Spontaneous emission luminescent. And there are different forms of luminescence depending on how excited atoms are produced (electroluminescence, chemiluminescence etc.). If the excitation is affected by the absorption of radiation the spontaneous emission is called fluorescence. Sometimes molecules have a metastable level and continue to fluoresce long after the exciting radiation is turned off; this is called phosphorescence. Figurines that glow in the dark are phosphorescent. Lasers start via spontaneous emission, then during continuous operation work by stimulated emission. Spontaneous emission cannot be explained by classical electromagnetic theory and is fundamentally a quantum process. The first person to derive the rate of
Stimulated emission given by where the proportionality constant "B" is known as the "Einstein B coefficient" for that particular transition, and "ρ"("ν") is the radiation density of the incident field at frequency "ν". The rate of emission is thus proportional to the number of atoms in the excited state "N", and to the density of incident photons. At the same time, there will be a process of atomic absorption which "removes" energy from the field while raising electrons from the lower state to the upper state. Its rate is given by an essentially identical equation, The rate of absorption is thus proportional