Relaxation effects on clean metal surfaces are at this time well known. Most metal surfaces exhibit an interlayer contraction at room temperature or below. This effect is physically understood from the loss of coordination of the surface atoms. Quite intuitive is the notion that the broken symmetry and loss of coordination at the surface leads to an enhanced anharmonicity experienced by the surface atoms. Therefore as the temperature is increased, an enhanced expansion of the first interlayer spacing is expected. MEIS studies of Ag(111) showed a dramatic effect. A smaller effect was observed on Cu(111). Theoretical modeling [ref] of the Ag(111) surface revealed that the anharmonicity normal to the surface was not responsible, and that the expansion is rather explained by a softening of in-plane vibrational modes.

      The much less closed packed fcc(110) surfaces exhibit numerous interesting phenomena, other than enhanced vibrations and thermal expansions, as the temperature is increased. These effects include order-disorder transitions such as roughening, pre-melting, and surface melting. Again, the loss of coordination and enhanced anharmonicity are usually held responsible. The detailed microscopic mechanisms of these disordering processes are not well understood yet. Molecular dynamics simulations are probably the most straightforward method for investigating this type of problem. Experimental studies are important for testing predictions of the simulations, which in turn ultimately test the validity of the potentials used. Recently, we have studied the temperature dependent vibrations and interlayer spacings of Ag(110) and Al(110).

Example scattering data from Ag(110) (surface peak versus scattering angle) for several temperatures. The solid lines are best-fit simulation results. The dashed vertical line indicates the bulk blocking direction at 60o.

      In the case of Al(110), evidence was found to support a novel vacancy formation mechanism predicted by ab initio molecular dynamics simulations [ref] in which atoms from the second metal layer can pop out onto the surface.