Electron Paramagnetic Resonance Absorption Spectra Research Articles

EPR Spectrum Analysis of DPPH and MnCl2

This paper presents the findings of electron paramagnetic resonance (EPR) spectroscopy experiments conducted on two different samples: 2,2-diphenyl-1-picrylhydrazyl (DPPH) and Manganese Chloride (MnCl2) dissolved in H2O. The experiments were conducted at Dr. D. Petasis EPR lab located in Allegheny College, Meadville, PA, USA. A Varian X-Band spectrometer was utilized for the analysis of the EPR absorption spectra. The primary objective was to determine the g-values and hyperfine structure of the two samples, which can be crucial in characterizing their electronic properties. The g-values obtained for both DPPH and MnCl2 were found to be remarkably close to the theoretically calculated values, suggesting the high accuracy and success of the experiments. This agreement validates the reliability of the EPR measurements and demonstrates the precision of the instruments and experimental procedures employed. For the MnCl2 sample, a hyperfine structure with 6 visible EPR lines was observed. By analyzing the number of EPR lines, the nuclear spin of the Mn(II) ion was determined to be I = 5/2, consistent with the expected value for Mn(II). The hyperfine constant (A) was also calculated from the field position differences between adjacent lines, providing further insights into the sample's magnetic properties. Comparisons were made between the obtained g-values and hyperfine constant with the expected values for Mn(II) in the literature, confirming the accuracy of the measurements and reinforcing the credibility of the experimental data. Overall, the EPR spectroscopy experiments conducted on DPPH and MnCl2 in H2O have provided valuable information about the electronic and magnetic properties of these paramagnetic materials. The results demonstrate the effectiveness of EPR spectroscopy as a powerful experimental technique for investigating various bio-chemical samples and understanding the effects of solvents on the EPR spectra of compounds. The close agreement between the experimental and theoretical values for the g-factors and hyperfine constants reinforces the reliability of the data obtained from the EPR experiments.

Temperature dependences of paramagnetic response in oxidized spark plasma sintered carbon nanotubes: The way of understanding the local electronic structure

Electron paramagnetic resonance (EPR) spectroscopy was used to study an electronic structure of carbon nanotubes (CNTs) consolidated by spark plasma sintering and additionally oxidized in vapor of boiling nitric acid. Two components were distinguished in the EPR absorption spectra: narrow and broad lines with different g-factors. This testified to the different surrounding of paramagnetic centers associated with the lines. Temperature dependence of spin numbers, associated with the narrow line, transfers from Curie-Weiss law to Curie law with increasing temperature. EPR susceptibility, associated with the broad line, can be attributed to a paramagnetic matrix with superparamagnetic inclusions.

S-band electron spin resonance spectroscopy using a short-circuited coplanar waveguide resonator

In this work, we study the development of a coplanar waveguide (CPW) resonator and its use in an electron spin resonance (ESR) spectrometer. The CPW resonator is designed to operate in S-band (2-4 GHz), with a short circuit configuration leading to miniaturization. It is so constructed such that it has a characteristic impedance of 50 ohms. The resonator supports quasi-TEM mode of propagation owing to its uniplanar nature, demanding detailed electromagnetic simulation. The design parameters and the electromagnetic field distribution are obtained from the simulation. The resonator is fabricated using optical lithography with a rapid prototyping technique. The characteristic response of the resonator is measured by coupling it to a Vector Network Analyzer (VNA). The ESR absorption spectrum of free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) is captured by using this resonator in reflection geometry. The microwave magnetic field distribution at the sample position is investigated. The ascertained value of Lande g-factor is consistent with that reported in the literature. Spin sensitivity of the order of 1015 spins/gauss is achieved upon using this resonator at room temperature. The quality factor of this resonator is found to be low and this makes it suitable for use in a Pulsed ESR spectrometer.

