Electron Spin Resonance Peak Research Articles

ЭПР-исследование кинетики распада свободных радикалов облученной сахарозы

Electron paramagnetic resonance (EPR) spectrometry makes it possible to study the formation of free radicals during the transfer of energy to the object. The decay kinetics of free radicals is an integral part of the study since this phenomenon directly interferes with the goals of accurate identification of the irradiation fact and dosimetry using EPR spectrometry. In this work, we have studied the time dependence of the intensity and characteristics of the EPR signal of analytical sucrose irradiated with gamma rays at doses from 0.3 to 9 kGy. It has been shown that irradiated sucrose gives a stable EPR peak after 60 days. A change in the spectral characteristics of the signal was recorded in the first 23 hours after irradiation. The signal intensity tends to increase in the first 72 hours after irradiation for a sample with an irradiation dose of 300 Gy and 48 hours for 1000 Gy. The dependence of the signal intensity on the absorbed dose is linear in the range of studied doses. The g-factor value at the intersection point of the derivative contour with the zero line is 2.013. Based on the data obtained, it can be assumed that sucrose (sugar) is one of the best candidates among solid radiation-sensitive materials for identifying the fact of irradiation using EPR spectrometry

Focused-Ion-Beam Induced Paramagnetic Defects in FAMn:PbI3 Perovskite Films

FAMn:PbI3 perovskite films were synthesized and probed mainly through electron spin resonance (ESR) spectroscopy. FAMn:PbI3 with low (~1%) Mn concentration showed a hyperfine sextet line originated from Mn++ ions. FAMn:PbI3 with high (10%) Mn concentration showed broad resonance (~500 G peak-to-peak linewidth). However, after bombardment of FAMn:PbI3 with high Mn concentration by focused ion beams (FIB), a sharp ESR peak appeared. The peak-to-peak linewidth (ΔHpp) was ~8 G regardless of the temperature. The FIB-induced defect showed Curie behavior at low temperatures (5 K - 50 K), which indicates the presence of localized electrons at the defect sites at low temperatures. The g-value increased from g = 2.0002 to 2.0016 as the temperature increased from 5 K to 50 K. Together with the ongoing search for electron spin echo (ESE), this could potentially provide a platform for realizing magnetic bits, information storage, and increased manipulation speed.

EPR Investigation on Electron Transfer of 2D/3D g‐C3N4/ZnO S‐Scheme Heterojunction for Enhanced CO2 Photoreduction

AbstractPhotocatalytic conversion of CO2 into useful products is a promising technology from environmental and economic viewpoints. However, the efficiency of current photocatalytic materials is still unsatisfactory. In this work, a robust S‐scheme heterojunction composed of 3D ZnO hollow spheres wrapped by 2D g‐C3N4 layers is fabricated. The electrostatic attraction between both components not only facilitates the exfoliation of g‐C3N4 layers but also strengthens the photocatalyst framework. The prepared g‐C3N4/ZnO photocatalyst afforded an enhanced CH4 production rate, which is ≈40 and 7 times higher than those of pure ZnO and g‐C3N4, respectively. The improved activity is attributed to the extended light absorption and suppressed charge carrier recombination. Moreover, apart from traditional techniques, electron paramagnetic resonance (EPR) is used to probe charge movement in the fabricated S‐scheme heterojunction. An observable shift of the relevant EPR peak is noticed after the construction of g‐C3N4/ZnO heterostructure, indicating the electron transfer process. This study sheds light on S‐scheme heterojunctions as efficient candidates for CO2 photocatalytic conversion.

