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Physics Today

Molecules Are Magnetically Trapped

Gloria B. Lubkin

Published Nov 1, 1998• 0 citations
Publisher

Abstract

The techniques of magnetic trapping and evaporative cooling of atoms to nanokelvin temperatures have led to Bose–Einstein condensation in alkali atoms and atomic hydrogen and to studies of ultracold atomic collisions. But none of the optical cooling methods has worked for molecules because it hasn't seemed possible to keep the molecules from being optically pumped into inaccessible states.

DOI
10.1063/1.882077

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The techniques of magnetic trapping and evaporative cooling of atoms to nanokelvin temperatures have led to Bose–Einstein condensation in alkali atoms and atomic hydrogen and to studies of ultracold atomic collisions. But none of the optical cooling methods has worked for molecules because it hasn't seemed possible to keep the molecules from being optically pumped into inaccessible states.
Slowing down atoms using laser cooling, together with techniques like magnetic trapping and evaporative cooling, has led to numerous exciting results over the last decade [1], with the realization of Bose-Einstein condensation as the most spectacular achievement [2]. Laser-cooling schemes are applicable to atoms since closed multilevel systems can be realized and many consecutive absorptionemission cycles can be induced. This leads to a significant momentum transfer from the photons of the laser to the atom, resulting in a change of the velocity of the atom. There currently is a great interest in producing cold molecules for the study of cold molecule-molecule collisions, molecular quantum collective effects, Doppler-free spectroscopy, frequency standards, as well as many other applications already realized for atoms [3]. Slowing down of neutral molecules, a prerequisite for being able to trap them, has proven to be considerably more difficult to be achieved, however. The above-mentioned laser-cooling scheme cannot be applied to slow down molecules as no simple closed-level systems can be found; the unavoidable off-resonant radiative decay of molecules from the excited state to other vibrational levels in the electronic ground state hampers efficient momentum transfer [4]. Once molecules are present at sufficiently low kinetic energies, below a fraction of a cm21, they can be trapped in various ways. Trapping of neutral molecules in inhomogeneous magnetic [3] and electric fields [5] and trapping based on the use of evanescent fields close to the dielectricvacuum interface have been proposed [6]. It has also been suggested that extremely high laser powers might be used to induce a dipole moment in polarizable molecules, leading to trapping of these molecules in the laser beam [7]. Other methods to produce slow molecules have been put forward and have recently been successfully applied to produce the first samples of trapped molecules. By injecting molecules in a cold buffer gas, sufficiently low kinetic energies of the injected molecules can be obtained after thermalization that they can subsequently be trapped in an inhomogeneous magnetic field. This has recently been demonstrated for CaH injected in 3He buffer gas at temperatures of around 0.3 K [8]. Associative collisions between ultracold trapped atoms [9] have proven to be
In recent years, much progress has been made in producing and manipulating ultracold atoms in Bose-Einstein condensates (BECs). But, as Williams and Julienne explain in this Perspective, molecules are poor candidates for the laser cooling techniques commonly used to produce BECs. Many groups are trying to devise techniques for making ultracold molecules. A technique reported by Wynar et al. (page 1016) has allowed the first observation of molecule formation in a BEC, and may ultimately be able to produce a pure molecular condensate.
Laser radiations are used to slow down atoms by the process o f momentum transfer.This leads to reducing the temperature to microkclvin region.Gas phase atoms are trapped by using magnetic fields.The recent advances have led to the realization of the dream of physicists o f confining the atoms and reducing their velocities to the limit imposed by quantum mechanics.A number of new experiments arc possible with the cooled and trapped atoms and ions that would be useful to solve many problems of theoretical physics. 1 unher cooling by the process o f evaporative technique has led to the observation of Bose-Hinslcin Condensation predicted by Einstein and Rose nearly seventy-five years ago.A brief review of the method of laser cooling, magnetic trapping and evaporative cooling methods used for obtaining ultracold atoms are discussed It is possible to obtain temperature in the nanokelvin region without using cryogenic methods thus simplifying the experimental methods to a great extent.
While cold molecule experiments are rapidly moving towards their promised benefits of precision spectroscopy, controllable chemistry, and novel condensed phases, heretofore the field has been greatly limited by a lack of methods to cool and compress chemically diverse species to temperatures below ten millikelvin. While in atomic physics these needs are fulfilled by laser cooling, magneto-optical trapping, and evaporative cooling, until now none of these techniques have been applicable to molecules. In this thesis, two major breakthroughs are reported. The first is the observation of evaporative cooling in magnetically trapped hydroxyl (OH) radicals, which potentially opens a path all the way to Bose-Einstein condensation of dipolar radicals, as well as allowing cold- and ultracold-chemistry studies of fundamental reaction mechanisms. Through the combination of an extremely high gradient magnetic quadrupole trap and the use of the OH Λ-doublet transition to enable highly selective forced evaporation, cooling by an order of magnitude in temperature was achieved and yielded a final temperature no higher than 5mK. The second breakthrough is the successful application of laser cooling and magneto-optical trapping to molecules. Motivated by a proposal in this thesis, laser cooling of molecules is now known to be technically feasible in a select but substantial pool of diatomic molecules. The demonstration of not only Doppler cooling but also two-dimensional magneto-optical trapping in yttrium (II) oxide, YO, is expected to enable rapid growth in the availability of ultracold molecules--just as the invention of the atomic magneto-optical trap stimulated atomic physics twenty-five years ago.