Dynamic nuclear polarization by the solid effect in systems with large crystal-field splitting of the ESR spectrum

Abstract

It is shown that the dynamic nuclear polarization (DNP) of the solid effect can exhibit unusual behaviors when the energy levels of the electron spins ($S\ensuremath{\ge}1$) exhibit a large crystal-field, fine-structure splitting. When the splitting between the energy levels associated with an ESR fine-structure line decreases (converges) as a function of the external magnetic field $H$, we predict that the DNP produced by microwave saturation of the ESR line will display a magnetic-field dependence proportional to the positive of the first-derivative ESR line shape rather than to the negative of the first-derivative ESR line shape as is usually observed. If the energy levels are inflected ($\frac{\mathrm{dE}}{\mathrm{dH}}$ changing sign) as a function of $H$, the DNP in certain cases can be totally positive and roughly proportional to the ESR absorption line shape. These predictions have been experimentally confirmed in single crystals of ruby (${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$: ${\mathrm{Cr}}^{3+}$) and sapphire (${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$: ${\mathrm{Fe}}^{3+}$).