Mean Displacement of Surface Atoms in Palladium and Lead Single Crystals

The intensity of specularly reflected [(00) reflection] low‐energy (5‐500‐eV) electrons has been measured as a function of temperature for the (100) and (111) faces of palladium and for the (111) face of lead in the temperature ranges 25°‐600°C and 25°‐225°C, respectively. From the data, the root‐mean‐square displacements, $\mathrm{〈u\perp 〉}$ , of surface atoms perpendicular to the surface planes and the surface Debye temperatures have been calculated. The mean vibrational amplitudes of surface atoms are 40%→100% larger than for bulk atoms. There is little difference between the surface mean displacements of different crystal orientations; $\mathrm{〈u\perp 〉}$ appears to be relatively insensitive to changes of surface structure or surface density. The experimental results correlate well with those obtained for platinum and silver single‐crystal surfaces. It is believed that all monatomic face‐centered cubic crystal surfaces should have mean displacements markedly larger than those in the bulk.