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Physical Review Letters

Decelerating Neutral Dipolar Molecules

Hendrick L. Bethlem, Giel Berden, Gerard Meijer

Published Aug 23, 1999• 664 citations
Publisher

Abstract

It is experimentally demonstrated that a beam of neutral dipolar molecules can be efficiently decelerated with a time-varying electric field. A pulsed beam of neutral metastable CO molecules is slowed down from 225 m/s ( ${E}_{\mathrm{kin}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}59{\mathrm{cm}}^{\ensuremath{-}1}$) to 98 m/s ( ${E}_{\mathrm{kin}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}11{\mathrm{cm}}^{\ensuremath{-}1}$) upon passage through an array of 63 synchronously pulsed electric field stages.

DOI
10.1103/physrevlett.83.1558

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It is experimentally demonstrated that a beam of neutral dipolar molecules can be efficiently decelerated with a time-varying electric field. A pulsed beam of neutral metastable CO molecules is slowed down from 225 m/s ( Ekin=59cm\ensuremath−1{E}_{\mathrm{kin}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}59{\mathrm{cm}}^{\ensuremath{-}1}Ekin​=59cm\ensuremath−1) to 98 m/s ( Ekin=11cm\ensuremath−1{E}_{\mathrm{kin}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}11{\mathrm{cm}}^{\ensuremath{-}1}Ekin​=11cm\ensuremath−1) upon passage through an array of 63 synchronously pulsed electric field stages.
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.
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.
Inspired by the spectacular successes in the field of cold atoms, there is currently great interest in cold molecules. Cooling molecules is useful for various fundamental physics studies and gives access to an exotic regime in chemistry where the wave property of the molecules becomes important. Although cooling molecules has turned out to be considerably more difficult than cooling atoms, a number of methods to produce samples of cold molecules have been demonstrated over the last few years. This paper aims to review the application of cold molecules and the methods to produce them. Emphasis is put on the deceleration of polar molecules using time-varying electric fields. The operation principle of the array of electrodes that is used to decelerate polar molecules is described in analogy with, and using terminology from, charged-particle accelerators. It is shown that, by applying an appropriately timed high voltage burst, molecules can be decelerated while the phase-space density, i.e. the number of molecules per position-velocity interval, remains constant. In this way the high density and low temperature in the moving frame of a pulsed molecular beam can be transferred to the laboratory frame. Experiments on metastable CO in states that are either repelled by or attracted to high electric fields are presented. Loading of slow molecules into traps and storage rings is discussed.
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
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.