Probing the behaviour of surface-adsorbed free radicals using ALC μSR

Preliminary investigations show that the avoided level crossing muon spin rotation technique is well suited to the study of surface-adsorbed free radicals. The samples used were monolayer films of benzene and anisole on small (7 nm and 14 nm) amorphous silica grains. The observed cyclohexadienyl radical resonances indicate a reduction in mobility down to a temperature of about 125 K, beyond which measurements are unreliable due to weak signals. However, the failure to observe any anisotropy in the resonances shows that the residual mobility is mainly translational. For a benzene multilayer, on the other hand, we found clear evidence of anisotropic behaviour, co-existing with signals from a mobile phase. It seems likely that this arises from a thin fluid surface layer in equilibrium with frozen bulk. (orig.)