SPIN-LATTICE RELAXATION IN HYPERFINE-COUPLED SYSTEMS - APPLICATIONS TO INTERSTITIAL DIFFUSION AND MOLECULAR-DYNAMICS

The solid state diffusion of hydrogen, or of its pseudo-isotope muoniu m, provides an interesting example of spin-lattice relaxation in a 2-s pin, 4-level system. The local field experienced by the interstitial a tom fluctuates as it moves, inducing transitions between the coupled e lectron and nuclear spin slates. Rate equations governing the populati ons of these states may be solved numerically to simulate the differen t relaxation functions which would be displayed by ESR, ENDOR and mu S R spectroscopies and to assist in extracting motional correlation time s from the experimental data. Spin relaxation in molecular radicals ma y be treated similarly, with different selection rules for different m echanisms: this paper treats the spin rotation mechanism and perturbat ion to anisotropic or isotropic components of the hyperfine interactio n, caused by inter or intra-molecular motion. Conventional magnetic re sonance monitors the population differences appropriate to particular transitions; only in sufficiently high fields do these distinguish the electronic and nuclear response. Muon spin relaxation is remarkable i n separating out the nuclear spin projection whatever the degree of mi xing of the spin states, via the asymmetry in the muon radioactive dec ay. Experimentally it has the advantage that measurements can be made over a wide range of field, from null external field up to the level c rossing where the relaxation rate exhibits a striking peak.