Studies of the Mean Displacement of Surface Atoms in the (100) and (110) Faces of Silver Single Crystals at Low Temperatures

Abstract

The intensities of back-diffracted low-energy (10-300-eV) electrons have been measured as a function of temperature for the (110) and (100) faces of single-crystal silver in the temperature range -195 to 85°C. From these data, the rms displacements $〈u_{\perp }〉$ of surface atoms perpendicular to the surface planes have been calculated and the magnitudes of the parallel components $〈u_{\parallel }〉$ estimated. The perpendicular components of the surface rms displacements on silver (110) and (100) surfaces are 60 and 100% larger than those of bulk atoms. There is little difference in magnitude between the parallel and perpendicular components for these faces, as has also been reported for the (111) face. The parallel components of the rms displacements did not exhibit anisotropy within the accuracy of the measurements, and the physical adsorption of several noble gases had no effect on any of these measurements. The surface rms displacement for silver and those other fcc metals studied are larger than predicted by theory, which assumes bulk force constants for the surface atoms. This suggests that force constants for surface atoms are smaller than those of bulk atoms.