Phonons in a quasi-two-dimensional solid: Cesium monolayer on Cu(001)

The phonon dispersion curves of a well-ordered Cs monolayer epitaxially gro wn on Cu(001) were measured over the entire Brillouin zone using high-resol ution He atom scattering. In addition to the Rayleigh mode an additional pe rpendicular resonance near the <(Gamma)over bar> point and a longitudinal f ilm mode were observed. The appearance of a distinct longitudinal mode, whi ch was not observed for the corresponding monolayer films of Na or K, is at tributed to the small interface corrugation seen by the Cs atoms and the ph onon velocity mismatch between the film and substrate. Thus the lateral mot ion of the film is effectively decoupled from the substrate and reveals a q uasi-two-dimensional phonon behavior.