Colloidal processing of thulium-yttria microceramics

The development of new dosimetric materials is essential for the safe and effective use of nuclear technology. In the present study, an eco-friendly bio-prototyping approach was developed for preparing thulium-yttria microceramics with potential applications in radiation dosimetry. Micro-powder compacts were obtained by casting colloidal thulium-yttria suspensions prepared with 20 vol% particles in thin-walled tube templates. Samples were sintered at 1600 °C for 2 h under the environmental pressure and atmosphere to obtain thulium-yttria microceramics with dimensions of 3.33 ± 0.01 mm × 2.27 ± 0.01 mm (height × diameter), as well as a cubic C-type structure, pycnometric density of 4.79 g cm−3 (95.61% theoretical density), and surface microstructure comprising hexagon-like grains bonded at the boundaries. The use of thulium as an activator of yttria greatly improved the electron paramagnetic resonance (EPR) response of the microceramics, where the main EPR peak (p1) was recorded at 351.24 mT and the g factor was 2.0046. The innovative findings obtained in this study may facilitate the production of new solid state dosimeters.

Reactive sputtered ZnO thin films: Influence of the O2/Ar flow ratio on the oxygen vacancies and paramagnetic active sites

Zinc oxide thin films were prepared under oxygen-deficient and oxygen-rich conditions by changing the oxygen to argon ratio (O2/Ar) during the reactive sputtering deposition at room temperature. The effects of increasing the partial pressure of oxygen in the sputtering gas from 20 to 70% O2/Ar on the thin film composition, crystallinity and defects that can act as active sites for gas reactions were studied using X-rays diffraction, X-rays photoelectron spectroscopy (XPS), Raman scattering and electron paramagnetic resonance (EPR). All the films exhibited a textured (0002) wurtzite phase and a crystallite size that increases as the partial pressure of oxygen in the sputtering gas increases. The XPS analysis showed that the number of oxygen vacancies decreases as the oxygen percent in the sputtering gas increases. The Raman spectra of the samples contained a band related to a hydroxide, OH, bond in addition to the vibrational modes associated with the wurtzite structure. A strong EPR signal, consistent with the OH acting as a paramagnetic center, was detected in all the films. An additional, but very weak EPR peak, was observed in the film grown at 20% O2/Ar, which was assigned to singly ionized oxygen vacancies located in the crystallite lattice. These paramagnetic centers are highly reactive because of their unpaired electrons and their formation will have important effects on the physical and chemical properties of the thin films.

Spectroscopic study for a chromium-adsorbed montmorillonite

Samples of purified montmorillonite with trace amounts of quartz were subjected to different concentrations of chromium sulphate solutions for one week to allow cation exchange. The chromium-bearing montmorillonites were verified and tested using powder X-ray diffractometry (XRD), X-ray fluorescence spectrometry, electron spin resonance (ESR) spectrometry and Fourier transformation infrared (FTIR) spectroscopy to explore the occupation sites of the chromium. The ESR spectra recorded before and after the chromium exchange show clear differences: a strong and broad resonance with two shoulders at the lower magnetic field side was present to start, and its intensity as well as that of the ferric iron resonance, increased with the concentration of added chromium. The signals introduced by the chromium, for example at g=1.975 and 2.510 etc., suggested that the chromium had several occupational sites. The ESR peak with g=2.510 in the second derivative spectrum suggested that Cr3+ was weakly bounded to TOT with the form of [Cr(H2O)3]3+ in hexagonal cavities. This was verified by comparing the FTIR spectra of the pure and modified montmorillonite. The main resonance centred at g=1.975 indicated that the majority of Cr3+ occupied the interlayer region as [Cr(H2O)6]3+. The substitution of Ca2+ by Cr3+ also greatly affected the vibration of the hydrogens associate to water, ranged from 3500 to 2600cm-1 in FTIR. Furthermore, the presence of two diffraction lines in the XRD results (specifically those with d-values of 1.5171 and 1.2673nm) and the calculations of the size of the interlayer space suggested the presence of two types of montmorillonite with different hydration cations in the sample exposed to 0.2M chromium sulphate. The two diffraction lines were assigned to [Cr(H2O)6]3+ and [Cr(H2O)3O3]3+, respectively. This also suggested that the species of hydration cation was constrained by the concentration of the chromium solution.

