Vibrational modes of the K/Si(1 1 1)-(3×1) surface studied by high-resolution electron energy loss spectroscopy

We have investigated the vibrational modes of the K/Si(1 1 1)-(3×1) surface using high-resolution electron energy loss spectroscopy (HREELS). An energy loss peak was clearly observed in the HREELS spectra at 55 meV. The origin of the energy loss peak is assigned to be a surface phonon mode which has negligible dispersion. Taking our calculation based on a density-functional method into account, we have determined the atomic vibrational motions that produce the 55-meV peak.