  Helium atom scattering (HAS) studies of the H-covered Mo(110) and W(110)
surfaces reveal a twofold anomaly in the respective dispersion curves. In order
to explain this unusual behavior we performed density functional theory
calculations of the atomic and electronic structure, the vibrational
properties, and the spectrum of electron-hole excitations of those surfaces.
Our work provides evidence for hydrogen adsorption induced Fermi surface
nesting. The respective nesting vectors are in excellent agreement with the HAS
data and recent angle resolved photoemission experiments of the H-covered alloy
system Mo_0.95Re_0.05(110). Also, we investigated the electron-phonon coupling
and discovered that the Rayleigh phonon frequency is lowered for those critical
wave vectors. Moreover, the smaller indentation in the HAS spectra can be
clearly identified as a Kohn anomaly. Based on our results for the
susceptibility and the recently improved understanding of the He scattering
mechanism we argue that the larger anomalous dip is due to a direct interaction
of the He atoms with electron-hole excitations at the Fermi level.
