Muonium formation as a probe of radiation chemistry in sub- and supercritical carbon dioxide

Muonium (Mu = mu(+)e(-)), which can be considered a light isotope of the H atom, has been observed for the first time in supercritical CO2 (ScCo2). It is Unreactive on a time scale of a few microseconds and over a wide density range from well below to well above the CO2 critical density rho(c) = 0.47 g/cm(3). The fraction of muon polarization in muonium, P-Mu, does not vary significantly at low densities but changes quickly at the highest densities, approaching zero. This density dependence is reflected in a concomitant increase observed in the lost fraction of polarization, P-L, demonstrating that the dynamics of Mu formation and depolarization in ScCO2 is a direct probe of radiolysis effects in the terminal muon radiation track. In marked contrast to previous studies in hydrogen-containing solvents, C2H6 and H2O, over comparable density ranges, the diamagnetic fraction, P-D, was found to be almost independent of density in CO2, attributed to the formation of the stable solvated MuCO(2)(+) molecular ion in this hydrogen-free solvent. The differing density dependences of both the Mu and the diamagnetic fraction in Co-2, in comparison with the rather similar trends seen for both in C2H6 and H2O, supports previous claims of a significant role played by proton (muon) transfer reactions in the competing processes involved in Mu formation in hydrogen-containing solvents. In addition to this being the first report of radiolysis effects accompanying energetic positive muons stopping in ScCO2, it is the only report of end of track effects in this solvent, which has many applications in nuclear waste management and green chemistry. With a mass intermediate between that of the electron, which has provided most radiation-chemistry studies in ScCO2 to date, and the proton (or alpha-particle), implanted muons provide a unique data set, characteristic of higher LET radiation, that may be relevant to radiolysis effects induced in ScCO2 by alpha decay from heavy nuclei, for which there are no comparable studies.