Surface-adsorbed free radicals observed by positive-muon avoided-level-crossing resonance

Heterogeneously catalysed reactions usually proceed at outer or inner surfaces of materials that have large and often non-uniform surface area. Many intermediates of such chemical transformations are short-lived and escape direct detection; such transient species may include free radicals. The structure, adsorption and surface dynamics of diamagnetic intermediates have been well studied by thermodynamic or spectroscopic methods, but paramagnetic species (that is, radicals) recombine at catalytically relevant temperatures so that they are never present in sufficiently high concentrations to allow their observation by these techniques. The otherwise powerful laser methods are also hampered by the non-transparent nature of heterogeneous systems. Here we demonstrate for the model case of cyclohexadienyl radicals on silica that a novel technique, positive-muon avoided-level-crossing spin resonance, which uses positive muons as spin probes, is sufficiently sensitive to allow the detection and study of radicals under such constrained conditions. The technique can detect radical concentrations many orders of magnitude less than can be studied by conventional magnetic spectroscopy. (author)