Molecular dynamics of. mu. sup + -C sub 60 radical in solid C sub 60

The molecular dynamics and electronic structure of the {mu}{sup +}-C{sub 60} radical in crystalline C{sub 60} have been studied using muon spin rotation and relaxation. At room temperature {mu}{sup +}-C{sub 60} appears to be in a state of quasifree rotation. At the critical temperature {ital T}{sub {ital S}}=260 K the local electronic structure and molecular dynamics change discontinuously as expected for a first-order phase transition. The correlation times for reorientation are remarkably close to those determined by recent NMR experiments on C{sub 60}, suggesting that the molecular dynamics of {mu}{sup +}-C{sub 60} are strongly coupled to those of its C{sub 60} neighbors.