We considered the experiments, in which longitudinal electromagnetic oscillations are excited by electron (Reflected Electron Energy Loss Spectrometry or REELS) and X-ray (X-ray Photo Electron Spectroscopy or XPS) scanning of solids. Peaks in the spectra of electron energy losses associated with the excitation of plasma oscillations of free (valence) electrons of a solid body were found. We considered (e, 2e) the experimental analysis of secondary electron energy spectra showing peaks corresponding to the energy of plasma oscillations (Langmuir waves). We also studied the electron bombardment-induced photon emission experiments in the energy range coinciding with the energy of plasmon excitations; we observed the energy of plasmon excitations. We noted that the introduction of the “plasmon vacuum” concept provided a consistent quantum-mechanical definition of plasmon relaxation by the emission of longitudinal photons. We identified the wavelength range in which the most intense emission of longitudinal quanta is observed. 1. Afanas’ev, V.P., Gryazev, A.S., Efremenko, D.S., Kaplya, P.S. Differential inverse inelastic mean free path and differential surface excitation probability retrieval from electron energy loss spectra // Vacuum, 2017, Vol. 136, pp. 146–155. Doi: 10.1016/j.vacuum. 2016.10.021.
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• longitudinal plasmons
• energy spectra
• secondary electrons
• electron bombardment-induced photon emission
• plasmon vacuum
• longitudinal photons