Characterization of radicals arising from oxidation of commercially-important essential oils

Inappropriate use of essential oils may entail risks to human health due to mutational events, carcinogenic effects, genetic damages and sensitizing effect caused by generation of reactive oxygen species. In order to detect radicals that are expected to form during their oxidation, we measured the electron spin resonance (ESR) spectra of a standard reaction mixture (I) containing 25 μM flavin mononucleotide, 0.018% several essential oils (or 0.015% geraniol), 1.9 M acetonitrile, 20 mM phosphate buffer (pH 7.4), 0.1 M α-(4-pyridyl-1-oxide)-N-tert-butylnitrone (4-POBN) and 1.0 mM FeSO4(NH4)2SO4 irradiated with 436 nm visible light (7.8 J/cm2). The ESR peak heights of the standard reaction mixture (I) of the essential oils increased in the following order: tea tree > palmarosa >geranium > clary sage > petitgrain > lavender > bergamot > frankincense > ravintsara > ylang ylang > lemongrass > niaouli > eucalyptus globulus > peppermint. The ESR peak height of the standard reaction mixture (I) of geraniol, a main component of palmarosa, was comparable to the one of palmarosa (97 ± 19% of palmarosa). Furthermore, high performance liquid chromatography (HPLC)-ESR analyses of the standard reaction mixture (I) of palmarosa and geraniol gave the same peaks. The results suggest that the radicals formed in the standard reaction mixture (I) of palmarosa are derived from geraniol. HPLC-ESR-mass spectrometry analyses detected m/z 294 ions, 4-POBN/5-hydroxy-3-methyl-3-pentenyl radical adducts and m/z 320 ions, 4-POBN/C7O2H9 radical adducts in the standard reaction (I) of geraniol. The 5-hydroxy-3-methyl-3-pentenyl and C7O2H9 radicals may be implicated in the sensitizing effect of palmarosa.

EPR spectroscopy of spices

From 01 January 2017 you enter the interstate standard GOST 33271-2015 “Dry Spices, herbs and vegetable seasonings. Manual exposure in order to combat pathogens and other microorganisms” which States that the absorbed dose of radiation to the spices should be from 3 to 30 kGy. The study found that before the introduction of permissive legislative framework in the consumer market of Russia there are irradiated food products (chili, ground chili, ground spicy chili, black pepper). For radiation monitoring of food safety, we used the method of electron paramagnetic resonance (EPR), which allows quickly and with a high degree of reliability to establish the fact of irradiation. It is established that all samples of spices irradiated with dose of 12 kGy (technology radappertization) gave typical spectra of the signals established by the method of electron paramagnetic resonance in the domestic EPR spectrometer, the intensity, amplitude and peak width of the EPR signal of samples of spices with the increase of irradiation dose increases. It is proven that repeated exposure no effect accumulation. Integration with 2017 Russia in the global practi ce of using radiation technologies of processing of food products and food raw materials with the purpose of extending shelf life confirms the need for a data Bank on the radiation sensitivity of various food products to determine the optimal doses and the eff ect of radiation doses on the shelf life and quality of products.

Low symmetry configurations of vacancy-oxygen complexes in irradiated silicon

Two new electron paramagnetic resonance (EPR) spectra, labeled KU2 and KU3, emerge as a result of annealing of γ-ray irradiated silicon crystals in the temperature range 360–560 °C. The EPR intensities of KU2 and KU3 increase in the upper range of the annealing temperature where the EPR peaks of the vacancy-oxygen complex (A-center) diminish. The angular dependence study shows that the EPR spectra of KU2 and KU3 resemble that of SL1, the excited triplet (spin S = 1) state of A-centers, but with slightly different g and D-tensors representing lowering in symmetries. Microscopic lattice distortions caused by capturing of extra interstitial oxygen atoms by the A-centers are proposed to be the source of lowering of the symmetries associated with KU2 and KU3.

