Surface Adsorbed Free Radicals Observed by Positive Muon Avoided Level Crossing Resonance
Cyclohexadienyl type radicals are produced by positive muon irradiation of benzene adsorbed at the surface of silica. They contain the muon as a polarized spin label. Cross relaxation effects are observed when a longitudinal magnetic field is scanned through regions of avoided crossings of magnetic energy levels. This permitts the identification of the radicals and the study of surface motional dynamics. Complete randomisation of orientations on a timescale of 10 ns is ascribed to translational diffusion around the grains.