Termolecular kinetics for the Mu + C O + M recombination reaction:A unique test of quantum rate theory

The room-temperature termolecular rate constants, k 0 , for the Mu + C O + M ⇌ Mu C O + M ( M = He , N 2 , Ar) recombination reaction have been measured by the μ SR technique, and are reported for moderator gas pressures of up to ∼ 200 bar (densities ≲ 0.4 × 10 22 molec cm − 3 ). The experimental relaxation rates reveal an unusual signature, in being dominated by the electron spin-rotation interaction in the MuCO∙ radical that is formed in the addition step. In N 2 moderator, k 0 = 1.2 ± 0.1 × 10 − 34 cm 6 s − 1 , only about 30% higher than found in Ar or He. The experimental results are compared with theoretical calculations carried out on the Werner-Keller-Schinke (WKS) surface [ Keller , J. Chem. Phys. 105 , 4983 ( 1996 ) ], within the framework of the isolated resonance model (IRM). The positions and lifetimes of resonance states are obtained by solving the complex Hamiltonian for the nonrotating MuCO system, using an L 2 method, with an absorbing potential in the asymptotic region. Accurate values of the vibrational bound and resonance states of MuCO reveal unprecedented isotope effects in comparisons with HCO, due to the remarkable effect of replacing H by the very light Mu atom ( m Mu ≈ 1 9 m H ) . Due to its pronounced zero-point energy shift, there are only two ( J = 0 ) bound states in MuCO. Contributions from nonzero J states to the termolecular rate constants are evaluated through the J -shifting approximation, with rotational constants evaluated at the potential minimum. The value of the important A constant ( 181 cm − 1 ) used in this approximation was supported by accurate J = K = 1 calculations, from which A = 180 cm − 1 was obtained by numerical evaluation. The calculations presented here, with a "weak collision factor" β c = 0.001 , indicative of the very sparse density of MuCO states, give a very good account of both the magnitude and pressure dependence of the experimental rates, but only when the fact that the two initially bound ( J = 0 ) states become resonances for J > 0 is taken into account. This is the first time in IRM calculations of atom-molecule recombination reactions where J ≠ 0 states have proven to be so important, thus providing a truly unique test of quantum rate theory.