μSR spectroscopy on free radicals: A complement to ESR spectroscopy

After a brief description of the types of experiments that constitute these two forms of spectroscopy, based on a consideration of the Breit Rabi diagram, the major powers of the two techniques for probing radical structure are discussed and compared. It is shown that although the μSR and related experiments are apparently more limited in scope than are those involving ESR spectroscopy, nonetheless there are certain unique aspects of the μSR studies which cannot be achieved otherwise. The major importance of current work, however, lies in the ability to compare the behaviour of chemically similar systems, using μSR and ESR as ideally complementary techniques. Large hyperfine isotope effects are found, due principally to the greater zero-point energy of muonium relative to hydrogen, when covalently bound in a molecular radical, or when confined within an interstitial cavity. These effects are compared and contrasted for organic radicals and for the hydrogen/muonium defect centres in crystalline solids. The different categories of radical studied to date by μSR are reviewed; the unique results in semiconductors are discussed in terms of molecular radical models, and the possible preparation of novel species, including muonated radical cations and radical anions, is considered. (orig.)