  Neutral molecules, isolated in the gas-phase, can be prepared in a long-lived
excited state and stored in a trap. The long observation time afforded by the
trap can then be exploited to measure the radiative lifetime of this state by
monitoring the temporal decay of the population in the trap. This method is
demonstrated here and used to benchmark the Einstein $A$-coefficients in the
Meinel system of OH. A pulsed beam of vibrationally excited OH radicals is
Stark decelerated and loaded into an electrostatic quadrupole trap. The
radiative lifetime of the upper $\Lambda$-doublet component of the $X
^2\Pi_{3/2}, v=1, J=3/2$ level is determined as $59.0 \pm 2.0$ ms, in good
agreement with the calculated value of $57.7 \pm 1.0$ ms.
