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The 'quantum yield' of a radiation-induced process is the number of times that a defined event (usually a chemical reaction step) occurs per photon absorbed by the system. Thus, the quantum yield is a measure of the efficiency with which absorbed light produces some effect.
For example, in a chemical photodegradation process, when a molecule falls apart after absorbing a light quantum, the quantum yield is the number of destroyed molecules divided by the number of photons absorbed by the system. Since not all photons are absorbed productively, the typical quantum yield will be less than 1.
Quantum yields greater than 1 are possible for photo-induced or radiation-induced chain reactions, in which a single photon may trigger a long chain of transformations. One example is the reaction of hydrogen with chlorine, in which a few hundred molecules of hydrochloric acid are typically formed per quantum of blue light absorbed.
In optical spectroscopy, the 'quantum yield' is the probability that a given quantum state is formed from the system initially prepared in some other quantum state. For example, a singlet to triplet transition quantum yield is the fraction of molecules that, after being photoexcited into a singlet state, cross over to the triplet state. The 'fluorescence quantum yield' is defined as the ratio of the number of photons emitted to the number of photons absorbed.

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Quantum Efficiency