Reaction Kinetics of Muonium with N2O in the Gas Phase

The thermal reaction Mu + N 2 O has been studied by the muon spin rotation (μSR) technique at temperatures from 303 to 593 K and pressures up to 60 atm. The overall reaction rate coefficient depends on the N 2 O pressure quadratically in pure N 2 O and is proportional to both the N 2 O partial pressure and the total pressure in mixtures, confirming the theoretical prediction of Diau and Lin that the analogous H atom reaction proceeds through two channels in this temperature range, forming different products, MuN 2 O and MuO + N 2 . The measured total rate coefficients are much larger than those reported by Marshall et al. for H(D) + N 2 O, indicating a dramatic kinetic isotope effect, which is mainly due to the enhanced quantum tunneling of the ultralight Mu atom. Even at room temperature (and low pressure), k Mu / k H ≈ 120, the largest yet seen in comparisons of gas-phase Mu and H reactivity at such relatively high temperatures. The addition reaction forming MuN 2 O (and by implication, HN 2 O) contributes significantly to the total reaction rate at higher pressures but with the thermal rate coefficient remaining in the termolecular regime even at the highest pressures measured.