HArF in solid argon revisited: transition from unstable to stable configuration.

The thermal conversion of HArF configurations in solid argon has been investigated both experimentally and theoretically. The matrix isolation experiments have been concentrated on temperatures 25-27 K, promoting the transition from the unstable to stable HArF configuration. The combined quantum mechanical-molecular mechanical and temperature-accelerated dynamics approach has been developed to study the real-time evolution of HArF trapped in different matrix-site morphologies. Two realistic pathways of the stable HArF formation are found for annealing at 25-27 K. The conversion mechanism in both pathways involves the local mobility of matrix vacancies in the vicinity of the HArF molecule. These two relaxation processes occurring within different timescales can cause the multiexponential decay of unstable HArF observed experimentally. The theoretical values of the activation energy of 64 meV as well as the corresponding pre-exponential factor of exp(28) s(-1), obtained for one of the unstable HArF configurations, are well consistent with the experimental estimates of 70 meV and exp(30 +/- 3) s(-1), respectively.