Effect of phonons on the electron spin resonance absorption spectrum

The unavoidable presence of vibrations in solid-state devices can drastically modify the expected electron spin resonance (ESR) absorption spectrum in magnetically active systems. In this work, we model the effect of phonons and temperature on the ESR signal in molecular systems with strong E ⊗ e Jahn–Teller (JT) effect and an electronic spin-1/2. Our microscopic model considers the linear JT interaction with a continuum of phonon modes, the spin–orbit coupling, the Zeeman effect, and the response of the system under a weak oscillating magnetic field. We derive a Lindblad master equation for the orbital and spin degrees of freedom, where one- and two-phonon processes are considered for the phonon-induced relaxation, and the thermal dependence of Ham reduction factors is calculated. We find that the suppression of ESR signals is due to phonon broadening but not based on the common assumption of orbital quenching. Our results can be applied to explain the experimentally observed absence of the ESR signal in color centers in diamond, such as the neutral nitrogen-vacancy and negatively charged silicon-vacancy color centers in diamond.

Magnetic Resonance in Collective Paramagnets with Gapped Excitation Spectrum

Some magnets due to particular geometry of the exchange bonds do not undergo transition to the conventional magnetically ordered state despite of the presence of significant exchange couplings. Instead, a collective paramagnetic state is formed. The later state can remain stable down to $T=0$ if the ground state of this magnet turns out to be nonmagnetic singlet separated from the excited triplet states by an energy gap. Low-temperature spin dynamics of the collective paramagnets with gapped excitations spectrum (or spin-gap magnets) can be described in terms of a dilute gas of the triplet excitations. Applied magnetic field can suppress the energy gap, resulting in the formation of the gapless spin-liquid state or even leading to the unusual phenomenon of field-induced antiferromagnetic order. Introduction of defects in the crystallographic structure of the spin-gap magnet can result either in the formation of multi-spin paramagnetic center or in the formation of randomly distributed modified exchange bonds in the crystal. This review includes results of electron spin resonance (ESR) spectroscopy study of several representative quantum paramagnets with gapped excitations spectrum: quasy-two-dimensional magnet \phcc{}, quasy-one-dimensional magnets of "spin-tube" type \sul{} and "spin-ladder" type \dimpy{}. We will demonstrate that ESR absorption spectra reveal common features of these systems: ESR spectroscopy allows to observe and characterize fine structure if the triplet energy levels, to identify many-particles relaxation processes in the gas of triplet excitations and to observe collective spin-wave oscillations in the field induced antiferromagnetically ordered state, as well as to observe some individual features of the studied systems.

Electron spin resonance study of spin relaxation in the strong-leg spin ladder with nonmagnetic dilution

We have studied electron spin resonance (ESR) absorption spectra for the nonmagnetically diluted strong-leg spin ladder magnet ({C}$_{7}$H$_{10}$N)$_{2}$Cu$_{(1-x)}$Zn$_{x}$Br$_{4}$ (abbreviated as DIMPY) down to 450 mK. Formation of the clusters with non-zero net magnetization is confirmed; the cluster-cluster interaction is evidenced by the concentration dependence of ESR absorption. High-temperature spin-relaxation time was found to increase with non-magnetic dilution. The ESR linewidth analysis proves that the Dzyaloshinskii-Moriya (DM) interaction remains the dominant spin-relaxation channel in diluted DIMPY. Experimental data indicate that the dilution results in the weakening of the effective DM interaction, which can be interpreted as total suppression of DM interaction in the close vicinity of impurity atom.

Free Radical Characteristics and Classification of Coals and Rocks Using Electron Spin Resonance Spectroscopy

Coal–rock interface recognition is one of the key unaddressed problems in unmanned mining, so a novel method for it is proposed. Firstly, electron spin resonance (ESR) is used to directly measure 10 kinds of coals/rocks common in China. Secondly, the free radical characteristics of different particle coals/rocks such as the Lande factor g, line width ΔH, and the concentration of the free radical Ng in the X-band ESR are studied. Lastly, the statistical classifier method of support-vector machine is employed to build a classification model with the input of the parameters of the ESR absorption spectra. Based on the ESR-SVM model, the recognition rate of coals/rocks reaches 100%, the recognition rate of different coals reaches 100%, and the recognition rate of different bituminous coals reaches 88.3%. The experimental results demonstrate that the proposed method is fast, stable, and accurate for the detection of the coal–rock interface and can be a promising tool for the classification of different coals.

