Avoided crossing of vibrational modes in Ag(110) observed by He time-of-flight measurements

The phonon spectrum of the Ag(110) surface along the 〈11̄0〉 direction is studied by He inelastic scattering with time-of-flight detection. The Rayleigh mode and two surface vibrational resonances are described up to the surface Brillouin-zone boundary. The dispersion curves of the resonances show the avoided crossing behavior predicted but never observed for the (110) surfaces of fcc metals. Surface lattice-dynamics calculations using a simple central nearest-neighbor forceconstant model are performed and found to be in close agreement with the experiment.