Reaction Kinetics of Muonium with N2O in the Gas Phase

The thermal reaction Mu + N2O 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 N2O pressure quadratically in pure N2O and is proportional to both the N2O 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, MuN2O and MuO + N2. The measured total rate coefficients are much larger than those reported by Marshall et al. for H(D) + N2O, 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 MuN2O (and by implication, HN2O) 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.