Anomalous inelastic He-atom scattering intensities for the vibrational modes of alkali metals on graphite

Low-energy (2.3–4.4 meV) vibrational modes have been observed from K, Rb, and Cs chemisorbed on the basal plane of graphite using inelastic He-atom scattering. These modes are interpreted as phonons propagating in the sagittal plane and polarized parallel to the surface plane. Modes having perpendicular polarization were undetectable. This anomalous result may be due to the coupling of the He atoms to the conduction electrons in the surface.