Rotating-Frame Double Resonance and Nuclear Cross Relaxation in LiF

Abstract
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.