Electronic structure of hydrogen-like impurities in nearly-free-electron metals

Recent developments in our understanding of the electronic structure of hydrogen and its isotopes in simple metals is reviewed. The role of nonlinear electron density distribution in the interpretation of the electronic properties of hydrogen-like impurities is emphasized. Calculated Knight shifts and hyperfine fields at μ+ site and electric field gradients at cubic host nuclear sites due to interstitial μ+ are compared with recent experimental data. The feasibility of using positive muon as a probe of defect structure is discussed. Future experiments and theoretical investigations aimed at a deeper understanding of the electronic properties of μ+ are suggested. (Auth.)