Electrong-factor anisotropy in an AlAs quantum well probed by ESR

Electron spin resonance (ESR) was studied in an asymmetrically doped 16-nm AlAs quantum well grown in the [001] direction. Surprisingly the ESR was detectable even if the magnetic field was parallel to the surface of the two-dimensional system. This allowed us to investigate precisely the in-plane anisotropy of the electron $g$ factor. In the case of the magnetic field aligned along the [110] or $[1\overline{1}0]$ crystallographic axes only one ESR peak was observed, whereas it tended to split into two well-separated peaks when the in-plane component of the magnetic field was between these directions. This fact clearly indicates that two in-plane valleys at $X$ points of the Brillouin zone were occupied with electrons and each valley was characterized by the anisotropic $g$ factor with the [100], [010], and [001] principal axes. The principal $g$-factor values were extracted.

Electrical characterization of hydrogenated amorphous silicon oxide films

The electrical characterization of hydrogenated amorphous silicon oxide (a-SiOx:H) films was performed by electron spin resonance (ESR) and electrical conductivity measurements. In the ESR spectra of the a-SiOx:H films, two ESR peaks with g-values of 2.005 and 2.013 were observed. The ESR peak with the g-value of 2.013 was not observed in the ESR spectra of a-Si:H films. The photoconductivity of the a-SiOx:H films decreased with increasing spin density estimated from the ESR peak with the g-value of 2.005. On the other hand, photoconductivity was independent of spin density estimated from the ESR peak with the g-value of 2.013. The optical absorption coefficient spectra of the a-SiOx:H films were also measured. The spin density estimated from the ESR peak with the g-value of 2.005 increased proportionally with increasing optical absorption owing to the gap-state defect.

Structural phase transformations in radiolytically synthesized Al–Cu bimetallic nanoparticles

The structural changes of radiolytically prepared aluminium–copper (Al–Cu) bimetallic nanoparticles by adjusting the precursors’ mole ratio and gamma radiation dose were investigated by transmission electron microscopy, field emission scanning electron microscopy/energy dispersive spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy (FTIR), and X-band continuous wave electron paramagnetic resonance (EPR). The EPR spectrum was also analysed through the simulation of the powder-like EPR spectra. The results note that in prepared samples with higher Al contents, formation of core–shell structure is dominant, whereas in Cu-rich samples, the final structures are primarily in alloy and oxide forms. According to the analysis of data obtained from X-ray diffraction, FTIR, and EPR, we found that the unpaired electron of the Cu2+ ion in various phases play the main role in structural phase transformation of Al–Cu nanoparticles. Additionally, based on the information extracted from simulated EPR peaks of Cu–Cu, the diameter of the Cu core in core–shell structures was obtained. We showed that by increasing the gamma radiation dose from 80 to 120 kGy, the overall size of nanoparticles decreases from 9.47 to 3.75 nm, but the contribution of copper core increases from 11 to 22 % of overall particle size.

Detection and identification of 1-methylethyl and methyl radicals generated by irradiating tea tree (Melaleuca alternifolia) oil with visible light (436 nm) in the presence of flavin mononucleotide and ferrous ion

Here, we determined the electron spin resonance (ESR) spectra of standard reaction mixtures (I) containing 25 μM flavin mononucleotide (FMN), 0.018% tea tree (Melaleuca alternifolia) oil, 1.9 M acetonitrile, 20 mM phosphate buffer (pH 7.4), 0.1 M α-(4-pyridyl-1-oxide)-N-tert-butylnitrone (4-POBN), and 1.0 mM FeSO4(NH4)2SO4 irradiated with 436 nm visible light (7.8 J/cm2). Prominent ESR signals (αN = 1.58 mT and αHβ = 0.26 mT) were detected, suggesting that free radicals form in the standard reaction. In order to know whether singlet oxygen (1O2) is involved in the radical formation or not, ESR measurement was performed for the standard D2O reaction mixture (I) which contained 25 μM FMN, 0.0036% tea tree oil, 1.9 M acetonitrile-d3, 20 mM phosphate buffer (pH 7.4), 0.1 M 4-POBN and 1.0 mM FeSO4 in D2O. The ESR peak height of the standard D2O reaction increased to 169 ± 24% of the control. Thus, 1O2 seems to be involved in the formation of the radicals because D2O increases the lifetime of singlet oxygen. High-performance liquid chromatography-ESR-mass spectrometry analyses detected 1-methylethyl and methyl radicals in the standard reaction. The radicals appear to form through the reaction of ferrous ion with α-terpinene endoperoxide (ascaridole), which generated from the reaction of α-terpinene with 1O2. The 1-methylethyl and methyl radicals may exert a pro-oxidant effect under these conditions.

