natural lifetime

of an excited state
symbol: $\Delta t$, unit: $\pu{s}$
https://doi.org/10.1351/goldbook.08640
Lifetime of an excited state of a molecule that is isolated from radiation fields and other molecules or entities.
Notes:
  1. The lifetime is limited by the probability of spontaneous emission to a lower state. If an excited state, \(n\) can only emit spontaneously to a single lower state \(m\), the natural lifetime of state \(n\) equals the reciprocal of \(A_{mn}\), the Einstein transition probability of spontaneous emission.
  2. SI unit: \(\pu{s}\)
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)