A molecular radical model for hydrogen and muonium in graphite

Detection of muon spin relaxation in graphite above room temperature, toget her with reports of a temperature-dependent muon Knight shift, suggest that the muon state in graphite is not electronically diamagnetic, as previousl y supposed. The involvement of a molecular radical formed by the chemical r eaction and bonding of interstitial muonium is proposed. These consideratio ns should, with due regard for isotope effects, apply similarly to hydrogen and are supported by simulations of hydrogen addition to a graphene fragme nt. Density functional calculations provide hyperfine parameters as well as a visualization of the singly occupied molecular orbital, This allows inte rpretation of the data in terms of the temperature-dependent occupancy of t his orbital and its rate of exchange with conduction electrons.