Surface-phonon dispersion curves of Mo(001) were measured with high-resolution helium-atom scattering at three different surface temperatures, 1000, 750, and 400K, respectively. Time-of-flight spectra in the <100> and <110> directions reveal two distinct phonon modes. The mode at higher energies could only be observed in the first third of the Brillouin zone and disappeared for larger phonon wave vectors. The low-energy mode showed a remarkable softening in the <110> direction from the 8meV at 1000K to about 2meV at 400K at the wave vector Q = 1.1/Å, whereas, within experimental accuracy, no softening occurred in the <100> direction.

Evidence for a Soft-Phonon Mechanism in the Reconstruction of the Mo(001) Surface. E.Hulpke, D.M.Smilgies: Physical Review B, 1989, 40, 1338-40