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The Journal of Physical Chemistry A

A Linear AC Trap for Polar Molecules in Their Ground State

Melanie Schnell, Peter Lützow, Jacqueline van Veldhoven, Hendrick L. Bethlem, Jochen Küpper, Břetislav Friedrich, Monika Schleier-Smith, Henrik Haak, Gerard Meijer

Published Jun 13, 2007• 37 citations
Publisher

Abstract

A linear AC trap for polar molecules in high-field seeking states has been devised and implemented, and its characteristics have been investigated both experimentally and theoretically. The trap is loaded with slow 15ND3 molecules in their ground state (para-ammonia) from a Stark decelerator. The trap's geometry offers optimal access as well as improved loading. We present measurements of the dependence of the trap's performance on the switching frequency, which exhibit a characteristic structure due to nonlinear resonance effects. The molecules are found to oscillate in the trap under the influence of the trapping forces, which were analyzed using 3D numerical simulations. On the basis of expansion measurements, molecules with a velocity and a position spread of 2.1 m/s and 0.4 mm, respectively, are still accepted by the trap. This corresponds to a temperature of 2.0 mK. From numerical simulations, we find the phase-space volume that can be confined by the trap (the acceptance) to be 50 mm3 (m/s)3.

DOI
10.1021/jp070902n

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A linear AC trap for polar molecules in high-field seeking states has been devised and implemented, and its characteristics have been investigated both experimentally and theoretically. The trap is loaded with slow 15ND3 molecules in their ground state (para-ammonia) from a Stark decelerator. The trap's geometry offers optimal access as well as improved loading. We present measurements of the dependence of the trap's performance on the switching frequency, which exhibit a characteristic structure due to nonlinear resonance effects. The molecules are found to oscillate in the trap under the influence of the trapping forces, which were analyzed using 3D numerical simulations. On the basis of expansion measurements, molecules with a velocity and a position spread of 2.1 m/s and 0.4 mm, respectively, are still accepted by the trap. This corresponds to a temperature of 2.0 mK. From numerical simulations, we find the phase-space volume that can be confined by the trap (the acceptance) to be 50 mm3 (m/s)3.
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 15ND(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 15ND(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 mm(3).
Polar molecules in high-field seeking states cannot be trapped in static traps as Maxwell's equations do not allow a maximum of the electric field in free space. It is possible to generate an electric field that has a saddle point by superposing an inhomogeneous electric field to an homogeneous electric field. In such a field, molecules are focused along one direction, while being defocused along the other. By reversing the direction of the inhomogeneous electric field the focusing and defocusing directions are reversed. When the fields are being switched back and forth at the appropriate rate, this leads to a net focusing force in all directions. We describe possible electrode geometries for creating the desired fields and discuss their merits. Trapping of 15ND3^{15}\mathrm{ND}_{3}15ND3​ ammonia molecules in a cylindrically symmetric ac trap is demonstrated. We present measurements of the spatial distribution of the trapped cloud as a function of the settings of the trap and compare these to both a simple model assuming a linear force and to full three-dimensional simulations of the experiment. With the optimal settings, molecules within a phase-space volume of 270mm3(m∕s)3270\phantom{\rule{0.3em}{0ex}}{\mathrm{mm}}^{3}\phantom{\rule{0.2em}{0ex}}(\mathrm{m}∕\mathrm{s}{)}^{3}270mm3(m∕s)3 remain trapped. This corresponds to a trap depth of about 5mK5\phantom{\rule{0.3em}{0ex}}\mathrm{mK}5mK and a trap volume of about 20mm320\phantom{\rule{0.3em}{0ex}}{\mathrm{mm}}^{3}20mm3.
Cold molecules are potentially beneficial for several areas of research. They have applications in spectroscopy, collision studies and, for instance, the study of the effects of dipole-dipole interactions in BECs. One method to decelerate and thereby cool down neutral dipolar molecules is to use a Stark decelerator. In my thesis, I show the advantages of cold, Stark-decelerated molecules in a high-resolution spectroscopy experiment, using the pure inversion motion of ammonia molecules. The transition associated with this motion is measured both on a warm and on a cold beam, resulting in an increased resolution in the latter case. For many of the applications of cold molecules mentioned above, molecules in high-field seeking states are necessary. To induce molecules to form a BEC, ground state molecules, which are always high-field seeking, are necessary for the use of preliminary cooling methods like evaporative cooling, as trap loss due to inelastic collisions is absent in this state. Many of the more interesting spectroscopy experiments, like experiments with biomolecules or experiments involved with the testing of fundamental physics theories, need to be performed on heavy molecules. Most states in these molecules are high-field seeking as well. To be able to better study high-field seeking molecules, it is important that we are able to confine a cold sample of such molecules in a trap. For this, a so-called AC trap is needed. The first electrical AC trap for molecules is demonstrated in this thesis. It utilizes two fields that are shaped like a saddle point. One of the fields is rotated over 90 degrees with respect to the other. By switching between these two fields, a molecule will alternately experience a focusing (maximum in the electric field strength) and defocusing force (minimum in the electric field strength) in each direction. At the appropriate switching frequency, the overall force is focusing and the molecules remain trapped
We here report on the implementation of a microwave lens for neutral polar molecules suitable to focus molecules in both low-field-seeking and high-field-seeking states. By using the TE{11m} modes of a 12 cm long cylindrically symmetric microwave resonator, Stark-decelerated ammonia molecules are transversally confined. We investigate the focusing properties of this microwave lens as a function of the molecules' velocity, the microwave power, and the frequency detuning Δ from the molecular resonance. Such a microwave lens can be seen as a first important step towards further microwave devices, such as decelerators and traps.
We present experiments on decelerating and trapping ammonia molecules using a combination of a Stark decelerator and a traveling wave decelerator. In the traveling wave decelerator, a moving potential is created by a series of ring-shaped electrodes to which oscillating high voltages (HV) are applied. By lowering the frequency of the applied voltages, the molecules confined in the moving trap are decelerated and brought to a standstill. As the molecules are confined in a true 3D well, this kind of deceleration has practically no losses, resulting in a great improvement on the usual Stark deceleration techniques. The necessary voltages are generated by amplifying the output of an arbitrary wave generator using fast HV amplifiers, giving us great control over the trapped molecules. We illustrate this by experiments in which we adiabatically cool trapped NH3 and ND3 molecules and resonantly excite their motion.