Optical Pumping of Trapped Neutral Molecules by Blackbody Radiation

Optical pumping by blackbody radiation is a feature shared by all polar molecules and fundamentally limits the time that these molecules can be kept in a single quantum state in a trap. To demonstrate and quantify this, we have monitored the optical pumping of electrostatically trapped OH and OD radicals by room-temperature blackbody radiation. Transfer of these molecules to rotationally excited states by blackbody radiation at 295 K limits the $1/e$ trapping time for OH and OD in the $X^2{\Pi }_{3/2}$, $v^{\prime \prime }=0$, $J^{\prime \prime }=3/2(f)$ state to 2.8 and 7.1 s, respectively.

Figure 1

In the upper part, the room-temperature spectrum of blackbody radiation is shown. The relevant region for rotational excitation of OH and OD radicals is shown, with sticks indicating the lowest rotational excitation frequency. In the lower part the relevant rotational levels of OH and OD are depicted. Each rotational level is split into two $\Lambda$-doublet components with opposite parity, denoted by $f$ and $e$. The size of the splitting is exaggerated for clarity. The most relevant transition rates (in ${\mathrm{s}}^{-1}$) due to blackbody radiation connecting the $f$ components of rotational levels are indicated next to the arrows.

Figure 2

Scheme of the experimental setup. OH (OD) radicals are created by photodissociation of supersonically expanded (deuterated) nitric acid, and subsequently decelerated in a Stark decelerator. The slowed molecules are trapped in an electrostatic trap, and the population in the different rotational states is probed by laser-induced fluorescence detection. In the inset the arrival of undecelerated OD molecules followed by the steady signal of the trap-loaded OD molecules is shown, up to 20 ms after production of the molecules. We have detected the trapped molecules up to 15 s.

Figure 3

The population of electrostatically trapped OH and OD radicals in the $3/2(f)$ and $5/2(f)$ levels as a function of time. The squares are the measured data: red (gray) for OH, black for OD. The curves are the outcome of the rate-equation model using a constant background-gas collision rate of $0.17{\mathrm{s}}^{-1}$.