Abstract:The thermal reaction rate of muonium with methane (and ethane) in the gas phase Muon polarizations are reported for nitrogen and ethane over a wide pressure range from below 1 to 200 atm for N z and up to 245 atm for C Z H 6 • The N2 measurements were made at ambient temperature, while those for C 2 H 6 were made at temperatures both above and below the critical temperature (305.3 K). This is the first ,uSR study of muonium and diamagnetic muon formation to cover the entire range from a low pressure gas to den… Show more
“… 6 Diamagnetic ( P D ) fractions in pure CO 2 , H 2 O, and C 2 H 6 as a function of density (for clarity, the error bars for C 2 H 6 and H 2 O data are not shown). Data for H 2 O and C 2 H 6 taken from refs and , respectively. The lowest density data point for water, represented by the black diamond, is from ref .…”
Section: Discussionmentioning
confidence: 99%
“…It therefore serves as a unique probe of the mechanisms of inhomogeneous, nonequilibrium radiation chemistry process, here in ScCO 2 in particular. Since previous μSR studies of the density dependence of radiolysis effects and the measurements of polarization fractions as a function of density over a broad range have only been carried out in hydrogen-containing fluids, , the present study is unique in that it allows an interpretation of these fractions in comparison with those in a hydrogen-free solvent.…”
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.
“… 6 Diamagnetic ( P D ) fractions in pure CO 2 , H 2 O, and C 2 H 6 as a function of density (for clarity, the error bars for C 2 H 6 and H 2 O data are not shown). Data for H 2 O and C 2 H 6 taken from refs and , respectively. The lowest density data point for water, represented by the black diamond, is from ref .…”
Section: Discussionmentioning
confidence: 99%
“…It therefore serves as a unique probe of the mechanisms of inhomogeneous, nonequilibrium radiation chemistry process, here in ScCO 2 in particular. Since previous μSR studies of the density dependence of radiolysis effects and the measurements of polarization fractions as a function of density over a broad range have only been carried out in hydrogen-containing fluids, , the present study is unique in that it allows an interpretation of these fractions in comparison with those in a hydrogen-free solvent.…”
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.
“…Indeed, more circumstantial evidence suggests that the principal muonium formation mechanism varies with density, with radiolysis favoured in dense liquids and charge exchange in low pressure gases. 13 Of particular importance to water as a medium for muonium formation, is rapid proton transfer. As indicated in eqn.…”
Muonium has been studied in muon-irradiated water over a wide range of conditions, from standard temperature and pressure (STP) up to 350 bar and up to 420[degree]C, corresponding to water densities from 1.0 down to 0.1 g cm. This is the first report of muonium in supercritical water. Muonium was unambiguously identified from its spin precession frequencies in small transverse magnetic fields. The hyperfine constant was determined and found to be similar to the published values for muonium in water at STP and in vacuum. Muonium was found to be long-lived over the whole range of conditions studied. The fraction of muons which form muonium was found to vary markedly over the density range studied. Correlation of the muonium fraction with the ionic product of water suggests a common cause, such as the rate of proton transfer between molecules involved in the radiolysis of water and the formation of MuOH, which competes with muonium formation.Peer reviewe
“…radiolysis track) and other methane molecules could occur, possibly leading then to an increase in the diamagnetic fraction as a function of density, as seen in the present data for methane above « 3 mol L-1 (Figure 4) and also in the ethane data reported earlier. 2 The energetics of reaction R2 have not been calculated…”
Section: Discussionmentioning
confidence: 99%
“…The muon spin rotation (gSR) technique, using spin-polarized positive muons, has proven to be a valuable tool in the field of (chemical) reaction dynamics, especially for thermal reaction processes.1 While the interpretation of the rate constants and mechanisms in thermal muonium (Mu = µ+ß") kinetics is reasonably straightforward, the interpretation of data for epithermal reactions may be difficult due to the many possible contributing mechanisms. 2 For the saturated-bond systems of interest herein, the muon polarization is observed in the Mu atom, with initial polarization ?Mu, or in a diamagnetic state, with PD. In addition, there will often be a "missing" or "lost" polarization PL, such that Pm + PD + Pi = 1…”
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