We present a theoretical and experimental study on the phonon dynamics of the low-temperature Sn/Ge(1 1 1)-(3 x 3) structure. High-resolution helium atom scattering (HAS) data show that, besides the Rayleigh wave, there are three surface phonon branches with low dispersion related to the (3 x 3.) surface phase. Their energies are approximately 6.5, 4, and 3 meV at the Gamma point. In addition, we detect phonon peaks in the Q range 0.4-0.5 Angstrom(-1) at similar to2 meV, which correspond to (3 x 3) folding of the Rayleigh wave. Ab initio DFT-GGA total energy calculations have been performed to determine the frequencies associated with the vertical displacements of the three Sn atoms in the unit cell. The values obtained are in good agreement with the experiment. (C) 2004 Elsevier B.V. All rights reserved.