PNNL

Abstract

The muonic atom 4Heu has the composition a++u-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 (0.113amu) 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 Heu + H2 - HeuH + H reaction in the gas phase. The first measurements, at 295K, give a rate constant of kHei = 4:1 - 0:7 x 10-16 cm3 molec-1 s-1. In comparison, variational transition state calculations give a value of 2:46 x 10-16 cm3 molec-1 s-1, some what below the measurement, despite the large error bar, raising the possibility that the calculations, on a nessentially exact potential energy surface, have underestimated the amount of quantum tunneling involved, even for this heavyH-atom isotope.