  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 electronic susceptibility 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 compared to the clean surfaces. 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
electron-hole excitations by the He scattering.
