Thermionic emission is the process whereby charged particles (usually electrons), known as thermions, are emitted from the surface of a heated material, usually a metal. The process is driven by the extra thermal energy that is added to the system. As a result, energised electrons can overcome the surface barrier of the material and escape into the surrounding space. This phenomenon has been used in various applications, including vacuum diodes, cathode ray tubes and, more recently, mass spectrometry.
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