Release of electrons from parent atoms on absorption of energy in excess of normal average. This can arise from: 1 thermal thermionic agitation, as in valves, Coolidge X-ray tubes, cathode-ray tubes; 2 secondary emission of electrons, which are ejected by impact of higher energy primary electrons; 3 photoelectric release on absorption of quanta above a certain energy level; 4 field emission by actual stripping from parent atoms by a strong electric field.

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