Narrow Electron Paramagnetic Resonance Line Research Articles

Electronically Tunable Surface-Coil-Type Resonator for L-Band EPR Spectroscopy

The automatic frequency control (AFC) circuit in conventional electron paramagnetic resonance (EPR) spectrometers automatically tunes the microwave source to the resonance frequency of the resonator. The circuit works satisfactorily for samples stable enough that the geometric relations in the resonance structure do not change in a significant way. When EPR signals are measured during in vivo experiments with small rodents, however, the distance between the signal source and the surface-coil detector can change rapidly. When a conventional AFC circuit keeps the oscillator tuned to the resonator under those conditions, the resultant frequency change may exceed ±5 MHz and markedly shift the position of the EPR signal. Such a shift results in unacceptable effects on the spectra, especially when the experimenter is dealing with narrow EPR lines. The animal movement also causes a mismatching of the resonator and the 50-ohm transmission line. Direct results of this mismatching are increased noise; shifts in the position of the baseline; and a high probability of overdriving the signal preamplifier with consequent loss of the EPR signal. We therefore designed, built, and tested a new surface-coil resonator using varactor diodes for tuning the resonance frequency to the fixed frequency oscillator and for capacitive matching of the resonator to the 50-ohm transmission line. The performance of the automatic matching system was tested in vivo by measuring EPR spectra of lithium phthalocyanine implanted in rats. Stability and sensitivity of the spectrometer were evaluated by measuring EPR spectra with and without the use of the automatic matching system. The overall experimental performance of the spectrometer was found to significantly improve during in vivo experiments using the automatic matching system. Excellent matching between the 50-ohm transmission line and the resonator was maintained under all experimental circumstances that were tested. This should allow us now to carry out experiments that previously were not possible.

EPR and DNP Properties of Certain Novel Single Electron Contrast Agents Intended for Oximetric Imaging

Parameters of relevance to oximetry with Overhauser magnetic resonance imaging (OMRI) have been measured for three single electron contrast agents of the triphenylmethyl type. The single electron contrast agents are stable and water soluble. Magnetic resonance properties of the agents have been examined with electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), and dynamic nuclear polarization (DNP) at 9.5 mT in water, isotonic saline, plasma, and blood at 23 and 37°C. The relaxivities of the agents are about 0.2–0.4 mM−1s−1and the DNP enhancements extrapolate close to the dipolar limit. The agents have a single, narrow EPR line, which is analyzed as a Voigt function. The linewidth is measured as a function of the agent concentration and the oxygen concentration. The concentration broadenings are about 1–3 μT/mM and the Lorentzian linewidths at infinite dilution are less than 1 μT in water at room temperature. The longitudinal electron spin relaxation rate is calculated from the DNP enhancement curves. The oxygen broadening in water is about 50 μT/mM O2at 37°C. These agents have good properties for oximetry with OMRI.

Bistable Conduction Electron Spin Resonance in Metallic Lithium Particles

This paper reports the study by electron spin resonance (ESR) and electron nuclear double resonance (ENDOR) of lithium particles in Li2O powders irradiated by 1-MeV electrons. The metallic lithium particles are characterized by a narrow ESR line (ΔBpp = 0.013 mT) due to conduction electrons and an ENDOR signal from 7Li nuclei interacting with these electrons. The ESR of conduction electrons in lithium particles exhibits a bistable hysteresis of 0.18 mT width at T = 4 K due to steady-state bistable dynamic nuclear polarization by the Overhauser effect, in good agreement with theoretical predictions. The analysis of the bistable conduction electron spin resonance (BCESR) line shape showed the existence of two populations of particles differing by their electron spin relaxation times T2 and characterized by a distribution of nuclear relaxation times.

Trapped electrons in solid deuterium.

We report a narrow electron-spin-resonance (ESR) signal near the free-electron frequency in solid deuterium containing 2% tritium. We speculate that this narrow ESR line is due to localized electrons in bubbles. \textcopyright{} 1996 The American Physical Society.

