Muonium addition reactions in the gas phase : Quantum tunneling in Mu + C 2 H 4 and Mu + C

The reaction kinetics for the addition of the muonium (Mu = Jl, + e) atom to C2H4 and C2D4 have been measured over the temperature range 150-500 K at (N2) moderator pressures near 1 atm. A factor of about 8 variation in moderator pressure was carried out for C2H4, with no significant change seen in the apparent rate constant kapp , which is therefore taken to be at the high pressure limit, yielding the bimolecular rate constant k Mu for the addition step. This is also expected from the nature of the Jl,SR technique employed, which, in favorable cases, gives kapp = k Mu at any pressure. Comparisons with the H atom data of Lightfoot and Pilling, and Sugawara et al. and the D atom data of Sugawara et al. reveal large isotope effects. Only at the highest temperatures, near 500 K, is k Mulk H given by its classical value of2.9, from the mean velocity dependence ofthe collision rate but at the lowest temperatures kMulkH ~ 3011 is seen, reflecting the pronounced tunneling ofthe much lighter Mu atom (mIL = 1/9 mp )' The present Mu results should provide accurate tests of reaction theories on currently available ab initio surfaces.