Randall J. Mikula

Muonium addition reactions in the gas phase: Quantum tunneling in Mu + C2H4 and Mu + C2D4

The reaction kinetics for the addition of the muonium (Mu=μ+e−) atom to C2H4 and C2D4 have been measured over the temperature range 150–500 K at (N2) moderator pressures near 1 atm. A factor of about 8 variation in moderator pressure was carried out for C2H4, with no significant change seen in the apparent rate constant kapp, which is therefore taken to be at the high pressure limit, yielding the bimolecular rate constant kMu for the addition step. This is also expected from the nature of the μSR technique employed, which, in favorable cases, gives kapp=kMu at any pressure. Comparisons with the H atom data of Lightfoot and Pilling, and Sugawara et al. and the D atom data of Sugawara et al. reveal large isotope effects. Only at the highest temperatures, near 500 K, is kMu/kH given by its classical value of 2.9, from the mean velocity dependence of the collision rate but at the lowest temperatures kMu/kH≳30/1 is seen, reflecting the pronounced tunneling of the much lighter Mu atom (mμ=1/9 mp). The present M...

Kinetics of the Mu + H2 and Mu + D2 reactions from 610 to 850 K

Abstract The bimolecular rate constants for the thermal chemical reactions of muonium (Mu): Mu + H 2 → MuH + H and Mu + D 2 → MuD + D, have been measured in the temperature range 610–850 K. For the hydrogen reaction, log A (cm 3 molecule −1 s −1 ) = −9.6 ± 0.4 and E a = 13.8 ± 1.5 kcal mole −1 , while for the deuterium reaction log A (cm 3 molecule −1 s −1 ) = −9.2 ± 1.2 and E a = 17.2 ± 4.2 kcal mole −1 . These results compare well with variational transition state theory calculations carried out on the Liu—Siegbahn—Truhlar—Horowitz potential surface.

The formation and reactivity of the μ+ molecular ion NeMu+

Abstract Evidence for the formation and reactivity of the positive muon molecular ion NeMu + at room temperature in a low-pressure Ne moderator to which trace amounts of Xe, CH 4 , NH 3 or He have been added, is reported. A two-component relaxation of the diamagnetic muon spin rotation (μSR) signal is seen upon the addition of trace amounts of Xe to Ne: a fast relaxing component with bimolecular rate constant (3.6 ±) 0.6 × 10 −10 cm 3 atom −1 s −1 is thought to be due to thermal muonium formation in a charge-exchange process while the other slow relaxing component is attributed to a muon-transfer reaction, as in proton-transfer studies. With CH 4 or NH 3 added to Ne there is, at most, only a very slow relaxation seen, even though thermal muonium formation is expected, in analogy with Xe. These latter results may be due to very fast, possibly tunneling assisted, muon-transfer reactions, the first time that such processes have been at all characterized.

Muon capture in oxides using the lifetime method

Abstract The relative capture rate has been measured for muons stopping in 22 oxides. The experimental method was to detect the decay electrons and to use the unique lifetime signature to determine which element captures the muon.

Muonium spin‐exchange in low pressure gases: Mu+O2 and Mu+NO

The spin‐exchange cross sections σSE for muonium collisions with O2 and NO have been measured in weak transverse magnetic fields in low pressure moderator at 300 K. Average values from runs at 1 and 2.5 atm are σSE(Mu+O2)?1.2)×10−16 cm2 and σSE(Mu+NO) ?1.2)×10−16 cm2. These numbers are in qualitative agreement with those determined earlier by Mobley et al. in ∼40 atm. moderator pressure using a longitudinal field technique. Comparison with recent measurements of the corresponding cross sections for H atom spin exchange at 300 K reveals no isotope effect, consistent with expectations based on a random‐phase approximation.

A temperature dependent study of the spin exchange reactions of muonium with O2 and NO in the range 295 to 478 K

The temperature dependence of the gas phase spin exchange reaction rates for muonium collisions with O2 and NO has been measured over the temperature range 295 to 478 K. In both cases the bimolecular rate constant k(T) was found to vary essentially as T1/2, leading to the following temperature independent spin exchange cross sections: σSE(Mu+NO) = (10.5±0.6)×10−16 cm2 and σSE(Mu+O2) = (9.1±0.7)×10−16 cm2. These results are in quantitative agreement with the H+O2 and H+NO results reported by Gordon et al., demonstrating that there is no isotope effect in spin exchange in this temperature range.

Muonium formation and the “missing fraction” in vapors

The vapor phase fractional polarizations of positive muons thermalizing as the muonium atom (PM) and in diamagnetic environments (PD) has been measured in H2O, CH3OH, C6H14, C6H12, CCl4, CHCl3, CH2Cl2 and TMS, in order to compare with the corresponding fractions measured in the condensed phases. There is a marked contrast in every case, with the vapor phase results being largely understandable in terms of a charge exchange/hot atom model. Unlike the situation in the corresponding liquids, there is no permanent lost fraction in the vapor phase in the limit of even moderately high pressures (≈1 atm); at lower pressures, depolarization is due to hyperfine mixing and is believed to be well understood. For vapor phase CH3OH, C6H14, C6H12, and TMS therelative fractions are found to be pressure dependent, suggesting the importance of termolecular hot atom (or ion) reactions in the slowing down process. For vapor phase H2O and the chloromethanes, the relative fractions are pressure independent. For CCl4,PM=PD≈0.5 in the vapor phase vs.PD=1.0 in the liquid phase; fast thermal reactions of Mu likely contribute significantly to this difference in the liquid phase. For H2O,PM ≈ 0.9 andPD≈0.1 in the vapor phase vs.PD≈ 0.6 andPM≈0.2 in the liquid phase. Water appears to be the one unequivocal case where the basic charge exchange/hot atom model is inappropriate in the condensed phase, suggesting, therefore, that radiation induced “spur” effects play a major role.

μ+ Thermalization and muonium formation in noble gases

+One energy loss mechanism in u thermalization (in gases) is that due to charge exchange, in which muonium is repeatedly formed and lost in a series of charge-exchange cycles u*+e- ~ Mu, a process which depends on the ionization potential of the moderator gas but one in which no depolarization of the u+ is expected at ~I atm. pressure. However, if the time between collisions in a given energy regime can be made sufficiently long then additional depolarization may occur, which can provide further information on the chargeexchange process itself. Extensive data showing this effect has been found in gases; results for the noble gases are presented.