ABSTRACT
This chapter examines what needs to be known of the muon or muonium chemical state in order to understand the evolution of the spin states. Paramagnetic muonium states are characterised by hyperfine and super-hyperfine interactions which greatly alter the energy levels and give rise to distinctive muon spin rotation or level-crossing spectra when the combined muon-electron spin eigenstates are reasonably long-lived. In diamagnetic metals and in closed-shell molecular species, the muon spin rotation or resonance spectrum displays a single line, displaced from the Larmor frequency by a small Knight shift (metals) or chemical shift (molecules) and broadened by any nuclear dipolar interaction. Site assignments are usually possible from the orientation dependence of these features. More information is available on local electronic structure when the muon has quadrupolar neighbours, via level crossing resonance. Spin-lattice relaxation, when it occurs on a measurable timescale, gives dynamical rather than structural information.
