Title: quantum yield
Long Title: IUPAC Gold Book - quantum yield
DOI: 10.1351/goldbook.Q04991
Status: current
Index: quantity

Number of defined events occurring per photon absorbed by the system. 

Definition
The integral quantum yield is \[\mathit{\Phi}(\lambda) = \frac{\rm{number\ of\ events}}{\rm{number\ of\ photons\ absorbed}}\] For a photochemical reaction, \[\mathit{\Phi}(\lambda) = \frac{\rm{amount\ of\ reactant\ consumed\ or\ product\ formed}}{\rm{number\ of\ photons\ absorbed}}\] The differential quantum yield is \[\mathit{\Phi}(\lambda) = \frac{{\rm{d}}x/{\rm{d}}t}{q_{n,{\rm{p}}}^{0}[ 1 - 10^{-A(\lambda)} ]}\] where \({\rm{d}}x/{\rm{d}}t\) is the rate of change of a measurable quantity (spectral or any other property), and \(q_{n\rm{,p}}^{0}\) the amount of photons (\(\pu{mol}\) or its equivalent \(\rm{einstein}\)) incident (prior to absorption) per time interval (photon flux, amount basis). \(A(\lambda)\) is the absorbance at the excitation wavelength.

Notes
1) Strictly, the term quantum yield applies only for monochromatic excitation. Thus, for the differential quantum yield, the absorbed spectral photon flux density (number basis or amount basis) should be used in the denominator of the equation above when \(x\) is either the number concentration (\(C = N/V\)), or the amount concentration (\(c\)), respectively.
2) \(\mathit{\Phi}\) can be used for photophysical processes (such as, e.g., intersystem crossing, fluorescence and phosphorescence) or photochemical reactions.

Related Terms
- absorbance: https://goldbook.iupac.org//terms/view/A00028
- amount concentration: https://goldbook.iupac.org//terms/view/A00295
- fluorescence: https://goldbook.iupac.org//terms/view/F02453
- intersystem crossing: https://goldbook.iupac.org//terms/view/I03123
- measurable quantity: https://goldbook.iupac.org//terms/view/M03788
- phosphorescence: https://goldbook.iupac.org//terms/view/P04569
- photochemical reaction: https://goldbook.iupac.org//terms/view/P04585
- photophysical processes: https://goldbook.iupac.org//terms/view/P04647
- wavelength: https://goldbook.iupac.org//terms/view/W06659

Source
- PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 406 (https://doi.org/10.1351/pac200779030293)

Other Outputs
- html: https://goldbook.iupac.org/terms/view/Q04991/html
- json: https://goldbook.iupac.org/terms/view/Q04991/json
- xml: https://goldbook.iupac.org/terms/view/Q04991/xml

Citation: Citation: 'quantum yield' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.Q04991
License: The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms.
Collection: If you are interested in licensing the Gold Book for commercial use, please contact the IUPAC Executive Director at executivedirector@iupac.org .
Disclaimer: The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using.
Accessed: 2026-05-18T09:25:21+00:00