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Excerpt from Field Guide to Spectroscopy
Emission occurs when a substance goes from a higher energy state to a lower energy state, giving off a photon whose energy is equal to the difference in the energies of the states:
In 1917, Albert Einstein defined a rate of spontaneous emission as
rate of spontaneous emission = A · c
where c is the concentration of absorbers in the upper energy state and A is the Einstein coefficient of spontaneous emission and is given by
Einstein also demonstrated that there is also a rate of stimulated emission:
where B' is the Einstein coefficient of stimulated emission and ρ(ν) is the density of radiation having frequency ν. Einstein was able to show that B' = B, the coefficient of stimulated absorption. Further, the ratio of spontaneous emission to stimulated emission is proportional to ν3: