Resonance Phenomenon ...

Magnetic Interactions ...

The EPR Spectrum ...

... Electronic Structure

... Molecular Structure

... Molecular Motions

... Magnetic Properties

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EPR: Electron Paramagnetic Resonance

ESR: Electron Spin Resonance

EMR: Electron Magnetic Resonance

... Enhancement of Spectral Resolution: g-factor, ENDOR

... Some spin systems such as transition metal ions with large zero-field splittings only have transitions at high frequencies

... Suppression of second-order effects

... High orientation selectivity for powder samples and disordered sytems

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Mn²⁺ and V²⁺ impurities in a 1.7 mg single crystal of MgO

The two groups of hyperfine lines are completely separated in the EPR spectrum taken at 220 GHz.

Chromium ions in ruby are another example: See spectra shown in the demonstration of phase control.

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The enhancement of spectral resolution at high frequencies can be exploited to study nitroxide free radicals in polystyrene. The X-band (9.7 GHz) and D-band (220 GHz) EPR spectra from a thin film (0.2 mm, 580 mg) of 1 mM nitroxide in polystyrene show the clear resolution of the g-tensor components in the HFEPR spectrum while in the X-band spectrum both, spin-orbit and hyperfine couplings are on top of the same features.

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Spin saturation experiments reveal the spin relaxation behaviour at a certain temperature. This is an indirect path to the relaxation times T₁ and T₂ which are otherwise determined by pulsed EPR experiments.

The spectra in the diagram are obtained from a 1 mM nitroxide (DPN) diluted in polystyrene in a non-resonant probe at 20 K. The amplitude of the aquired spectra has been multiplied by 2 (6 dB), 4 (12 dB), 8 (18 dB), and 16 (24 dB) to compensate for the normal intensity increase with microwave power in the unsaturated region. The sequence of spectra clearly demonstrates the effect of spin saturation at higher microwave power.

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The spectra in the diagram are obtained from a 3 mM nitroxide (DPN) diluted in polystyrene in a non-resonant probe at 5 K. The microwave frequency used was 218.004 GHz. The series of spectra shown is obtained at a number of different static magnetic fields. The bottom trace starts at the low-field portion of the EPR spectrum representing molecules being oriented with their x-axes parallel to the external magnetic field. From bottom to top the magnetic field increases in equidistant steps. The top spectrum is mesured at the high-field end of the EPR spectrum representing molecules being oriented with their z-axes parallel to the external field. This example nicely demonstrates the high spectral ... and as a consequence the high orientational resolution.

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