08641 IUPAC Gold Book - natural linewidth 10.1351/goldbook.08641 08641 current 1 Line width of a molecular spectral band that arises from the probability of spontaneous emission in the absence of radiation fields and interaction with other molecules or entities. The natural linewidth, measured as the full width half maximum equals \(1/(2\pi c_{0} \Delta t)\), where \(\Delta t\) is the natural lifetime of the excited state. If the excited state \(n\) can only emit spontaneously to a single lower state \(m\) the natural linewidth is \(\pu{1.86E-38}\ \tilde{\nu}_{mn}^3 \langle m|\boldsymbol{\mu}|n \rangle^{2}\), when the transition wavenumber \(\tilde{\nu}_{mn}\) is in \(\pu{cm-1}\) and the dipole moment \(\mu\) is in Debye (\(\pu{1D \approx 3.335E-30 C m}\)). In the infrared, typical natural linewidths are approximately \(\pu{E-7 cm-1}\). full width half maximum https://goldbook.iupac.org//terms/view/08251 natural lifetime https://goldbook.iupac.org//terms/view/08640 spectral band https://goldbook.iupac.org//terms/view/08285 PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 755 (https://doi.org/10.1515/pac-2019-0203) https://goldbook.iupac.org/terms/view/08641/html https://goldbook.iupac.org/terms/view/08641/json https://goldbook.iupac.org/terms/view/08641/plain Citation: 'natural linewidth' 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.08641 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. If you are interested in licensing the Gold Book for commercial use, please contact the IUPAC Executive Director at executivedirector@iupac.org . 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. 2026-06-28T16:25:48+00:00