Q.What do you mean by electron emission? Explain briefly various methods of electron emission.

Electron emission is the general process by which free electrons, normally trapped inside a metal by an attractive potential (surface) barrier set up by the metal's positive nuclei near its surface, are given enough extra energy to cross that barrier and leave the surface altogether. Inside any metal, the outermost-shell electrons are loosely bound and move about freely and randomly even at room temperature, but this freedom is confined to the interior -- an electron reaching the surface is pulled back by the attraction of the nearby positive nuclei, and only electrons that acquire sufficient additional energy from some outside source can actually escape.

Four distinct types of electron emission are recognised, classified by the form of energy supplied. Thermionic emission heats the metal (for example, a filament in a vacuum tube) so that its surface electrons gain enough thermal energy to escape; this is the electron source in cathode-ray tubes, electron microscopes and X-ray tubes. Field emission applies a very strong external electric field directly across the metal surface, mechanically pulling free electrons out through the barrier without any heating at all; this is exploited in field-emission scanning electron microscopes and field-emission displays. Photoelectric emission shines electromagnetic radiation of a suitable frequency on the metal, transferring photon energy directly to individual free electrons and ejecting them; the number of electrons emitted this way scales with the intensity of the incident light, and this specific type of emission -- the subject of the whole photoelectric-effect discussion -- underlies photodiodes and photoelectric cells. Secondary emission occurs when a beam of already fast-moving electrons strikes the metal surface, transferring kinetic energy to the target's own free electrons and knocking some of them loose as secondary electrons; this mechanism is used in image intensifiers and photomultiplier tubes to amplify a weak initial signal into a much larger cascade of electrons.