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Stimulated emission is the emission of a photon by an excited atom, but specifically due to a photon of the same wavelength approaching and passing by, which can happen if the exited atom happens to be ready to spontaneously emit a photon of the same photon energy as the passing photon. In the "stimulated" case, the emitted photon has the same characteristics as the photon that stimulated the emission, including its direction. Such stimulated emission is also referred to as negative absorption: on a macro scale, absorption suggests EMR passing through some material is attenuated, but if stimulated emission is occurring, it is possible that more EMR at the wavelength exits than entered. Masers and lasers are based on this effect, by setting up a configuration so that the "doubling" of photons happens repeatedly.
Generally, the number of atoms (or molecules) at the level of excitement to produce stimulated emission is such that it does no more than contribute to the balance of atoms absorbing and emitting the wavelength. But if many more atoms are in the excited state (a condition termed population inversion), then photons triggering occurrences result in more such photons, positive feedback (a runaway process, lasting until the population inversion is sufficiently reduced or the photon(s) exit the region). Given population inversion, any single photon produced by spontaneous emission can trigger a cascade. Processes that produce such a population inversion are termed pumping, the processes requiring the energy to produce the necessary state of excitation.
Albert Einstein predicted stimulated emission, deducing the need while working out the effects of absorption and spontaneous emission: without the existence of such stimulated emission, a parcel of gas undergoing spontaneous emission could heat a second parcel to a temperature higher than its own, which the laws of thermodynamics do not allow.
Astrophysical masers occur in regions where such a population inversion has formed.