  The problem of longitudinal oscillations of an electric field and a charge
polarization density in QED vacuum is considered. Within the framework of
semiclassical analysis, we calculate time-periodic solutions of bosonized
(1+1)-dimensional QED (massive Schwinger model). Applying the Bohr--Sommerfeld
quantization condition, we determine the mass spectrum of charge-zero bound
states (plasmons) which correspond in quantum theory to the found classical
solutions. We show that the existence of such plasmons does not contradict any
fundamental physical laws and study qualitatively their excitation in
(3+1)-dimensional real world.
