Mechanisms of Double Resonance in Solids

A study of electron-nuclear double resonance (ENDOR) in ruby and other solids demonstrates the existence of the "distant-ENDOR" effect, which involves a change in the electron paramagnetic resonance (EPR) signal caused by the depolarization of "distant" nuclei (nuclei having negligible hyperfine interaction with the paramagnetic centers). In order to obtain interpretable data on the mechanism, it proved necessary to perform most of the experiments without modulation, observing not the derivatives but the functions ${\chi }^{\prime }$ and ${\chi }^{\prime \prime }$ themselves, the dispersive and absorptive parts of the spin susceptibility. The former shows a large decrease upon application of rf power at a nuclear transition frequency; the latter shows a moderate increase. Both the distant ENDOR (${\mathrm{Al}}^{27}$ nuclear Zeeman frequencies) and local ENDOR (${\mathrm{Cr}}^{53}$ hyperfine frequencies) affect the EPR with a response time comparable to the spin-lattice relaxation time of the distant aluminum nuclei. Nuclear-nuclear double-resonance experiments show that applied rf corresponding to ${\mathrm{Cr}}^{53}$ nuclear transitions depolarizes ${\mathrm{Al}}^{27}$ nuclei. Both of these observations are consistent with a mechanism involving dynamic nuclear polarization. A theoretical analysis of this mechanism, based on forbidden transitions involving distant nuclei, gives good agreement with observed nuclear polarizations and with the observed behavior of ${\chi }^{\prime }$, but predicts small increases in ${\chi }^{\prime \prime }$. The increased absorption signal may be explained by enhanced spectral spin diffusion or by a spin packet considerably wider than assumed. Distant ENDOR is expected to occur quite generally.