In recent years, the availability of high-resolution surface-sensitive experimental
techniques such as Helium atom scattering (HAS) and electron energy loss spectroscopy
(EELS) has opened new perspectives in the study of the dynamical properties
of solid surfaces. In particular, a great deal of experimental data are now
available on the phonon dispersions of surfaces of insulator, semiconductors, and
metals [1]. Among transition metals, the vibrational properties of the hydrogenated
W(llO) surface have attracted much attention because they display unusual and
unexpected features which are still poorly understood [2-6]. When a full monolayer
of hydrogen is adsorbed, the surface phonon spectrum undergoes a dramatic change
and an anomalous behavior appears
