The muonic He atom and a preliminary study of the ⁴He μ + H₂ reaction

The muonic atom ⁴He μ has the composition α⁺⁺ μ^- e^-, and is formed by stopping negative muons in He doped with a small amount of NH₃ (or Xe). It may be regarded as a unique heavy H-atom isotope with a mass of 4.1 amu. As such, the study of its chemical reaction rates and comparison with those of the well-known light Mu atom (0.113 amu) allows unprecedented tests of kinetic isotope effects over a range of 36 in mass. As a first example, and one which is of most fundamental interest, we have begun kinetics studies of the He μ + H₂ → He μ H + H reaction in the gas phase. The first measurements, at 295 K, give a rate constant of k_{He μ} = 4.1 ± 0.7 × 10^{- 16} cm³ molec^{- 1} s^{- 1}. In comparison, variational transition state calculations give a value of 2.46 × 10^{- 16} cm³ molec^{- 1} s^{- 1}, somewhat below the measurement, despite the large error bar, raising the possibility that the calculations, on an essentially exact potential energy surface, have underestimated the amount of quantum tunneling involved, even for this heavy H-atom isotope.