The solid state diffusion of hydrogen, or of its pseudo-isotope muonium, provided an interesting example of spin-lattice relaxation in a 2-spin 4-level system. The local field experienced by the interstitial atom fluctuates as it moved, inducing transitions between the coupled electron and nuclear spin states. Rate equations governing the populations of these states may be solved numerically to simulate the different relaxation functions which would be displayed by ESR, ENDOR and μSR spectroscopies and to assist in extracting motional correlation times from the experimental data. Spin relaxation in molecular radicals may be treated similarly, with different selection rules for different mechanisms: this paper treats the spin rotation mechanism and perturbation to anisotropic or isotropic components of the hyperfine interaction, caused by inter or intra-molecular motion. Conventional magnetic resonance monitors the population differences appropriate to particular transitions; only in sufficiently high fields did these distinguish the electronic and nuclear response. Muon spin relaxation was remarkable in separating out the nuclear spin projection whatever the degree of mixing of the spin states, via the asymmetry in the muon radioactive decay. Experimentally it had the advantage that measurements could be made over a wide range of field, from null external field up to the level crossing where the relaxation rate exhibited a striking peak.

Spin-Lattice Relaxation in Hyperfine-Coupled Systems: Applications to Interstitial Diffusion and Molecular Dynamics. Cox, S.F.J., Sivia, D.S.: Applied Magnetic Resonance, 1997, 12[2-3], 213-26