Correlation of spin and structure in doped bismuth ferrite nanoparticles

The mutiferroic Bi1−xEuxFeO3 nanoparticles with x = 0 to 0.4 are studied by x-ray diffraction (XRD), Raman spectra and electron spin resonance (ESR) with X-band (9.53 GHz), in order to investigate the doping effect on crystalline and spin structures. Both XRD and Raman spectrum reveal a structural transformation at x = 0.15, which is associated with the shortening of Bi—O bond length. These structural data are further related to the variation of ESR peak position and peak area, providing evidence for the enhancement of ferromagnetic coupling as x < 0.3.

Spectroscopic study of plasma-facing graphite tiles modified by electron and proton irradiation

We have carried out Raman, electron spin resonance (ESR), and X-ray photoemission (XPS) spectroscopy measurements on the graphite tiles modified by electron and proton irradiation. While increase in local order was observed in the electron-irradiated sample, structural disorder and amorphization were revealed in the proton-irradiated sample, with much reduced electrical conductivity. Besides, a narrow ESR peak attributed to localized spins, closely associated with C–OH bonds as identified by the XPS measurements, was detected in the proton-irradiated sample.

Inhomogeneous magnetic cluster states in the magnetoresistance materialLu2V2O7

We report Raman scattering and multifrequency (54--336 GHz) electron-spin-resonance (ESR) measurements on ${\text{Lu}}_{2}{\text{V}}_{2}{\text{O}}_{7}$ single crystals in the temperature range $4--300\text{ }\text{K}$ with the view of understanding the origin of the large magnetoresistance in this lattice. The $307\text{ }{\text{cm}}^{\ensuremath{-}1}$ phonon mode undergoes a hardening at temperatures below ${T}_{m}=150\text{ }\text{K}$. Concomitantly, the single ESR peak arising from the paramagnetic ${\text{V}}^{4+}$ ions becomes distorted. This is ascribed to the development of short-range magnetic correlations leading to a formation of magnetic clusters. Below the Curie temperature ${T}_{C}=70\text{ }\text{K}$ the ESR line develops with fine structures, indicative of an inhomogeneous ground state. Between ${T}_{m}$ and ${T}_{C}$ the magnetic cluster concentration decreases strongly as a function of the applied magnetic field, suggesting that the large magnetoresistance of this material could be due to the magnetic inhomogeneity in this lattice. The origin of what causes the initiation of such clusters is still unclear.

Singlet Diradical Character from Experiment

Understanding the electronic structure of the singlet diradical state tackles the fundamentals of the chemical bond itself. The singlet diradical character is a key factor, which determines the chemical reactivity, the reaction products, as well as the chemical/physical (electronic, optical and magnetic) properties, although this quantity is defined in a purely theoretical manner. In this Letter, on the basis of the valence configuration interaction scheme, we present an explicit connection between the singlet diradical character and measured quantities obtained from one- and two-photon absorption spectra as well as from phosphorescence and electron spin resonance peaks. This enables us to estimate the singlet diradical character experimentally.

