PNNL

Abstract

The neutral muonic helium atom 4Heµ, in which one of the electrons of He is replaced by a negative muon, may be effectively regarded as the heaviest isotope of the hydrogen atom, with a mass of 4.115 amu. We report details of the first muon spin rotation (µSR) measurements of the chemical reaction rate constant of 4Heµ with molecular hydrogen, 4Heµ + H2 ? 4HeµH + H, at temperatures of 295.5, 405, and 500 K, as well as a µSR measurement of the hyperfine coupling constant of muonic He at high pressures. The experimental rate constants, kHeµ, are compared with the predictions of accurate quantum mechanical (QM) dynamics calculations carried out on a well converged Born–Huang (BH) potential energy surface, based on complete configuration interaction calculations and including a Born–Oppenheimer diagonal correction. At the two highest measured temperatures the agreement between the quantum theory and experiment is good to excellent, well within experimental uncertainties that include an estimate of possible systematic error, but at 295.5 K the quantum calculations for kHeµ are below the experimental value by 2.1 times the experimental uncertainty estimates. Possible reasons for this discrepancy are discussed. Variational transition state theory calculations with multidimensional tunneling have also been carried out for kHeµ on the BH surface, and they agree with the accurate QM rate constants to within 30% over a wider temperature range of 200–1000 K. Comparisons between theory and experiment are also presented for the rate constants for both the D + H2 and Mu + H2 reactions in a novel study of kinetic isotope effects for the H + H2 reactions over a factor of 36.1 in isotopic mass of the atomic reactant. © 2011 American Institute of Physics. [doi:10.1063/1.3657440]