Lifetime of a molecular entity, which decays by first-order kinetics, is the time
needed for a concentration of the entity to decrease to
of its original value,
i.e.,
.
Statistically, it represents the life expectation of the entity. It is equal to the
reciprocal of the sum of the first-order rate constants of all processes causing the
decay of the molecular entity.
Notes:
-
Mathematical definition:
with
the first-order rate constants for all decay processes of the decaying state.
-
Lifetime is used sometimes for processes, which are not first order. However, in such
cases, the lifetime depends on the initial concentration of the entity, or of a quencher and, therefore, only an initial or a mean lifetime can be defined. In this case it
should be called decay time.
-
Occasionally, the term half-life
(
)
is used, representing the time needed for the concentration of an entity to decrease
to one half of its original value, i.e.,
.
For first-order reactions,
.
Source:
PAC, 2007, 79, 293
(Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006))
on page 363
Cite as:
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by
A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997).
XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic,
J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8.
https://doi.org/10.1351/goldbook.
