Observation of endohedral muonium in C61H2

A study is presented of endohedral muonium (Mu = μ+e−) in a functionalized fulleroid, the 6, 5 isomer of C61H2. The temperature independence of the muonium depolarization rate implies that in the timescale of the μSR experiment no isotropic reorientation regime sets in even at 280 K. The depolarization appears dominated by the interaction of Mu with methylene protons. No anisotropy of the hyperfine interaction was directly detected within experimental accuracy; an upper limit for the anisotropy can be fixed to 600 kHz by a transverse field measurement.