Physical Characterization of Organic Radicals by Low-Field Epr Spectrometry
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PHYSICAL CHARACTERIZATION OF ORGANIC RADICALS BY LOW-FIELD EPR SPECTROMETRY MEHDI MOUSSAVI; MARC BERANGER; DENIS DURET; NELLY KERNEVEZ and LILIANE SECOURGEON LETI, A Division of CEA. CENG 85 X 38041 GRENOBLE, FRANCE
ABSTRACT A low-field (0.6 Gauss, 2 MHz) EPR spectrometer providing quantitative parameters is described. It gives informations such as frequency dependence of the linewidth, anisotropy of inhomogeneous line and also the values of Tl, T2 and the magnetic susceptibility using simple hypothesis. The behaviour of different typical w-systems, as well metallic as semiconducting is compared to a radical anion salt, quinolinium bis tetracyanoquinodinemethane Qn(TCNQ)2 considered as a reference. The materials physical characteristics will be provided by a new variable field (0.6-60 Gauss, 2-200 MHz) which is under realization, for an extended family of organic radicals.
INTRODUCTION For reasons of sensitivity, CW EPR Spectrometry is mainly performed in the frequency range 1 to 35 GHz. Different classes of material with exceptional spin dynamics or magnetic properties have already been developed. For such systems, an EPR study at low field becomes of high interest. The earth field magnetometry emphasizes the need of a spectrometer in the same range to characterize the materials. The performance of the spectrometer must be optimized for the sensitivity which decreases quadratically with the frequency. The described spectrometer is automated and reduced in size (and price) in order to be used in a non specialized laboratory.
I.
PRINCIPLE
Considering EPR, the electron has a spin and therefore a colinear magnetic moment. The resultant magnetic moment of the spin population can precess around the static field Ho at the Larmor frequency : Co = y po Ho (1) where 11 7 1 -y = 1.76.10 s.-iT-I and vo = 4 ur.10 Hm- . The classical arrangement to measure a resonance is known as the Bloch configuration [Ei. The method we used exploits the derivative of the resonant curves. It is obtained by superimposing a dither modulation of a frequency CN (uw
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