Plasma Resonance in the Electrodynamics of Metal Films

When a fast electron passes through a thin metal film it can excite plasma oscillations which then re-radiate the energy in the form of photons. The emitted radiation will appear in a more or less sharp spectral line whose width is the sum of the intrinsic damping rate and the radiative natural line breadth. A completely equivalent formalism is provided by the extension of the theory of transition radiation (semi-infinite metal), to the case of a slab of metal bounded by two parallel planes. In the ideal case of a metal whose dielectric constant contains only a real lossless part, and no imaginary absorptive part, the predicted breadth and strength of the spectral line are the same in both theories. A further phenomenon which is suggested by the plasma-oscillation point of view is the possibility of a resonance in the reflection coefficient of a thin metal film at the plasma frequency, brought about by the collective response of the conduction electrons. The height and breadth of this resonance have been computed and should be observable in relatively ideal cases.