Defects induced in cerium dioxide single crystals by electron irradiation

Micro-Raman spectroscopy, X-band electron paramagnetic resonance (EPR) spectroscopy, and UV-visible optical absorption spectroscopy were used to study the damage production in cerium dioxide (CeO2) single crystals by electron irradiation for three energies (1.0, 1.4, and 2.5 MeV). The Raman-active T2g peak was left unchanged after 2.5-MeV electron irradiation at a high fluence. This shows that no structural modifications occurred for the cubic fluorite structure. UV-visible optical absorption spectra exhibited a characteristic sub band-gap tail for 1.4-MeV and 2.5-MeV energies, but not for 1.0 MeV. Narrow EPR lines were recorded near liquid-helium temperature after 2.5-MeV electron irradiation; whereas no such signal was found for the virgin un-irradiated crystal or after 1.0-MeV irradiation for the same fluence. The angular variation of these lines in the {111} plane revealed a weak g-factor anisotropy assigned to Ce3+ ions (with the 4f1 configuration) in a high-symmetry local environment. It is concluded that Ce3+ ions may be produced by a reduction resulting from the displacement damage process. However, no evidence of F+ or F0 center or hole center formation due to irradiation was found from the present EPR and optical absorption spectra.

Magnetic behavior of Ni substituted LiCoO2 – Magnetization and electron paramagnetic resonance studies

Single phase Ni substituted LiCo1-xNixO2 solid solutions with x ≤ 0.15 have been synthesized to study the effect of substitution on the magnetic behavior. Two different techniques, magnetic susceptibility and electron paramagnetic resonance (EPR) which provide information in two different time windows have been used. The solid solutions have been found to be single phase with large grains conforming to R3¯ m rhombohedral structure. The lattice parameters ‘c’ and ‘a’ increase with increasing Ni substitution but with a nearly constant c/a ratio of ≈4.99 in all the cases indicating that the CoO6 and LiO6 octahedra do not undergo any Jahn-Teller distortions. The room temperature EPR absorption spectra clearly shows a peak in all the compounds at a field of 314 mT corresponding to a g-factor of 2.14. The peak width however is found to be a strong function of Ni substitution; increasing with increasing Ni from 3.6 mT to 6.5 mT for x = 0.08. The magnetization increases with decreasing temperature in all the compounds, a paramagnetic behavior, unlike Li-deficient compounds which show a Pauli paramagnetic susceptibility. Also, the magnetization exhibits thermal irreversibility which vanishes at large magnetic fields in all the compounds. The unsubstituted compound has discontinuities at 200 K and 50 K corresponding to magnetic transitions which disappear on substitution with 2% Ni for Co. The effective magnetic moment μeff is found to increase from 0.32 μB to 0.66 μB on increasing the substitution to 0.15. The unique feature however is that all the compounds exhibit a clear magnetic hysteresis at room temperature with a finite coercivity. The coercivity increases from 31 Oe to 168 Oe for x = 0.04 and then decreases on further increasing of x. A deconvolution of the hysteresis loops clearly shows an increasing paramagnetic component with substitution.

Angular dependence of electron spin resonance for detecting the quadrupolar liquid state of frustrated spin chains

Spin nematic phase is a phase of frustrated quantum magnets with a quadrupolar order of electron spins. Since the spin nematic order is usually masked in experimentally accessible quantities, it is important to develop a methodology for detecting the spin nematic order experimentally. In this paper we propose a convenient method for detecting quasi-long-range spin nematic correlations of a quadrupolar Tomonaga-Luttinger liquid state of $S=1/2$ frustrated ferromagnetic spin chain compounds, using electron spin resonance (ESR). We focus on linewidth of a so-called paramagnetic resonance peak in ESR absorption spectrum. We show that a characteristic angular dependence of the linewidth on the direction of magnetic field arises in the spin nematic phase. Measurments of the angular dependence give a signature of the quadrupolar Tomonaga-Luttinger liquid state. In our method we change only the direction of the magnetic field, keeping the magnitude of the magnetic field and the temperature. Therefore, our method is advantageous for investigating the one-dimensional quadrupolar liquid phase that usually occupies only a narrow region of the phase diagram.

Dynamic nuclear polarization-magnetic resonance imaging at low ESR irradiation frequency for ascorbyl free radicals.

