Absorption of Electromagnetic Radiation by an Electron Gas

The total absorption cross section for a photon by an electron gas is calculated in the high-density limit. The calculation makes use of the Green's function formulation of the many-body problem. Only the simplest final states leading to a nonvanishing cross section, consisting of an electron-hole pair and a plasmon, are taken into account. This contribution has a threshold near the plasmon energy and is peaked owing to the initially rapid rise of the available phase space, with a total cross section for real metals of 10^-17-10^-19 cm² per electron. This compares favorably with other absorption mechanisms.