Term: natural linewidth
https://doi.org/10.1351/goldbook.08641

Definition:
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.

Notes:
1) 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.
2) 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 \(\pu{E-7 cm-1}\).

Related Term: full width half maximum (http://doi.org/10.1351/goldbook.08251).
Related Term: natural lifetime (http://doi.org/10.1351/goldbook.08640).
Related Term: spectral band (http://doi.org/10.1351/goldbook.08285).

Source: 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)

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. https://doi.org/10.1351/goldbook.08641

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