Mu@C₇₀: Monitoring the dynamics of fullerenes from inside the cage

Atomic muonium (Mu, the light isotope of hydrogen) is easily encapsulated inside fullerene cages and acts as a highly sensitive microscopic probe of both the fullerene's rotational dynamics and the anisotropy in its electronic distribution. Zero-field muon spin relaxation studies of Mu@C₇₀ as a function of temperature reveal the appearance of the Mu triplet precession upon orientational ordering of the fullerene at 270 K, signaling the presence of an axially symmetric hyperfine interaction. A small 2p_z admixture is found in the ground-state wave function of Mu@C₇₀; as a result, the electron density reproduces perfectly the internal shape of the fullerene skeleton, including the small "pinching" in the equator.