High-resolution phonon study of the Ag(100) surface.

Using high-resolution electron energy loss spectroscopy the phonon dispersion of Ag(100) has been studied at two different sample temperatures of 86 and 300 K. The dominant feature in the spectra corresponds to the Rayleigh wave. Its full dispersion is determined along the ΓX high symmetry direction in the first and second Brillouin zones. The Rayleigh phonon maximum at the X point shows a redshift with increasing temperature. This is explained based on a surface anharmonicity with an anharmonicity constant of 0.014, comparable to the value reported for Cu(100). In the vicinity of the Γ point two additional phonon features have been discovered at about 110 and 160 cm(-1), which are tentatively assigned to high density of states features from the bulk phonon bands. However, the observed steep dispersion is in contrast to theoretical calculations. Along ΓX two surface resonance branches have been observed with maximum frequencies in the range of 90-110 cm(-1) near to the zone boundary. These branches agree with helium atom scattering data where available, but are not predicted by theory.