  Surface effects on the electronic energy loss of charged particles entering a
metal surface are investigated within linear-response theory, in the framework
of time-dependent density functional theory. Interesting phenomena occur in the
loss spectra originated by the boundary (bregenzung) effect, which is as a
consequence of the orthogonality of surface and bulk excitation modes. Our
calculations indicate that the presence of a non-abrupt electron-density
profile at the surface severely affects the nature of surface excitations, as
deduced from comparison with simplified models.
