quantumyield

Quantum yield, often denoted by Φ, is a dimensionless measure of the efficiency with which a photophysical or photochemical event proceeds per photon absorbed. In general, it is defined as the number of events of interest divided by the number of photons absorbed.

There are several contexts for quantum yield. For fluorescence, the fluorescence quantum yield ΦF is the fraction of absorbed photons that result in fluorescence; that is, the number of photons emitted divided by photons absorbed. For phosphorescence, ΦP similarly describes the fraction that result in phosphorescence. For photochemical reactions, the photochemical quantum yield Φpc is the number of molecules formed (or reactions that occur) per photon absorbed; in chain reactions Φpc can exceed 1.

Calculation can be expressed in terms of excited-state decay pathways. For a molecule in the excited state, the radiative decay rate is kf, nonradiative decay rate knr, and, for singlet states, intersystem crossing to a triplet state with rate kisc. The fluorescence yield is ΦF = kf/(kf + knr + kisc). Phosphorescence yields are described by analogous relationships involving the relevant radiative and nonradiative rates.

Measurement of quantum yields typically involves actinometry to determine the photon flux and, when possible, an integrating sphere to quantify absorbed photons and emitted photons or formed products. Yields are reported relative to standards and depend on factors such as solvent, oxygen content, temperature, and concentration. In photochemical contexts, yields can be less than, equal to, or greater than 1, depending on whether chain processes produce multiple products per absorbed photon.