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Physical Review A

Trapping polar molecules in an ac trap

Hendrick L. Bethlem, Jacqueline van Veldhoven, Melanie Schnell, Gerard Meijer

Published Dec 5, 2006• 47 citations
Publisher

Abstract

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 $^{15}\mathrm{ND}_{3}$ 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 $270\phantom{\rule{0.3em}{0ex}}{\mathrm{mm}}^{3}\phantom{\rule{0.2em}{0ex}}(\mathrm{m}∕\mathrm{s}{)}^{3}$ remain trapped. This corresponds to a trap depth of about $5\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$ and a trap volume of about $20\phantom{\rule{0.3em}{0ex}}{\mathrm{mm}}^{3}$.

DOI
10.1103/physreva.74.063403

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European Quantum Electronics Conference (EQEC)•Jan 1, 2003•0 citations
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.
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).
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.
A polar molecule experiences a force in an inhomogeneous electric field. Using this force, neutral molecules can be decelerated and trapped. It is shown here that this can in principle be done without loss in phase-space density. Using a series of 64 pulsed inhomogeneous electric fields a supersonic beam of ammonia molecules (14NH3,{(}^{14}{\mathrm{NH}}_{3},(14NH3​, 14ND3,{}^{14}{\mathrm{ND}}_{3},14ND3​, 15ND3){}^{15}{\mathrm{ND}}_{3})15ND3​) is decelerated. Subsequently, the decelerated molecules are loaded into an electrostatic quadrupole trap. Densities on the order of 107molecules/cm3{10}^{7} {\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{u}\mathrm{l}\mathrm{e}\mathrm{s}/\mathrm{c}\mathrm{m}}^{3}107molecules/cm3 at a temperature of 25 mK are obtained for 14ND3{}^{14}{\mathrm{ND}}_{3}14ND3​ and 15ND3{}^{15}{\mathrm{ND}}_{3}15ND3​ separately and simultaneously. This corresponds to a phase-space density in the trap of $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13},$ 50 times less than the initial phase-space density in the beam.
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
Abstract Strong-field-seeking states are the lowest-energy configurations for paramagnetic molecules in the magnetic field. Molecules in strong-field-seeking states cannot be trapped in a magnetostatic field because a magnetostatic maximum in free space is not allowed. In this paper, we propose an AC magnetic trap composed of two pairs of Helmholtz coils. The spatial magnetic field distribution is numerically calculated and the time-sequential control is depicted. We investigate the influence of the switching frequency and the electric current in the coils on the performance of our trap. Variations of the location and phase-space distribution during a whole switching cycle are simulated. Finally, we study the impact of time during which the field is switched off on the number of captured molecules in a switching cycle.
The interaction of polar molecules with an inhomogeneous, electrostatic field to filter molecules from an effusive room-temperature source is exploited to create a slow molecule beam. Whether a dipolar molecule is trapped in or expelled from the electric field minimum depends on the orientation of the molecular dipole with respect to the electric field. These molecules are called low and high-field seekers, respectively. Thus, an electric field configuration with a field minimum on axis acts as a 2-dimensional trap for low-field seeking molecules with small transverse velocity components. A linear quadrupolar is opted, which is one of the simplest suitable geometries. The longitudinal velocity is limited by a bent section, where the centripetal force due to the radius of curvature and electric field gradient guide only the slowest molecules around the bend.