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Deceleration and trapping of polar molecules using time-varying electric fields

Hendrick L. Bethlem

Published Jan 1, 2002• 3 citations

Abstract

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

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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

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.

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.

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.

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.

The interaction between inhomogeneous electric fields and the large electric dipole moments of atoms and molecules in Rydberg states of high principal quantum number can be used to efficiently accelerate and decelerate atoms and molecules in the gas phase. We describe here how hydrogen atoms and molecules initially moving with velocities of ∼600 m/s in supersonic beams can be decelerated to zero velocity and loaded into electric traps. The long observation times that are made possible by the electrostatic trapping enables one to study slow relaxation processes. Experiments are presented in which we have observed photoionization processes and transitions between Rydberg states induced by blackbody radiation at temperatures between 10 K and 300 K on a time scale of several milliseconds. Comparison of these processes in Rydberg states of H and H(2) suggests the importance, in H(2), of collisional processes and of the process of blackbody-radiation-induced predissociation.