Lattice dynamics of Kr films on the graphite (0001) surface by inelastic helium-atom scattering

Lattice dynamics of the Kr monolayer, bilayer, and trilayer physisorbed on graphite (0001) surface were studied by inelastic He-atom scattering (${\mathit{k}}_{\mathit{i}}$=5.77 A${\mathrm{̊}}^{\mathrm{-}1}$). The out-of-plane vibrational mode of the monolayer along both the Γ¯ M${\mathrm{¯}}_{\mathit{R}}$ and Γ¯ K${\mathrm{¯}}_{\mathit{R}}$ symmetry directions is essentially dispersionless with vibrational energy of 4.1 meV for Q≥0.3 A${\mathrm{̊}}^{\mathrm{-}1}$. For Q≤0.3 A${\mathrm{̊}}^{\mathrm{-}1}$, a strong interaction between adsorbate and substrate was indicated by the observation of an avoided-crossing behavior in the disperison curve and anomalous intensity distributions in the time-of-flight spectra. The disperison curves of the bilayer and trilayer, which are much lower in energy than that of the monolayer, appear to be the same and are not perturbed significantly by the substrate.