Highly water-soluble ubiquinone-0 (CoQ0) reacts with ascorbate monoanion (Asc) to mediate the production of ascorbyl free radicals (AFR). Using aqueous reaction mixture of CoQ0 and Asc, we obtained positively enhanced dynamic nuclear polarization (DNP)-magnetic resonance (MR) images of the AFR at low frequency (ranging from 515 to 530 MHz) of electron spin resonance (ESR) irradiation. The shape of the determined DNP spectrum was similar to ESR absorption spectra with doublet spectral peaks. The relative locational relationship of spectral peaks in the DNP spectra between the AFR (520 and 525 MHz), 14N-labeled carbamoyl-PROXYL (14N-CmP) (526.5 MHz), and Oxo63 (522 MHz) was different from that in the X-band ESR spectra, but were similar to that in the 300-MHz ESR spectra. The ratio of DNP enhancement to radical concentration for the AFR was higher than those for 14N-CmP, Oxo63, and flavin semiquinone radicals. The spectroscopic DNP properties observed for the AFR were essentially the same as those for AFR mediated by pyrroloquinoline quinone. Moreover, we made a success of in vivo DNP-MR imaging of the CoQ0-mediated AFR which was administered by the subcutaneous and oral injections as an imaging probe.

Formation of the Long-Lived Charge-Separated State of the 9-Mesityl-10-methylacridinium Cation Incorporated into Mesoporous Aluminosilicate at High Temperatures

The 9-mesityl-10-methylacidinium cation (Acr+-Mes) was incorporated into Na+-exchanged mesoporous aluminosilicate (AlMCM-41) by cation exchange. Incorporation of Acr+-Mes into AlMCM-41 has made the lifetime of the electron-transfer (ET) state (Acr•-Mes•+) produced upon photoexcitation of Acr+-Mes extremely long (e.g., 10 s) even at high temperatures (e.g., 373 K). The formation and decay of the ET state can be repeated by on–off photoirradiation cycles. Both intra- and intermolecular back electron-transfer processes of the ET state of Acr+-Mes were observed by electron spin resonance (ESR) and time-resolved nanosecond diffuse reflectance absorption spectra, respectively. The ET state has both strong oxidizing and reducing abilities to be able to oxidize neighboring 1-pyrenemethylammonium and reduce the hexyl viologen dication incorporated into AlMCM-41 by cation exchange, respectively.

Artifact suppression in electron paramagnetic resonance imaging of 14N- and 15N-labeled nitroxyl radicals with asymmetric absorption spectra

This article describes an improved method for suppressing image artifacts in the visualization of 14N- and 15N-labeled nitroxyl radicals in a single image scan using electron paramagnetic resonance (EPR). The purpose of this work was to solve the problem of asymmetric EPR absorption spectra in spectral processing. A hybrid function of Gaussian and Lorentzian lineshapes was used to perform spectral line-fitting to successfully separate the two kinds of nitroxyl radicals. This approach can process the asymmetric EPR absorption spectra of the nitroxyl radicals being measured, and can suppress image artifacts due to spectral asymmetry. With this improved visualization method and a 750-MHz continuous-wave EPR imager, a temporal change in the distributions of a two-phase paraffin oil and water/glycerin solution system was visualized using lipophilic and hydrophilic nitroxyl radicals, i.e., 2-(14-carboxytetradecyl)-2-ethyl-4,4-dimethyl-3-oxazolidinyloxy (16-DOXYL stearic acid) and 4-hydroxyl-2,2,6,6-tetramethylpiperidine-d17-1-15N-1-oxyl (TEMPOL-d17-15N). The results of the two-phase separation experiment verified that reasonable artifact suppression could be achieved by the present method that deals with asymmetric absorption spectra in the EPR imaging of 14N- and 15N-labeled nitroxyl radicals.

