Stark-Einstein law

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Stark–Einstein law is called after the German-born physicists Johannes Stark and Albert Einstein, who independently formulated the law between 1908 and 1913. Known also as the photochemical equivalence law or photoequivalence law. It states that for every quantum of radiation that is absorbed, one molecule of the substance reacts. A quantum is a unit of electromagnetic radiation with energy equal to the product of a constant (Planck's constant, h) and the frequency of the radiation, symbolized by the Greek letter ʋ (n).

The photochemical equivalence law is also restated as: for every mole of a substance that reacts 6.0225 $1023$ quanta of light are absorbed.

Δ E m o l = N A h ν

where N_A is Avogadro number.

The photochemical equivalence law applies to the part of a light-induced reaction that is referred to as the primary process (i.e. absorption or fluorescence).

In most photochemical reactions the primary process is usually followed by so-called secondary processes that are normal interactions between reactants not requiring absorption of light. As a result such reactions do not appear to obey the one quantum–one molecule reactant relationship.

The law is further restricted to conventional photochemical processes using light sources with moderate intensities; high-intensity light sources such as those used in flash photolysis and in laser experiments are known to cause so-called biphotonic processes; i.e., the absorption by a molecule of a substance of two photons of light.