Surface dynamics measurements of silica glass

We present helium atom scattering experiments from the vitreous silica surface. Time-of-flight data of scattered He-atoms have been recorded at varying temperature and as a function of parallel momentum transfer. A dominant elastic peak and two broad features at approximate to 4 meV, one each on the phonon annihilation and creation side, are observed in all spectra. The thermal measurements between 127.0 and 368.5 K unveil clear Debye-Waller attenuation behavior of the elastic peak and indicate that scattering occurs in the single-phonon regime below 200 K. The root-mean-square vibration amplitude at room temperature is estimated to be 0.014 nm with a corresponding surface Debye temperature of 340 +/- 20 K and an effective mass of 18 a.u. The obtained values agree well with bulk values under the assumption of force constants at the surface reduced by one-half. The wave-vector dependence of the Debye-Waller exponent indicates a well depth of the atom-surface interaction potential of 10 +/- 2 meV. The broad inelastic features on the phonon creation and annihilation sides are identified as the surface manifestation of the boson peak phenomenon in amorphous materials. An unusual shifting of the boson peak modes to higher energies with increasing temperature is observed. Measurements at different parallel momentum transfers unveil a nondispersive behavior of the boson peak mode.