Peierls transition of perylene radical cation salts with five-eighth filled conduction band

The Peierls transition of Perylene (PE) radical cation salts ( PE) 2X 1 · 2 3 THF (nominal composition, X = (PF 6) 1− x (AsF 6) x , 0 ⪯ x ⪯ 1, THF = tetrahydrofuran) was analysed by static magnetic susceptibility (χ), radiofrequency and microwave electron spin resonance (ESR), (g-factor, line width ΔB), and microwave conductivity (σ). According to crystal structure analyses, these salts have a five-eighth filled conduction band. The opening of the energy gap at 100 – 120 K (depending on x) was determined by the analysis of χ(T). Additional phase transitions were observed and analysed by the different experimental techniques (χ(T), σ(T), g(T)). Due to the narrow ESR line in the metallic phase ( ΔB pp ≅ 15 mG), a reorientation of the PE-stacks at 162 K can be derived precisely via the g-anisotropy. In the low-temperature phase the angular and frequency dependence of the ESR lines' boardening is used in order to analyse the interaction between thermally activated conduction electrons and localized defects in these quasi-onedimensional conductors.

A separate class of defects in silica: the natural EX center in thermal SiO 2 on Si

K-band electron spin resonance (ESR) studies of thermally grown Si/SiO 2 structures as a function of oxidation temperature have revealed a first type of natural defect in as-grown thin ( ∼ 100–600 A ̊ ) SiO 2 films. The defect (labelled EX center) is characterized by a narrow central ESR line of remarkably isotropic g = 2.00246 ± 0.00003, amid a doublet of 1.61 mT splitting. Etch back experiments show the defect to reside in the top ∼ 40 A ̊ of the SiO 2 layer, closely distributed along a Gaussian profile centered at the free SiO 2 surface. The generation of the defect has been analyzed over a broad range of preparation parameters. These show the generated EX concentration to depend on the preoxidation Si surface state. However, for a fixed Si surface condition, a universal type of behaviour is found in the sense that the as-grown oxide thickness, d ox, emerges as the sole factor controlling the EX incorporation; EX defects are observed from d ox ≈ 100 A ̊ onwards, reaching a maximum of ∼ 1.2 × 10 12 cm −2 at d ox ≈ 130 A ̊ , to decrease again for thicker oxides. From the ESR data, EX is inferred to be an intrinsic defect naturally generated and network stabilized during thermal oxidation as a result of the exclusive way thermal oxide is formed. The defect appears as a first member of a separate family of native intrinsic defects vis-à-vis the solitary class of damage centers sor far identified in fused silica. A tentative model pictures the defect as a delocalized unpaired electron at the site of an Si vacancy.

Kinetics of intercalation of tetraalkylammonium ions into polyacetylene

The doping kinetics of Shirakawa polyacetylene (PA), involving bulky R 4N + like Bu 4N +, Hep 4N + and Dodec 4N +, was undertaken. From the drastic influence of dopant concentration and film thickness on the kinetics, the interfibrillar diffusion of the dopant was identified as the rate-limiting step. However, a minimum value of 10 −15cm 2s 2−1 was estimated for the intrafibrillar diffusion constant. The relaxation of the narrow electron spin resonance line, produced during a short heterogeneous doping of cis-PA, was critically examined in the case of each of the above cations. The relaxation times were found to decrease in the order Bu 4N + < Hep 4N + < Dodec 4N +.

ESR gaussmeter for low-field applications

An ESR magnetometer has been developed for use at fields between 50 mu T and 10 mT. The organic conductor (FA)2PF6 is used as a magnetic field sensor. Due to its frequency-independent narrow electron spin resonance line of 2.6 mu T (FWHH), a measurement accuracy better than 10-4 or 0.1 mu T has been achieved. Frequency modulation and lock-in detection give rise to a differentiated Lorentzian lineshape whose zero of amplitude is referred to in a frequency control loop. The use of a lumped-parameter delay line as the probe head in place of a resonant circuit enables broad-band measurements to be performed.