Assembly and Function of the tRNA-Modifying GTPase MnmE Adsorbed to Surface Functionalized Bioactive Glass

Protein adsorption onto solid surfaces is a common phenomenon in tissue engineering related applications, and considerable progress was achieved in this field. However, there are still unanswered questions or contradictory opinions concerning details of the protein's structure, conformational changes, or aggregation once adsorbed onto solid surfaces. Electron paramagnetic resonance (EPR) spectroscopy and site-directed spin labeling (SDSL) were employed in this work to investigate the conformational changes and dynamics of the tRNA-modifying dimeric protein MnmE from E. coli, an ortholog of the human GTPBP3, upon adsorption on bioactive glass mimicking the composition of the classical 45S5 Bioglass. In addition, prior to protein attachment, the bioactive glass surface was modified with the protein coupling agent glutaraldehyde. Continuous wave EPR spectra of different spin labeled MnmE mutants were recorded to assess the dynamics of the attached spin labels before and after protein adsorption. The area of the continuous wave (cw)-EPR absorption spectrum was further used to determine the amount of the attached protein. Double electron-electron resonance (DEER) experiments were conducted to measure distances between the spin labels before and after adsorption. The results revealed that the contact regions between MnmE and the bioactive glass surface are located at the G domains and at the N-terminal domains. The low modulation depths of all DEER time traces recorded for the adsorbed single MnmE mutants, corroborated with the DEER measurements performed on MnmE double mutants, show that the adsorption process leads to dissociation of the dimer and alters the tertiary structure of MnmE, thereby abolishing its functionality. However, glutaraldehyde reduces the aggressiveness of the adsorption process and improves the stability of the protein attachment.

The Attachment Affinity of Hemoglobin toward Silver-Containing Bioactive Glass Functionalized with Glutaraldehyde

Bioactive glasses belonging to the 56SiO2·(40 - x)CaO·4P2O5·xAg2O system, with x = 0, 2, and 8 mol %, were surface functionalized with the protein coupling agent glutaraldehyde (GA) and further evaluated in terms of hemoglobin affinity. The bare and GA-functionalized samples were investigated before and after protein attachment, by electron paramagnetic resonance (EPR) spectroscopy combined with spin-labeling procedure. Methanethiosulfonate spin label was used to explore the local environment of β-93 cysteine in horse hemoglobin, in terms of spin label side chain mobility. The EPR simulation methods were employed to quantify the rotational correlational times and fraction of the immobilized spin labels. The EPR absorption spectrum was further exploited to estimate the amount of hemoglobin loaded on the substrates. The surface elemental composition obtained by X-ray photoelectron spectroscopy revealed similar tendency in terms of surface coverage. Changes in surface architecture, that is, changes in surface morphology after protein coverage, were observed by scanning electron microscopy. It was concluded that GA improves the stability of protein attachment and induces polymerization of hemoglobin molecules.

Quantum spin liquids in the absence of spin-rotation symmetry: Application to herbertsmithite

It has been suggested that the nearest-neighbour (NN) antiferromagnetic Heisenberg (HAF) model on the Kagome lattice may be a good starting point to understand the quantum spin-liquid (QSL) behaviour discovered in Herbertsmithite. We investigate possible QSL phases in the presence of experimentally relevant spin-rotation symmetry-breaking perturbations such as Dzyaloshinskii-Moriya and Ising interactions, as well as second-neighbour antiferromagnetic Heisenberg interactions. We use the projective symmetry group analysis within the slave-fermion framework of QSL phases and systematically classify possible QSLs in the presence of these perturbations. The dynamical spin-structure factor for relevant QSLs is computed and their effects are studied. Our calculations reveal dispersive features in the spin structure factor embedded in a generally diffuse background due to the existence of fractionalized S=1/2 excitations called spinons. For two of the previously proposed $Z_2$ states, the dispersive features are almost absent, and diffuse scattering dominates over a large energy window throughout the Brillouin zone. This resembles the structure factor observed in recent inelastic neutron scattering experiments on singlet crystals of Herbertsmithite. Furthermore, one of the $Z_2$ states with the spin structure factor with mostly diffuse scattering is gapped, and it may be adiabatically connected to the gapped QSL state observed in recent DMRG calculations for the NN HAF. The above perturbations are found to enhance the diffuse nature of the spin structure factor and reduce the momentum dependencies of the spin gap. We also calculate the electron spin resonance absorption spectra that further characterize the role of spin-rotation symmetry breaking perturbations, and can shed more light into the nature of the ground state in Herbertsmithite.

