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Electron energy distributions arising from 2-6-keV protons grazingly incident on an Al͑111͒ surface are reported for two different azimuthal angles of incidence and for a range of observation angles. Although the protons have energies that are an order of magnitude below the generally accepted threshold for plasmon excitation, the spectra strongly suggest that excitation and subsequent decay of bulk plasmons takes place. To explain this remarkable observation we propose that a subthreshold proton may excite a bulk plasmon in a process in which not only energy and momentum are transferred to the plasmon but also, mainly momentum, to the target.

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