Experimental determination of a longitudinal phonon dispersion curve in a quasi-two-dimensional system

The structural and dynamical properties of a monolayer of Xe atoms adsorbed on a Cu(001) surface have been studied with high-resolution He-atom scattering. Angular distributions reveal a phase-transition from the high-temperature in commensurate phase to a new, more densely packed, phase upon cooling below 65 K. For both phases the strongly dispersive longitudinal phonon mode could be detected. A lattice-dynamical analysis 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.