  We have investigated coupling constants in elementary electron-phonon
scattering processes on a graphite surface by the combined use of
high-resolution electron-energy-loss spectroscopy (HREELS) and very low-energy
electron diffraction (VLEED). HREELS is used to measure the modulations of
electron transition probabilities from incoming electrons in vacuum to outgoing
electrons in vacuum where the transition includes one-phonon scattering
processes inside a solid. Determining the electronic band structures of
graphite with VLEED, we defined electronic states of the solid surface that
electrons entered before and after scattering off phonons. Thus, we observed
that the measured electron transition probabilities significantly depended on
whether the electrons were in a bulk Bloch state or an evanescent state before
scattering off the phonons. This result clearly indicates that the measured
electron transition probabilities reflect the strength of the coupling
constants in the solid.
