Direct Measurement of the Radiative Lifetime of Vibrationally Excited OH Radicals

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 $59.0\pm 2.0\mathrm{ms}$, in good agreement with the calculated value of $58.0\pm 1.0\mathrm{ms}$.

Figure 1

Scheme of the experimental setup. A pulsed beam of OH ($v=0$) radicals is produced via photodissociation (193 nm) of ${\mathrm{HNO}}_3$ molecules seeded in Xe. Prior to entering the decelerator, part of the OH radicals are prepared in the $v=1$ state by Franck-Condon pumping via the $A{}^2\Sigma ^{+}$ state (upper left). OH radicals in the $J=3/2$ level of both the $v=0$ and the $v=1$ vibrational states of the $X{}^2\Pi _{3/2}$ electronic ground state are Stark decelerated and subsequently loaded into the electrostatic trap. Molecules in the trap are state-selectively detected by imaging the laser induced fluorescence onto a PMT.

Figure 2

Population of trapped OH radicals in the $X{}^2\Pi _{3/2}$, $v=0$, $J=3/2$ level (upper curve) and in the $X{}^2\Pi _{3/2}$, $v=1$, $J=3/2$ level (lower curve), as a function of the storage time $t$. The trapping potential is switched on at $t=0\mathrm{ms}$.

Figure 3

Excitation spectrum of trapped OH ($v=0$) radicals that is recorded 5 ms (lower curve) and 72 ms (upper curve) after switching on the trap. In the central inset, the population of trapped OH radicals in the $v=0$, $J=5/2$ level is shown as a function of the storage time.