Quantum diffusion of hydrogen on metal surfaces

A quantum mechanical theory is presented for the low temperature diffusion of atomic hydrogen on metal surfaces, based on a band model for the hydrogen motion. The theory is applicable to the diffusion of many interacting particles obeying quantum statistics. At low coverage the hydrogen band motion is limited by collisions between adsorbates, giving rise to a decrease of the diffusion constant with concentration. Other aspects of the hydrogen-hydrogen interaction are introduced to explain the coverage dependence at higher adsorbate concentrations. Comparison with recent low temperature diffusion measurements for H, D, and T on W(110) show that the above model reproduces satisfactorily the main features of the experimental coverage dependence of diffusion. The usefulness and limitations of band treatment for heavy particle diffusion are discussed in the light of these results.