Rotating-Frame Double Resonance and Nuclear Cross Relaxation in LiF

This paper resports our study of Hartmann and Hahn's technique of rotating-frame nuclear double resonance (RFDR). We consider the Li⁶-Li⁷ system in LiF and obtain the cross-relaxation dynamics from the Li⁷ free-induction-decay data via a general expression which we derive. We find that the presence of F¹⁹ spins, which constitute a third species interacting strongly with the Li systems in LiF, may easily be taken into account, and that the resulting RFDR behavior does not differ qualitatively from that in a simple two-species system. The cross-relaxation time dependence is well described by an exponential or sum of exponentials for times longer than the order of T₂, with nonexponential behavior for shorter times. The cross-relaxation rate WCR exhibits a Lorentzian dependence on the magnitude of the Li⁶ rf field, H₁₆, for the case where the Li⁷ system has been adiabatically demagnetized in the rotating frame; these results show that the Gaussian behavior previously assumed is incorrect. For the case where the Li⁷ rf field is of the order of the local field, WCR(H₁₆) is asymmetric about Hahn's double resonance (DR) condition, with the larger WCR corresponding to H₁₆ less than for the DR condition. The cross-relaxation times at the DR condition are on the order of 0.4 msec. We observe no spin-diffusion bottleneck in a sample of LiF with an isotopic abundance of 0.008% Li⁶. Finally, we draw some general conclusions about the application of RFDR to other problems.