Time-Resolved EPR Spectra of Spin-Correlated Radical Pairs: Spectral and Kinetic Modulation Resulting from Electron−Nuclear Hyperfine Interactions

This paper expands the established four-state model of spin-correlated radical pairs (SCRPs) to include local nuclear spins which are ubiquitous in real-world systems and essential for the radical pair intersystem crossing (RP-ISC) mechanism. These nuclei are coupled to the unpaired electron spins by hyperfine interaction and split their electron paramagnetic resonance (EPR) lines. Rather than enumerating all possible nuclear states, an algorithm is devised to sort out the net hyperfine offset 2Q, which, along with the electron spin-spin coupling 2J, characterizes the behavior of SCRPs. Using this algorithm, the EPR spectra of SCRPs coupled to arbitrary nuclear spins can be efficiently simulated with only 2J and the EPR spectra of individual radicals as the inputs. Particularly illustrative is the case of a SCRP resulting from photoinduced electron transfer comprised of a spectrally narrow anion radical signal having small hyperfine splittings and a broad cation radical signal having many large hyperfine splittings and a Gaussian width sigma, where the EPR peak of the anion radical exhibits an effective splitting of 2(1/2)J(2)/sigma. For SCRPs having singlet and triplet pathways for charge recombination, their kinetic behavior is obtained concisely by considering the decay rate constants k(S) and k(T) as imaginary energies, while adhering to the existing derivation of the four-state model. These models are employed to interpret the diverse array of spectral and kinetic modulation patterns observed in the experimental EPR spectra of photogenerated SCRPs and to extract the 2J value, which reflects the donor-acceptor electronic coupling. During the first several hundred nanoseconds following photoexcitation, the spectral and time domain characteristics of the measured time-resolved EPR spectra manifest the consequences of the Uncertainty Principle, and the modulation patterns in either domain result from hyperfine splittings between the unpaired electron and the nuclear spins.

Correlation between electron spin resonance and magnetization inGd5Ge4

Electron spin resonance (ESR) across the field-induced antiferromagnetic (AFM) to ferromagnetic (FM) transition in ${\text{Gd}}_{5}{\text{Ge}}_{4}$ has been studied. The AFM-FM transition can be characterized as an anomaly in the ESR spectrum. In the high-field FM state, a single strong ESR peak is observed while in the low-field AFM state, the ESR spectrum presents relatively weak double peaks. In the coexisting AFM and FM phases, the ESR spectrum is an asymmetrical double-peak structure due to a superposition of AFM and FM resonances. Thus, the ESR result is significantly correlated with magnetization. The $g$ value for FM resonance is found to be close to $g=1.992$ for the free ${\text{Gd}}^{3+}$ ion. The exchange interactions in the AFM state are discussed qualitatively.

Influence of ozone on traffic-related particulate matter on the generation of hydroxyl radicals through a heterogeneous synergistic effect

Epidemiologic studies suggest that ozone (O 3) and airborne particulate matter (PM) can interact causing acute respiratory inflammation and other respiratory diseases. Recent studies investigated the hypothesis that the effects of air pollution caused by O 3 and PM are larger than the effect of these two pollutants individually. We investigated the hypothesis that ozone and traffic-related PM (PM 10 and PM 2.5, diesel and gasoline exhaust particles) interact synergistically to produce increasing amounts of highly reactive hydroxyl radicals (HO ) in a heterogeneous aqueous mixture at physiological pH. Electron paramagnetic resonance (EPR) and spin trapping were used for the measurements. Results showed that HO radicals are generated by the catalytic action of PM surface area with ozone and that EPR peak intensities are two to three times higher compared to PM samples without ozone. Incubation of the nucleoside 2′-deoxyguanosine (dG) in aqueous mixtures of ozone and PM at pH 7.4 resulted in the hydroxylation at C(8) position of dG. The formation of 8-hydroxy-2′-deoxyguanosine (8-OHdG) showed a 2–2.5-fold increase over control (PM without O 3). These results suggest that PM and O 3 act synergistically generating a sustained production of reactive HO radicals. Partitioning of O 3 into the particle phase depends on the concentration, hygroscopicity and particle size.