ac Electric Trap for Ground-State Molecules

We here report on the realization of an electrodynamic trap, capable of trapping neutral atoms and molecules in both low-field and high-field seeking states. Confinement in three dimensions is achieved by switching between two electric field configurations that have a saddle point at the center of the trap, i.e., by alternating a focusing and a defocusing force in each direction. The ac trapping of ${{}^{15}\mathrm{N}\mathrm{D}}_3$ molecules is experimentally demonstrated, and the stability of the trap is studied as a function of the switching frequency. A 1 mK sample of ${{}^{15}\mathrm{N}\mathrm{D}}_3$ molecules in the high-field seeking component of the $|J,K⟩=|1,1⟩$ level, the ground state of para-ammonia, is trapped in a volume of about $1{\mathrm{m}\mathrm{m}}^3$.

Figure 1

Trajectories of molecules along the $z$ direction of the ac trap as a function of trapping time, for a switching frequency of 1100 Hz. The block wave above the graph indicates whether the electric field configuration is focusing or defocusing along $\widehat{\mathrm{z}}$. The gray area shows the size of a package of molecules, which is seen to be largest when the electric field configuration is focusing. The two dotted lines are at 79 and 79.5 periods of switching, when the measurements on the size of the trapped sample shown in Fig. 4 are performed.

Figure 2

Experimental setup. A molecular beam of ${{}^{15}\mathrm{N}\mathrm{D}}_3$ molecules in the low-field seeking component of the $|J,K⟩=|1,1⟩$ level is decelerated and brought to a standstill in the ac electric trap. Before turning on the trap a transition from low-field seeking to high-field seeking levels can be made by applying a microwave pulse. Cross sections of the trap are shown with lines of equal electric field strength [contours are shown at 5, 10, 12.5, 15, 17.5, 20, 25, and $30\mathrm{k}\mathrm{V}/\mathrm{c}\mathrm{m}$, increasing from blue to red (online only)] for both electric field configurations, together with plots of the electric field strength along the $r$ and the $z$ direction.

Figure 3

Density of ${{}^{15}\mathrm{N}\mathrm{D}}_3$ molecules in low-field seeking (lfs) and high-field seeking (hfs) levels of the $|J,K⟩=|1,1⟩$ state at the center of the trap as a function of the switching frequency, after the trap has been on for 72 ms. The signal of the high-field seekers is scaled up by a factor of 5.

Figure 4

Spatial distribution along $\widehat{\mathrm{z}}$ of ${{}^{15}\mathrm{N}\mathrm{D}}_3$ molecules in high-field seeking states after trapping at 1100 Hz for 79 (left) and 79.5 (right) switching periods. A Gaussian distribution is fitted to the measurements and the width (FWHM) of this distribution is given.