EXPERIMENTAL-DETERMINATION OF A LONGITUDINAL PHONON-DISPERSION CURVE IN A QUASI-2-DIMENSIONAL SYSTEM

The structural and dynamical properties of a monolayer of Xe atoms ads orbed on a Cu(001) surface have been studied with high-resolution He-a tom scattering. Angular distributions reveal a phase-transition from t he high-temperature in commensurate phase to a new, more densely packe d, phase upon cooling below 65 K. For both phases the strongly dispers ive longitudinal phonon mode could be detected. A lattice-dynamical an alysis yields a substrate mediated softening of the Xe-Xe radial force constant down to 25% of that obtained from gas-phase potentials and a fit of Xe bulk phonon data. This decrease is much larger than assumed in previous work for noble gas atoms adsorbed on metals. (C) 1996 Ame rican Institute of Physics.