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Momentum-resolved inelastic electron spectroscopy has been utilized to measure the surface-plasmon dispersion and damping for thick Hg films grown onto a Cu(100) substrate. Hg has been investigated since it offers an opportunity to examine the effects of the crystal potential on the surface-plasmon dispersion, and to correlate the measurements on a surface with those for collective excitations in clusters. The effects of the electronic band structure of Hg can be seen in the energy and width of the surface plasmon as a function of momentum transfer q∥. Dynamical response calculations were performed for a semi-infinite electron gas where several important aspects of the crystal potential are taken into account in an average manner. These effects are shown to lead to an overall lowering of the plasma frequency, to a flattening of the momentum dispersion, and to sizable broadening of the plasmon line width relative to the behavior obtained for the simple metals. Even though the band structure effects in Hg are significant, there is a quantitative correlation between the normalized energy and width of the surface plasmon as a function of q∥ and the Mie resonance seen in Hg clusters as a function of the inverse of the radius R.