OPTICAL PHONONS AND DELOCALIZATION IN HYDROGEN OVERLAYERS - LATTICE-DYNAMICS WITHOUT DISPLACEMENT COORDINATES

The excited vibrational states of adsorbed hydrogen on metals are usually delocalized. As a consequence the inelastic e-scattering signal is strongly broadened. At high coverage the phonon dispersion is superimposed on this broadening. We apply the random phase approximation to this situation. As long as the vibrational ground state has negligible band width the broadening and dispersion tum out to be independent. Within tight binding approximation for the excited vibrational states the dispersion can be calculated from standard lattice dynamics, although for the excited states the concept of a displacement coordinate cannot be used. Experimental dispersion curves for hydrogen on Ni(110) can be fitted using a model with first-, second- and third-nearest-neighbor forces. The dependence of force constants on distance turns out to be unusual, indicating strong indirect interactions and probably even non-central forces.