We report muon spin rotation/relaxation measurements of muonium in mesoporous silica (SBA-15) with a high specific surface area of 600 m2/g. Up to 70 percent of the incoming muons form muonium and escape efficiently into the open pores at all temperatures between 3 and 300K. We present evidence that the interaction with the silica surfaces involves both spin exchange and a transition to a diamagnetic state, possibly due to dangling bonds on the surface. At very low temperatures, below 20K, the interaction between muonium and the silica surfaces is suppressed due to a He film coating the surfaces. These results indicate that it should be possible to use muonium to probe the surfaces of uncapped nanoparticles supported in silica.

μSR and ALCR techniques have been used to investigate the structure and dynamics of the Mu-cyclohexadienyl radical interacting with Au and Pt metal nanoparticles (MNPs) supported in mesoporous silica (SBA-15). Surprisingly, coherent precession signals are observed and the isotropic hyperfine coupling constants are almost the same in loaded and unloaded samples, implying that the electronic structure of MuC6H6 is only weakly perturbed by the presence of the MNPs. We propose the observed radicals are shielded from the metallic surfaces by a benzene coating on the MNPs. The Δ1 resonance is observable in MNP-loaded samples at higher temperatures than in the unloaded SBA-15. This is attributed to stronger binding of MuC6H6 to the benzene coated MNPs.