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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.

In an experiment using the MSR (muonium spin rotation) technique, the rate of the chemical reaction Mu + Cl 2 → MuCl + Cl has been measured in Ar moderator gas at ≈ 1 atm and 22°C, yielding a bimolecular rate constant k = (5.2 ± 0.4) × 10 10 l mole −1 s −1 . This is 4.3 times faster than the most recent room temperature rate constant for the analogous II atom reaction.

The MSR (muonium spin rotation) technique was used to measure the chemical reaction rate for Mu + F 2 → MuF + F in N 2 moderator at ≈ 1 atm from 295 to 383 K giving the Arrhenius expression: log 10 k (l/mole s) = (10.83 ± 0.20) - (200 ± 50)/ T , with k = (1.46 ± 0.11) × 10 10 l/mole s at 300 K. This is at least 6.8 times the room temperature rate constant for the analogous H atom reaction. The measured activation energy and enhancement over the H reaction rate are indicative of significant tunnelling in the Mu reaction, in agreement with the recent collinear quantum mechanical calculations of Connor et al.

Muonium atoms are found to undergo spin-conversion (triplet-to-singlet) with high probability in encounters with paramagnetic (but not diamagnetic) ions in aqueous solution. The spin-flip or spin-exchange process is compared in magnitude with electron-transfer reactions for eleven transition metal ions.

The interaction between Mu and O 2 in water, which could be a spin-conversion process, has a rate constant of (2.4 ± 0.5) × 10 10 M −1 s −1 and is compared with the diffusion-controlled limit and the corresponding gas phase reaction.

Muon spin rotation and level-crossing spectroscopy have been used to measure the muon, proton, deuteron and 13C hyperfine coupling constants for the isotopically substituted ethyl radicals CH2CH2Mu, CD2CD2Mu and 13CH213CH2Mu in the gas phase.

A beam of surface muons (4.1 MeV) was used to generate thermalised muonium atoms (μ + e − ) in aqueous solutions. From measurements of the muonium atom relaxation time in the presence of various solutes, bimolecular rate constants were determined and compared with the corresponding reactions of the much heavier isotope, the hydrogen atom.