges from 60 nm up to 150 nm. For the lowest nominal concentrations of xMn=3 at. % and xCo=5 at. %, we detect the anisotropic EPR spectra of isolated Mn2+ (3d5, S6) and Co2+ (3d7, F4), respectively, on Zn sites. The detection of the well-resolved anisotropic spectra proves a coherent crystallographic orientation of the nanowires. The linewidth was larger than the best values reported in the literature. Nevertheless, it was possible to identify two different components, A and B, of the reported spectra. From the temperature dependence of the EPR intensity, we found that both components exhibit paramagnetic behavior and are present in a concentration ratio of NB∕NA=1.4. In the case of the Mn-doped ZnO wires, the linewidth increases with increasing Mn concentration due to the dipole-dipole interaction of the paramagnetic ions. At the highest used nominal concentration, xMn=10 at. %, an additional broad single line is observed.

Structures of the Hexafluorocyclopropane, Octafluorocyclobutane, and Decafluorocyclopentane Radical Anions Probed by Experimental and Computational Studies of Anisotropic Electron Spin Resonance (ESR) Spectra

Anisotropic electron spin resonance (ESR) spectra are reported for the radical anions of hexafluorocyclopropane (c-C(3)F(6)(-)), octafluorocyclobutane (c-C(4)F(8)(-)), and decafluorocyclopentane (c-C(5)F(10)(-)) generated via gamma-irradiation in plastically crystalline tetramethylsilane (TMS) and rigid 2-methyltetrahydrofuran (MTHF) matrices. By combining the analysis of these experimental ESR spectra involving anisotropic hyperfine (hf) couplings with a series of quantum chemical computations, the geometrical and electronic structure of these unusual perfluorocycloalkane radical anions have been characterized more fully than in previous studies that considered only the isotropic couplings. Unrestricted Hartree-Fock (UHF) computations with the 6-311+G(d,p) basis set predict planar ring structures for all three radical anions, the ground electronic states being (2)A(2)(") for c-C(3)F(6)(-) (D(3h) symmetry), (2)A(2u) for c-C(4)F(8)(-) (D(4h)), and (2)A(2)(") for c-C(5)F(10)(-) (D(5h)), in which the respective six, eight, and ten 19F-atoms are equivalent by symmetry. A successful test of the theoretical computation is indicated by the fact that the isotropic 19F hf couplings computed by the B3LYP method with the 6-311+G(2df,p) basis set for the optimized geometries are in almost perfect agreement with the experimental values: viz., 19.8 mT (exp) vs 19.78 mT (calc) for c-C(3)F(6)(-); 14.85 mT (exp) vs 14.84 mT (calc) for c-C(4)F(8)(-); 11.6 mT (exp) vs 11.65 mT (calc) for c-C(5)F(10)(-). Consequently, the same computation method has been applied to calculate the almost axially symmetric anisotropic 19F hf couplings for the magnetically equivalent 19F atoms: (-4.90 mT, -4.84 mT, 9.75 mT) for c-C(3)F(6), (-3.54 mT, -3.48 mT, 7.02 mT) for c-C(4)F(8)(-), and (-2.62 mT, -2.56 mT, 5.18 mT) for c-C(5)F(10)(-). ESR spectral simulations performed using the computed principal values of the hf couplings and the spatial orientations of the 19F nuclei as input parameters reveal an excellent fit to the experimental anisotropic ESR spectra of c-C(3)F(6)(-), c-C(4)F(8)(-), and c-C(5)F(10)(-), thereby providing a convincing proof of the highly symmetric D(nh) structures that are predicted for these negative ions. Furthermore, using the computed 19F principal values and their orientations, the effective 19F anisotropic hf couplings along the molecular symmetry axes were evaluated for c-C(3)F(6)(-) and c-C(4)F(8)(-) and successfully correlated with the positions of the characteristic outermost features in both the experimental and calculated anisotropic spectra. In addition, the electronic excitation energies and oscillator strengths for the c-C(3)F(6)(-) , c-C(4)F(8)(-), and c-C(5)F(10)(-) radical anions were computed for the first time using time-dependent density functional theory (TD-DFT) methods.

