Abstract <p>The paper examines the influence of the decay process of plasmon (Langmuir) excitations on secondary electron emission processes. An assessment of the lifetime of the plasmon excitation decay process is carried out. A connection is established between the relaxation process of plasmon excitations and the electron-photon emission yield. The electron-ion plasma of a solid body, interacting with an electron beam whose energy substantially exceeds the Fermi energy, is considered based on quantum electrodynamics. It is shown that the quantum description of plasmons leads to the concept of the electromagnetic vacuum of longitudinal Langmuir waves. The vacuum of longitudinal waves significantly alters the dielectric permeability of the solid-body plasma, resulting in the broadening and shifting of peaks associated with energy losses of fast electrons scattered by the solid. The interaction of plasmons with the plasmonic vacuum leads to the relaxation of plasma excitations and the generation of longitudinal photons. The relaxation mechanism of plasmons presented in this work helps to explain a number of seemingly anomalous phenomena, namely, the polarization of electron-photon emission observed at plasmonic frequencies and the features of the spectra of secondary electron emission at plasmonic energies observed during ion and electron bombardments. The paper presents a comparison of the spectra of electron-photon emission with the differential cross sections of inelastic energy losses of fast electrons due to plasmon excitation. Possible practical applications of the phenomena observed during the relaxation of collective excitations of solid-body plasma are discussed.</p>