Vibrational properties ofβ1-H andβ1-D on W(001): Electron-energy-loss measurements and lattice-dynamical calculations

High-resolution electron-energy-loss spectra are presented for ${\beta }_1$-H and ${\beta }_1$-D on W(001) for a variety of scattering geometries and scattering parameters. In addition to previously observed vibrational modes of this system corresponding to the symmetric stretch (130 meV), asymmetric stretch (160 meV), and wag (80 meV) modes, a new mode at 118 meV is observed and characterized. Angle-dependent studies of the vibrational peaks establish the dispersion of the hydrogen vibrational modes throughout the two-dimensional Brillouin zone. Lattice-dynamical calculations establish the origin of the 118-meV loss peak: It is found to be the optic version of the symmetric stretch mode, in which adjacent hydrogen atoms vibrate 180° out of phase.