The radio-frequency muon spin resonance technique (RF-μSR) is described, with examples drawn from muon studies of fullerences. Two distinct species can be detected by RF-μSR when solid C60 is irradiated with positive muons. Endohedral muonium (Mu@C60) is characterized by a muon hyperfine constant (A μ) close to the vacuum value. A remarkable feature of the RF-μSR spectrum is the double quantum transition, which appears when the allowed transitions are saturated. The exohedral muonium adduct (C60Mu) is also detected, and has a much smaller value ofA μ typical of a carbon-centred organic radical. It has been studied by RF-μSR in dilute solution, which is not possible for transverse field muon spin rotation (TF-μSR). There is a significant difference inA μ of C60Mu in the solid and in solution, a result of great import to the analysis of avoided-level crossing experiments on13C60Mu.
