Anomalous linewidth behavior of the S3 surface resonance on Ag(110)

We report detailed He atom scattering measurements of the phonon linewidth for the Rayleigh mode S-1 and the resonant mode S-3 of Ag(110) along the whole <(Gamma) over bar><(Y) over bar> side of the surface Brillouin zone as a function of the sample temperature. The novel behaviour observed for the resonance linewidth is explained by including the mixed-mode nature into the anharmonic calculations performed within the framework of the Born-von Karman model.