METAL AND ALLOY SURFACE STRUCTURE STUDIES USING MEDIUM-ENERGY ION SCATTERING

Many forms of disorder occur on metal surfaces as they are heated to their bulk melting temperature, and the microscopic physical mechanisms are not clear. Certain faces of single crystal metal surfaces are particularly prone to surface disordering, including melting at the surface. I have investigated the temperature dependent atomic structure on one such face of silver and aluminum using the technique of medium-energy ion scattering. From measurements of the interlayer separations, the degree and nature of the surface anharmonicity can be determined. For silver, a large surface thermal expansion (nearly 20 times larger than in the bulk) is followed by a pre-melting transition. On aluminum, component resolved measurements of the surface vibrational amplitudes reveal two possible precursors to disordering mechanisms. In-plane components of the vibrational amplitudes indicate that atoms on the surface move more easily in certain directions. Enhanced vibrational amplitudes in the second layer, normal iii to the surface, support a proposed vacancy formation mechanism where second layer atoms can jump onto the surface