Measurements of Mu+NO termolecular kinetics up to 520 bar: isotope effectsand the Troe theory

The recombination reaction Mu + NO + MMuNO + M (M = He, N-2, CH4) has been studied by the muon spin relaxation/rotation (mu SR) technique up to 520 ba r at room temperature. The reaction remains in the low pressure regime thro ughout. The measured termolecular addition rate constant in N-2, 8.8 +/- 0. 3 x 10(-33) cm(6) s(-1), is essentially the same as that found in our earli er study at pressures below 60 bar [J. J. Pan et al., J. Phys. Chem., 1995, 99, 17160]. It is somewhat smaller in He, 7.7 +/- 1.0 x 10(-33), but large r in CH4, 12.8 +/- 2.0 x 10(-33). The Mu + NO reaction is about five times slower than the corresponding H + NO reaction. The strong collision limits of the rate constants for three H-isotopes (Mu, H, D) reacting with NO have been estimated with Troe's formalism for unimolecular dissociation in the low pressure regime, based on the ab initio potential energy surface of Gua dagnini et al. [J. Chem. Phys., 1995, 102, 774]. The Troe calculations give less than satisfactory agreement with experiment with the corresponding we ak collision factor, beta(c), higher than expected by a factor of similar t o 2 for H + NO. The calculated kinetic isotope effect in the strong collisi on limit for Mu/H is weaker than the measured effect by a factor of two giv ing an apparent large isotope effect in this factor, beta(c)(Mu) approximat e to 1/2,beta(o)(H), possibly due to mode specific collisional stabilizatio n.