Electron spin resonance and exchange paths in the orthorhombic dimer system Sr2VO4

We report on magnetization and electron spin resonance (ESR) measurements of Sr$_{2}$VO$_4$ with orthorhombic symmetry. In this dimer system the $V^{4+}$ ions are in tetrahedral environment and are coupled by an antiferromagnetic intra-dimer exchange constant $J/k_B \approx$ 100 K to form a singlet ground state without any phase transitions between room temperature and 2 K. Based on an extended-H\"{u}ckel-Tight-Binding analysis we identify the strongest exchange interaction to occur between two inequivalent vanadium sites via two intermediate oxygen ions. The ESR absorption spectra can be well described by a single Lorentzian line with an effective g-factor $g$ = 1.89. The temperature dependence of the ESR intensity is well described by a dimer model in agreement with the magnetization data. The temperature dependence of the ESR linewidth can be modeled by a superposition of a linear increase with temperature with a slope $\alpha$ = 1.35 Oe/K and a thermally activated behavior with an activation energy $\Delta/k_B$ = 1418 K, both of which point to spin-phonon coupling as the dominant relaxation mechanism in this compound.

EPR and optical studies of Cu 2+ doped ammonium dihydrogen phosphate single crystals

Electron paramagnetic resonance (EPR) spectra of Cu 2+ ion in ammonium dihydrogen phosphate are studied at liquid nitrogen temperature (77 K). Four magnetically inequivalent Cu 2+ sites in the lattice are identified. The angular variation spectra of the crystal in the three orthogonal planes indicate that the paramagnetic impurity, Cu 2+ enters the lattice substitutionally in place of NH 4 + ions. The spin Hamiltonian parameters are determined with the fitting of spectra to rhombic symmetry crystalline field. The ground state wave function of Cu 2+ ion is constructed and found to be predominantly | x 2– y 2〉. The cubic field parameter ( Dq) and tetragonal parameters ( Ds and Dt) are determined from optical spectra at room temperature. By correlating EPR and optical absorption spectra, the bonding coefficients are calculated and nature of bonding of metal ion with different ligands in the crystal is discussed.

Gamma rays induced defect centers in phosphate glass for radio-photoluminescence dosimeter

Abstract Gamma induced defect centers of undoped phosphate glasses and the influence of these centers on the sensitivity and build-up time of silver-doped phosphate glasses were investigated. According to the ratio of oxygen and phosphorus ions (O/P) in phosphate glass, the basic glasses were polyphosphate glass (O/P = 3.07), metaphosphate glass (O/P = 3) and ultraphosphate glass (O/P = 2.93). Concentration of Ag + was 2 mol%. Defect centers induced by gamma rays of three base glasses were detected by Electron paramagnetic resonance (EPR) and optical absorption spectra. Three kinds of defect centers were observed and the characteristics of the hyperfine splitting doublets were quite similar in these glass samples. All the defect centers are phosphorus and oxygen related radiation-induced hole trapped defect centers (POHC) and electron centers PO 3 2 − and PO 4 4− . Based on EPR and optical absorption spectra, polyphosphate glasses have the highest relative POHC concentration. The fluorescence spectra and EPR spectra of silver-doped glasses were measured. The results indicate that polyphosphate glasses have the highest sensitivity and shortest build-up time of three silver-doped phosphate glasses which are related to the concentration of POHC.

Magnetic fluctuations and superconductivity in iron pnictides as probed by electron spin resonance

The electron spin resonance absorption spectrum of Eu^{2+} ions serves as a probe of the normal and superconducting state in Eu_{0.5}K_{0.5}Fe_2As_2. The spin-lattice relaxation rate 1/T_1^{\rm ESR} obtained from the ESR linewidth exhibits a Korringa-like linear increase with temperature above T_C evidencing a normal Fermi-liquid behavior. Below 45 K deviations from the Korringa-law occur which are ascribed to enhanced magnetic fluctuations within the FeAs layers upon approaching the superconducting transition. Below T_C the spin-lattice relaxation rate 1/T_1^{\rm ESR} follows a T^{1.5}-behavior without the appearance of a coherence peak.