| Motivated by the recent experimental report of the coverage dependent quantum delocalization on H/Cu(110) at low temperatures and coverage, where H tunnels through the "flat" potential energy barrier between the adjacent binding sites along a troughlike channel instead of hopping over the barrier, we have studied both H and D on the Cu(110) surface at an infinite dilution limit using the quantum Monte Carlo method and the embedded atom method (EAM) model potential. We have not found any evidence for the delocalization. In fact, they exhibit the much localized behavior owing to the harmonic nature of the potential energy surface (PES). However, the disagreement with the experimental results may not be contradictory in that the EAM potential used in the present study might have misrepresented the interactions between the H atom and the Cu(110) surface. In light of this, we have employed a first principles calculation method based on the density functional theory to investigate the energetics of H/Cu(110) at 1/8 monolayer (ML) coverage. The PES is markedly different from and significantly more complex than that predicted by the EAM calculations. In particular, in view of the relatively flat region between adjacent pseudo-threefold sites along the cross-channel direction, we speculate that the hydrogen atom motion at low coverages may be two-dimensional rather than quasi-one-dimensional in character. Finally, we have compared the chemisorption of H at the highly symmetric sites on Cu(100), (110), and (111) at 1 and 1/4 ML to investigate the effects of the crystal orientation and the coverage on the energetics, the bonding geometry, and the vibrational frequency. The strong, repulsive H-H interaction has been found to have the significant effect on the diffusion barrier. The effect is more pronounced on the most closely packed (111) surface. However, in comparison to the results on both P and Ni surfaces, the PES is relatively flat on all three surfaces. Also, we have studied the hydrogen induced reconstructions on the (110) surface. At the coverages above 1/2 ML, the missing-row reconstruction has been found to be energetically more stable than the adsorption on the unreconstructed surface. Below that coverage, the zigzag structure turns out to be energetically most favorable. |
