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Physical Review A

Deceleration and trapping of ammonia using time-varying electric fields

Hendrick L. Bethlem, Floris M. H. Crompvoets, Rienk T. Jongma, Sebastiaan Y. T. van de Meerakker, Gerard Meijer

Published May 10, 2002• 231 citations
Publisher

Abstract

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 ${(}^{14}{\mathrm{NH}}_{3},$ ${}^{14}{\mathrm{ND}}_{3},$ ${}^{15}{\mathrm{ND}}_{3})$ is decelerated. Subsequently, the decelerated molecules are loaded into an electrostatic quadrupole trap. Densities on the order of ${10}^{7} {\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{u}\mathrm{l}\mathrm{e}\mathrm{s}/\mathrm{c}\mathrm{m}}^{3}$ at a temperature of 25 mK are obtained for ${}^{14}{\mathrm{ND}}_{3}$ and ${}^{15}{\mathrm{ND}}_{3}$ 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.

DOI
10.1103/physreva.65.053416

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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×\else\texttimes\fi10\ensuremath−13,2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13},2\ifmmode×\else\texttimes\fi10\ensuremath−13, 50 times less than the initial phase-space density in the beam.
What we experience as an agreeable temperature is in fact due to a constant bombardment of our skin by very fast molecules. At room temperature the average velocity of air molecules around us is about 500 m/s, corresponding to about 2000 km/hour. Only at very low temperatures, close to the absolute zero (0 K = -273.15 C), will the molecules come to rest. The high velocities at ordinary temperatures limit our ability to study the molecules. This thesis describes a method to slow down a beam of polar molecules using time-varying electric fields. When the molecules are decelerated to low velocities they will have insufficient kinetic energy to escape from the electric fields, and they will be trapped. In this way molecules can be stored and studied over long periods of time. This thesis is organized as follows: In chapter I the motivation for studying cold molecules is made more explicit, and an overview is given of other methods to cool molecules. In Chapter II the interaction between molecules and electric fields is outlined. In Chapter III and IV deceleration of a beam of metastable CO is presented, using molecules in states which are either repelled by, or attracted to high electric fields. Finally, in Chapter V ammonia molecules are brought to a standstill and subsequently held in an electrostatic trap
The motion of polar molecules can be controlled by time-varying inhomogeneous electric fields. In a Stark decelerator, this is exploited to accelerate, transport, or decelerate a fraction of a molecular beam. When combined with a trap, the decelerator provides a means to store the molecules for times up to seconds. Here, we review our efforts to produce cold molecules via this technique. In particular, we present a new generation Stark decelerator and electrostatic trap that selects a significant part of a molecular beam pulse that can be loaded into the trap. Deceleration and trapping experiments using a beam of OH radicals are discussed.
A neutral polar molecule experiences a force in an inhomogeneous electric field. This electric field can be designed such that a beam of polar molecules is exposed to a harmonic potential in the forward direction. In this potential the longitudinal phase-space distribution of the ensemble of molecules is rotated uniformly. This property is used to longitudinally focus a pulsed beam of ammonia molecules and to produce a beam with a longitudinal velocity spread of 0.76 m/s, corresponding to a temperature of 250 mu K.
We propose a method to produce, in a pulsed or continuous way, cold samples of highly polar molecules. Using a pulsed or continuous standard (supersonic) beam of these molecules, our idea consists of transforming the molecules into their anionic counterparts, which are decelerated to a standstill by a well-controlled external electric field and ultimately neutralized. The neutral-to-anion transformation occurs through collisions with Rydberg atoms coming from an additional atomic beam. This Rydberg electron transfer process is possible provided that the molecular species has a sufficiently strong electric dipole ( D, i.e., cm). Whatever the mass of the species, the deceleration stage is realized by a temporally and spatially controlled electric field within a range of less than one centimeter, which is much shorter than in current deceleration experiments of neutral molecules. Once stopped, the molecular anions are neutralized by laser photodetachment or a pulsed electric field process. The resulting molecules might be held and accumulated, for instance, in a magnetic trap.
Neutral dipolar molecules can be decelerated and trapped using time-varying inhomogeneous electric fields. This has been demonstrated only for molecules in low-field seeking states, but can, in principle, be performed on molecules in high-field seeking states as well. Transverse stability is then much more difficult to obtain, however, since molecules in high-field seeking states always experience a force towards the electrodes. Here we demonstrate that an array of dipole lenses in alternate gradient configuration can be used to maintain transverse stability. A pulsed beam of metastable CO in high-field seeking states is accelerated from 275 to 289 m/s as well as decelerated from 275 to 260 m/s.