Muonium Formation as a Probe of Radiation Chemistry in Sub- and Supercritical Carbon Dioxide

Abstract

Muonium (Mu = μ+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 ρc = 0.47 g/cm3. 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 MuCO2 + molecular ion in this hydrogen-free solvent. The differing density dependences of both the Mu and the diamagnetic fraction in CO2, 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.

Presenting Muon Thermalization with Feynman QED

“…Our results are consistent with the experimental results by Percival group [10] and many experimental results by Brewer and Storchak (e.g. Refs [6] and [14]) as well as predictions in [11,13,14]. However, our preliminary results suggests that there is a significant probability of Mu forming in electronic excited states.…”

“…One model of Mu formation in condensed media is the spur model [9][10][11][12][13]. A simplified view of spur formation is presented in Fig.…”

Influence of confinement on free radical chemistry in layered nanostructures

“…The fact that we observe this electron/cation (muon) recombination (or interaction due to polaron generation) rates at the microsecond time scale suggests the existence of electron transfer processes with much slower rate than the ultrafast electron transfer leading to Mu formation, which is more than four orders of magnitude faster (faster than ~10 10 s −1 ) 19,20,23,54 .…”

“…The information on chemical dynamics is obtained in two ways; one is by following the sub-ns thermalization process that leads to the formation of different species, such as Mu and diamagnetic species 19,20,54,63 . This is obtained by the analysis of the amplitudes of time-dependent spectra of the different fractions (Fig.…”

“…The decay rates suggest that the diamagnetic fraction is mostly due to positive muons or to positive muons bound to oxygen lone pairs. Indeed, a molecular species like MuOH would have a slower reaction rate 23 . The spin relaxation of positive muons in a diamagnetic solid material is typically due to dipolar interactions between positive muons and dipoles.…”