Classical and Quantum Diffusion of Hydrogen Atoms on Cu(111) X.D. Zhu , Department of Physics, University of California at Davis Surface diffusion of light atoms such as hydrogen and its isotopes on metals serve as excellent model systems for studying classical and quantum dissipation effects of low energy excitations on a solid surface. Our earlier experimental study of hydrogen diffusion on Ni(111) revealed that the classical over-hopping motion (CL regime) of a hydrogen adatom crossed over to under-barrier tunneling (QM regime) between the vibrational excited states of the atom (black dash line). As a result, in the QM regime, the diffusion rates are characterized with an activation energy close to that of vibrational excitation energy. In the present study, we studied the diffusion of H adatom on Cu(111) from 85K to 210K where we again observed the transition from classical over-barrier hopping to under-barrier tunneling. However unlike hydrogen adatoms on Ni(111), the diffusion rates in the quantum regime for hydrogen on Cu(111) is characterized by an activation energy Ea = 22 meV and D0 = 6.6×10-14 cm2/sec. Since Ea = 22 meV in the QM regime is much smaller than the in-plane vibrational excitation energy of 100 meV for hydrogen atoms on Cu(111), the under-barrier tunneling must occur between the ground states of the atom at hcp sites and fcc sites (red dash line). The small activation energy can be either attributed to the polaron effect or the binding energy difference between an hcp site and a fcc site. QM regime CL regime E1(in-plane) Esaddle Eg hcp fcc