Dynamics of quantum tunneling: Effects on the rate and transition path of OH on Cu(110)

Recent low-temperature scanning-tunneling microscopy experiments [T. Kumagai et al., Phys. Rev. B 79, 035423 (2009)] observed the possible quantum tunneling of hydroxyl groups between two equivalent adsorption configurations on Cu(110). Here we analyze the quantum nuclear tunneling dynamics of hydroxyl on Cu(110) using density-functional theory based techniques. We calculate classical, semiclassical, and quantum-mechanical transition rates for the flipping of OH between two degenerate energy minima. The classical transition rate is essentially zero at the temperatures used in experiment and the tunneling rate along the minimum-energy path is also much too low compared to experimental observations. When tunneling is taken into account along a direct path connecting the initial and final states with only a minimum amount of the oxygen movement the transition rate obtained is in much better agreement with experiment, suggesting quantum tunneling effects cause a deviation of the reaction coordinate from the classical transition path.