Muons as Sensitive Isotopic H-atom Probes of Quantum Mass Effects in Chemical Reactivity and of the Motion of Muoniated Free Radicals in Nanoparticle Environments.

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The study of chemical reaction rates is central to the whole field of chemistry and in this study it is often the most simple elementary steps in a complex mechanism that are the most important. Chief among these are the reaction rates of the H-atom and its isotopic variants; deuterium (D), with a mass of 2 amu, being the most well known. Isotopes have the same number of electrons but different nuclear masses. Another common hydrogen isotope is tritium (T), with a mass of 3 amu, but being dangerously radioactive, it has seen little use in the study of thermal reaction rates. Thus it has fallen to nuclear science and particularly muon science to extend the isotopic H-atom mass scale. The lightest isotope of the H-atom is Muonium (Mu= u+e-), where `u+' is a positive muon, with a mass only 1/9th that of the proton. Though lacking a "nucleus'' in the conventional sense, the muon mass is still 200 times the electron mass, meaning that Mu and H atoms exhibit the same chemistry, but with very different reaction rates due to their factor of nine difference in mass. This application is concerned with new directions in Mu reactivity from state- selected reactants, using lasers to populate molecular excited states. At the other end of the mass scale is the Muonic He atom (4Heu), where negative muons from the TRIUMF accelerator are captured by a Helium atom, replacing one electron in the process, thereby endowing 4Heu with a mass of 4.1 amu, the heaviest isotope of the H-atom. We are now in a position to compare the reaction rates of Mu and 4Heu over an unprecedented ratio of a factor of 36 in mass, also a key aspect of the present application. A possible practical aspect of our study of Mu reactivity is the formation of ''muoniated radicals'' and in particular their motional effects in important industrial catalysts like Zeolites, for which there is no other technique that can yield the same detailed results. This too is an important aspect of the present application, but extended to other environments, the study of muoniated radicals on Au nanoparticles.