Electron spin resonance spectra powders

To simplify terminology of axial systems, gzz is defined to be g(l (the g-value observed with the symmetry axis of Cu + parallel to the applied field), and gxx (= gyy) is defined to be gA (the g-value observed with the symmetry axis perpendicular to the applied field). An elongated z-axis (depicted in Figure 11 for Cu(H20)5 +) results in gjj > gj. For axially symmetric Cu + rigidly bound in a crystal, the g-value can then vary between the minimum (gj.) and maximum (g(,), depending on orientation of the crystal within the magnetic field. However, for axial Cu + bound in a powdered clay sample, all possible orientations, and therefore all g-values between gA and gj are represented in the "powder" spectrum. Therefore, electron spin resonance occurs only for field values, H, between Hjj and H, where ... 

The present work deals with soUd CT complexes of phenazines which, in the most favorable cases, include the crystal structure, single-crystal electron spin resonance (epr) data, the temperature dependence of the magnetic susceptibility or of the powder conductivity, and a CT absorption spectrum. We have not pursued complexes that could not be crystallized, nor have we investigated completely similar materials. 

Fig. 18. The dashed curves represent the axially-symmetric powder pattern for the dangling bond observed at Si-SiOj interfaces (Caplan et al 1979) for several values of isotropic broadening W. The solid curve is the ESR spectrum in a-Si H. [Reprinted by permission of the publisher from Electron spin resonance studies of amorphous silicon, by D.K. Biegelsen, Proceedings of the Electron Resonance Society Symposium, Vol. 3, pp. 85 - 94. Copyright 1981 by Elsevier Science Publishing Co., Inc.]...

More advanced experiments, such as ENDOR, electron spin echo envelope modulation (ESEEM), or relaxation measurements by pulsed ESR rely on a selective excitation of spins close to the resonance field. Usually, the powder ESR spectrum is much broader than the excitation bandwidth of the pulses, which is in the range between 2 and 10 G. In cases where one anisotropic interaction dominates the spectrum, the experiments thus select contributions only from certain orientations of the molecule with respect to the external magnetic field. Such orientation selection is more efficient and easier to interpret at a field that is high enough for the g anisotropy to dominate. Finally, the size of mw resonators scales with wavelength and thus scales inversely with frequency. At higher frequency, spectra can thus be measured with much smaller sample volumes, yet the concentration does not need to be significantly increased. 

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