Radio-frequency muon spin resonance studies of endohedral and exohedral muonium adducts of fullerenes

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.