High resolution electron energy loss spectroscopy (HREELS) is normally used to probe the vibrational modes of adsorbed molecules, which typically lie in the 20 - 400 meV range (80 - 3000 wavenumbers). However, at Warwick we generally employ HREELS to study plasmon excitations in semiconductor materials. These are the oscillations of free carriers (electrons and holes) and lie in a similar energy range to molecular vibrational modes (dependent mainly on the doping level). The dispersion, damping and energy of plasmons contains an enormous amount of information on the electronic properties of the near-surface region of a semiconductor. Further details of this technique can be found here.