Structural, optical, and electronic properties of Li 3MS 3 compounds ( MIII = Ti, Zr, Hf)

The Li 3 MS 3 powder samples ( M III = Ti, Zr, Hf), chemically prepared under mild conditions, have been studied by X-ray diffraction, infrared and Raman vibrational spectroscopies, UV-visible absorption, and electron paramagnetic resonance (EPR) technics. From X-ray data we confirm the topochemical mechanism of three lithium insertions into the 2D- MS 3 host lattices already proposed by R. R. Chianelli and M. B. Dines ( Inorg. Chem. 14, 2417 (1975)). From infrared and Raman results new structural information is obtained: on the one hand, the cleavage of (S 2) 2− pairs induces a new arrangement of the basic structure and on the other hand, LiS interactions are by far predominant within strongly distorted two LiS 4 and one LiS 6 groups; moreover M-S interactions are markedly weakened. From UV-visible and EPR spectroscopies, the electronic perturbations upon lithiation are evidenced: new d-d-type transitions, in the 550- to 660-nm range, and strong broad EPR signals suggest that M 4+ ( d 0) ions are transformed into M 3+ ( d 1) centers. In addition, some unpaired electrons, responsible for a narrow EPR line, appear to be localized around not totally ionized Li centers. Finally, a structural model, which takes into account all the above experimental results, is proposed; this model allows us also to explain the reactivity and some physical properties of the Li 3 MS 3 phases when in short contact with air.

Radiation effects in a glass-ceramic

Electron spin resonance (ESR) and optical absorption techniques have been used to investigate the effects of ionizing radiation in a glass-ceramic. Samples of Zerodur were electron-irradiated either at 77 K or at room temperature, and five distinct ESR spectra and two optical absorption peaks were observed. Three of the ESR centers are holelike and two are electronlike. One of the electron centers, represented by a narrow ESR line at g = 1.9980, is easily produced by the 77-K irradiations and is suggested to be an oxygen-vacancy-related defect. The other electron center has a broad ESR line at g = 1.93 and is dominant after the room-temperature irradiations. This latter defect is created when electrons are trapped at high-valence cations such as zirconium or titanium, thus forming either Zr3+ or Ti3+ ions. These heavy-metal-ion electron centers decay in the 100–300 ° C range.

Gaussmeter application of an organic conductor

We present several examples for the application of organic conductors like (Fluoranthene)2PF6 as magnetic field probes. Due to their frequency independent, extremely narrow electron spin resonance line, magnetic flux densities ranging from the earth's field until thousands of Gauss can easily be evaluated with an accuracy better than 1.5 mG. The small sample size required — eg. 2 mm3 for a flux density of about 5 G (S/N ratio 10∶1) — allows to analyse strong magnetic field gradients. Examples for the adaptation of field or frequency-modulation and pulse techniques are given. We point to the requirement of single crystalline samples for high field applications.

Triplet spin excitons in organic charge transfer complexes

Electron paramagnetic resonance (EPR) spectra of single crystals of the 1 : 1 charge transfer (CT) complex of N, N'-dimethyldihydrophenazine ( M 2 P) and of tetracyanoquinodimethane (TCNQ) provide the first case of thermally activated, with ΔE p = 0.60 ± 0.06 eV, triplet spin excitons observed in an organic CT solid. The narrow EPR lines and resolved fine structure splittings indicate mobile triplets based on adjacent M 2 P +TCNQ - ion radicals above a largely neutral ground state.

Electron spin resonance and optical fluorescence of the chromia-alumina system

Polycrystalline α-phase Al 2O 3Cr 2O 3 solid solutions containing 0.01–4.98% Cr have been examined using electron spin resonance (ESR) and optical fluorescence techniques. The KU-band ESR spectrum of isolated Cr 3+ ions in the α-phase material is quantitatively explained using perturbation theory. Arguments against a previous explanation of one of the resonance lines from coupled Cr 3+ ions are given. The absence of sharp line fluorescence in the presence of the relatively narrow ESR line at g = 1.98 in the γ-phase material is further evidence for its assignment to Cr 5+ ions as opposed to Cr 3+ ions. Two new fluorescent lines are observed when the γ-phase and α-phase coexist and possible explanations are given.