Structural and dynamical studies of muonium substituted radicals: muon beam radiolysis and μSR spectroscopy

This article concerns the study of organic radicals which are labelled or substituted by muonium, Mu = μ+e-, the light pseudo-isotope of hydrogen. Such species are formed following the irradiation of certain non-saturated compounds with beams of positive muons. Atomic muonium may be formed during the primary radiolytic processes and then subsequently react by addition with the host molecules, but processes equivalent to protonation can provide an alternative precursor. The radical products are detected with considerable sensitivity and selectivity by μSR spectroscopy, which uses the muon spin as a probe of hyperfine fields. The technique known as level crossing resonance provides a particularly complete characterization. The example of the ethyl radical, and the comparison of the properties of the substituted species with those of its normal counterpart, serve to illustrate the various aspects of such studies. Isotope effects may be traced to the importance of zero-point energy, with surprisingly large effects on structure, and on the distribution of spin-density. Gas phase detection of the muonium substituted species promises to open a significant new field of study. In solid state physics, a topical example is muonium labelling, of C60 fullerene, illustrating studies of molecular dynamics