the emission of light or other radiation from atoms or molecules that are bombarded by particles, such as electrons, or by radiation from a separate source. The bombarding radiation produces excited atoms, molecules, or ions and these emit photons as they fall back to the ground state

such an emission of photons that ceases as soon as the bombarding radiation is discontinued

such an emission of photons for which the average lifetime of the excited atoms and molecules is less than about 10–8 seconds