Theoretical Studies of the Structure and the Dynamics on Clean and Chemisorbed Metal Surfaces

Molecular dynamics (MD) and lattice dynamics (LD) techniques are employed to investigate several phenomena related to the structure and vibrations at metal surfaces. The MD simulations are performed with the many-body interaction potentials obtained using the Embedded-Atom Method (EAM). As specific examples, we present the results for Ag(100) at 300 K and Cu(100) at 150 K, 300 K, and 600 K. The calculated frequencies and polarizations of all surface modes and resonances at the high-symmetry points in the two-dimensional Brillouin zone are in good agreement with available data, as well as, existing lattice dynamics results with force constants obtained from first-principles calculations. Our calculated surface relaxation is also in reasonable agreement with the data. We also test a much simpler lattice dynamics model with nearest neighbor central force interactions, and conclude that it can reproduce the main features of the phonon modes, but only when adjustable surface parameters are used. Additionally, the temperature dependent studies of the phonon line-widths and the mean-square displacement (MSD) of surface atoms are indicative of enhanced surface anharmonicity. On several chemisorbed metal surfaces, for which force constants are not available from first-principles calculations or the EAM, we perform lattice dynamics studies of phonon dispersion curves using simple force-constant models. These studies provide reliable mean-square displacement of surface atoms and can distinguish between possible reconstruction patterns, the results being insensitive to the exact values of the surface parameters. On c(2 times 2)S-Ni(100), it is found that the parallel component of the mean-square displacement for sulfur is around 50% larger than the vertical component, but for the mean-square displacement of oxygen atoms in the system c(2 times 2)O-Ni(100), the opposite is the case. As regards surface reconstruction, for both p(2 times 1)O-Ag(110) and p(2 times 1)O-Ni(110) surfaces, it is concluded that a substrate missing-row type reconstruction is induced by the adsorbates, but the local symmetry is C_{2v} with oxygen atoms at the long-bridge sites for the former and C_{s} with (110) being the only symmetry axis for the latter. In the above theoretical analysis, close contacts are made to many available experimental results such as surface phonon dispersion curves, interlayer relaxations, and Debye -Waller factors and adsorbate-substrate bond lengths.