Also contains definition of: natural lifetime
of an excited molecular entity in the absence of radiationless transitions.
The reciprocal of the first-order rate constant
for the radiative step, or the reciprocal
of the sum of these rate constants if there is more than one such step. The equivalent
natural lifetime, is discouraged.
PAC, 2007, 79, 293
(Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006))
on page 411