ELDOR enhancement arising from admixture of nuclear spin states

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Abstract
ELDOR enhancement effects arising from the admixture of nuclear spin states have been studied with the irradiated single crystals of potassium hydrogen maleate, which give strong forbidden ESR lines. When the magnetic field was applied along the c axis, all of the ELDOR transitions involving no level in common showed enhancement while those involving a level in common showed reduction. In the different orientations of the crystal, some of the enhanced lines changed into reduced ones with decreasing forbidden ESR transition probabilities. The temperature changes of the ELDOR spectra were also studied. With decreasing temperature, all of the enhanced lines at room temperature changed into reduced lines giving maximum R factors at around − 100°C. These results were interpreted in terms of large intramolecular cross relaxation probabilities that are due to the admixture of the nuclear spin states in our system. To calculate the R factors for our system, the generalized expression has been derived for the R factor in multiobserving and multipumping ELDOR experiments. The variation of the observed R factors with pumping transitions is qualitatively understood from the R factors calculated with appropriate assumptions on the relaxation probabilities. The calculated R factors also show that increasing nuclear spin relaxation probabilities with decreasing temperature are responsible for the temperature change of the observed R factors.

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