It is shown that the frequencies of logitudinal electric oscillations in solids satisfy the condition (ω)=0, where (ω) is the dielectric constant for the frequency ω/2π which can be obtained from the optical constants n, κ. Such longitudinal waves include so-called plasma oscillations. It is shown that energy losses of fast charged particles in discrete steps hωp (the plasma quantum) can be expected only if the quantity nκ/(n2 + κ2)2 has a sharp maximum at a frequency ωp. This holds for the alkali metals and for silver.
Fröhlich et al. (1955) studied this question.