An electron spin resonance study of γ-ray irradiated ginseng

Using electron spin resonance (ESR) spectroscopy, we revealed the presence of four radical species in γ-ray irradiated ginseng ( Agaliaceae). Before irradiation, the representative ESR spectrum of ginseng is composed of a sextet centered at g = 2.0, a sharp singlet at the same g-value, and a singlet at about g = 4.0. The first one is attributable to a hyperfine (hf) signal of Mn 2+ ion (hf constant: 7.4 mT). The second one is due to an organic free radical. The third one is originated from Fe 3+. Upon γ-ray irradiation, a new ESR (the fourth) signal was detectable in the vicinity of g = 2.0 region. The progressive saturation behaviors of the ESR signals at various microwave power levels were indicative of different relaxation time for those radicals. The anisotropic ESR spectra were detected by the angular rotation of the sample tube. This is due to the existence of anisotropic microcrystalline in the ginseng powder sample.

ESR of dislocation defects in MgF 2 crystals—with more than 130 line peaks

In this work, more than 130 line peaks in electron spin resonance (ESR) spectra have been discovered of the laser material MgF2 crystal in room-temperature experiments. A sample is cut from the shoulder part of the MgF2 crystal and another is from the MgF2:Co crystal. The samples were not treated by any irradiation. The same anisotropic ESR spectra of the two samples indicate that the dopant Co2+ introduces defects which induce the same multinuclear free radicals as in dislocations in the sample of MgF2. These paramagnetic solid multinuclear free radicals show good stability and their ESR spectra are found to be anisotropic. ESR signals are derived from three different types of multinuclear free radicals from a tentative simulation analysis. When the direction of the applied magnetic field is along the [100] or [010] orientation of the crystal, the magnetic field at which the ESR signals are detected ranges from 0.2294T to 0.4654T and the width of this range is 0.2362T (corresponding to an energy band of 0.233eV); the most narrow peak in the ESR spectra has a width ΔH about 1.28×10−3T. This width ΔH, equivalent to the energy difference of two neighbouring levels, is very small, only 1.85×10−7eV (or 1.46×10−3cm−1). This fact indicates that the ground state is highly degenerate and splits into nearly quasi-continuous energy levels like an energy band in an applied magnetic field. It may be served as a new starting point of solid laser exciter frequency modulation.

Restricted orientational motion of nitroxides in molecular glasses: Direct estimation of the motional time scale basing on the comparative study of primary and stimulated electron spin echo decays

A comparative study of anisotropic relaxation in two-pulse primary and three-pulse stimulated electron spin echo decays provides a direct way to distinguish fast (correlation time tau(c)<10(-6) s) and slow (tau(c)>10(-6) s) motions. Anisotropic relaxation is detected as a difference of the decay rates for different resonance field positions in anisotropic electron paramagnetic resonance spectra. For fast motion anisotropic relaxation influences the primary echo decay and does not influence the stimulated echo decay. For slow motion it is seen in both two-pulse echo and three-pulse stimulated echo decays. For nitroxide spin probes dissolved in glassy glycerol only fast motion was found below 200 K. Increase of temperature above 200 K results in the appearance of slow motion. Its amplitude increases rapidly with temperature increase. While in glycerol glass slow motion appears above glass transition temperature T(g), in ethanol glass it is observable below T(g). The scenario of motional dynamics in glasses is proposed which involves the broadening of the correlation time distribution with increasing temperature.

Electron spin resonance of the Kondo ion in YbRh 2Si 2

By means of electron spin resonance (ESR) we investigated the dynamic magnetic properties of the heavy fermion metal YbRh 2Si 2. This is the first observation of Yb 3+-ESR in YbRh 2Si 2 being also the first ESR signal of a Kondo ion in a dense Kondo lattice system. Below T=25 K strongly anisotropic ESR spectra observed in high-quality single crystals demonstrate the existence of local Yb 3+ magnetic moments even below a characteristic spin fluctuation temperature derived from thermodynamic properties.