  Recent experiments (R. A. Baragiola and C. A. Dukes, Phys. Rev. Lett. {\bf
76}, 2547 (1996)) with slow ions incident at grazing angle on metal surfaces
have shown that bulk plasmons are excited under conditions where the ions do
not penetrate the surface, contrary to the usual statement that probes exterior
to an electron gas do not couple to the bulk plasmon. We here use the quantized
hydrodynamic model of the bounded electron gas to derive an explicit expression
for the probability of bulk plasmon excitation by external charged particles
moving parallel to the surface. Our results indicate that for each ${\bf q}$
(the surface plasmon wave vector) there exists a continuum of bulk plasmon
excitations, which we also observe within the semi-classical infinite-barrier
(SCIB) model of the surface.
