56,352 research outputs found
STRUCTURE AND DYNAMICS OF RADICALS IN SOLIDS BY EPR AND ENDOR SPECTROSCOPIES
No full textEPR and ENDOR (Electron Nuclear DOuble Resonance) spectroscopies give complementary information in the study of paramagnetic species in solids. The ENDOR technique allows the determination of hyperfine tensors an order of magnitude smaller with respect to the EPR one. The molecular motions in solids affect the relaxation of both the transverse and longitudinal magnetization of the paramagnetic probe. The measurement of the transverse relaxation time is traditionally available by the EPR Lineshape analysis. Population variations due to the longitudinal relaxation processes can be monitored by the study of the amplitude of the ENDOR spectra
Electron Spin-lattice Relaxation-time and Spectral Diffusion In Gamma-irradiated L-alanine.
No full textWe performed continuous (CW) wave and pulsed ESR experiments to obtain information on the relaxation behavior of the 1-alanine radical in an irradiated single crystal. The analysis of the CW saturation behavior gives a relaxation time of 2.8 micro s. The echo detected saturation recovery was obtained for a number of different experimental conditions. In any case only a portion of the 120 G wide ESR spectrum can be affected by the microwave (MW) pulses, spectral diffusion is active and a multi-exponential decay is therefore obtained. We measured characteristic spectral diffusion times of 1-10 and 20-50 micro s. We found that a long time of about 200 micro s can be measured only by using a train of long selective saturating pulses and short detecting pulses. The stimulated echo decay is bi-exponential, and the characteristic times are very short. A variable temperature investigation in the range 200 to 290 K showed that the decay is governed by the spectral diffusion and by the transverse nuclear spin relaxation time T-2n of the methyl protons
CaCO3 Crystallite Evolution during CaO Carbonation: Critical Crystallite Size and Rate Constant Measurement by In-Situ Synchrotron Radiation X-ray Powder Diffraction
No full textSimulation of EPR and Time Resolved EPR lineshapes in partially ordered phases
No full textSimulation of magnetic resonance spectra of probes in partially ordered glasses requires in principle a numerical integration on the full set of three Euler angles Ohm=(alpha beta gamma) from a laboratory fixed to a molecule fixed reference frame. It is shown that it is possible to manage efficiently this problem by using the algebraic properties of the Wigner matrix elements. This analysis is applied to time resolved EPR (TREPR) spectra of a series of bis-adducts of C-60 in the ordered glass of a nematic liquid crystal solvent. A paramagnetic triplet state is created by light excitation and TREPR spectra are obtained with the external magnetic field set parallel or perpendicular to the director n of the mesophase. The preferred orientation in the mesophase of the triplet state zero field tensor is determined
Twisted Segre products
No full textWe introduce the notion of the twisted Segre product of
-graded algebras and with respect to a twisting map .
It is proved that if and are noetherian Koszul Artin-Schelter regular
algebras and is a twisting map such that the twisted Segre product
is noetherian, then is a noncommutative graded
isolated singularity. To prove this result, the notion of densely (bi-)graded
algebras is introduced. Moreover, we show that the twisted Segre product
of and with respect to a diagonal
twisting map is a noncommutative quadric surface (so in particular it is
noetherian), and we compute the stable category of graded maximal
Cohen-Macaulay modules over it.Comment: 23 pages, v2: a few typos were fixed, v3: minor update
AN ENDOR STUDY OF THE TEMPERATURE-DEPENDENCE OF METHYL TUNNELING
No full textWe have studied the motion of the methyl group in the free radical obtained by gamma-irradiation of 4-methyl-2,6-di-t-butylphenol (MDBP). The ENDOR spectra have been measured in the temperature range 4.5 to 200 K. The positions of the methyl proton lines are temperature dependent. Their variation is accounted for by considering the transition from the quantum regime of motion of the methyl group at low temperature to the classic regime at higher temperature. This transition is found to occur in a temperature range narrower than that anticipated by Allen's theory
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