QUANTUM EFFECTS IN ELEMENTARY SURFACE-REACTIONS - CO DIFFUSION ON NI(111)

Quantum and classical calculations of equilibrium rate constants for surface diffusion have been carried out using a model of CO adsorbed on Ni(111). Quantum transition-state theory (TST) rates were calculated using a Monte Carlo path integral method. Arrhenius plots of both the quantum and classical rate constants are linear from 100 to 300 K. Over this range, the quantum activation energy is slightly lower than the classical value due to a difference between zero-point energies in the reactant and transition states. At temperatures below 100 K, quantum rates are significantly higher than expected from the high-temperature Arrhenius plot. This is the result of quantum tunneling effects. The density matrix elements used to derive the quantum TST rate are also used to illuminate the differences between classical and quantum mechanics for this model.