Abstract
Some twenty years ago Ruthemann and Lang shot a monoenergetic beam of electrons in the keV range through a thin foil of aluminum and measured the energy distribution of the emergent electrons. They noticed that while most of the beam penetrated the metal without an appreciable loss of energy, there was a distinct group of electrons which lost very nearly 15 eV. When a different metal was substituted for aluminum, the same phenomenon was observed except that the characteristic energy loss differed from metal to metal. The explanation of this effect was not long in coming. Some of the electrons lost a certain definite amount of energy by exciting longitudinal plasma oscillations inside the metal at a frequency ωp characteristic of the particular metal. This cost the electron a “quantum” of energy ℏωp, where ℏ is Planck's constant; Pines called this quantum the “plasmon”. The experiment, and the Bohm and Pines theory of it, marked the beginning of research into the properties of solid-state plasmas.
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