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ChemPhysChem

Why are Cold Molecules so Hot?

Břetislav Friedrich, John M. Doyle

Published Feb 19, 2009• 70 citations
Publisher

Abstract

Abstract Brrrr! Cold molecule and cold atom research are juxtaposed and the challenges in cooling and trapping molecules are recounted. Both indirect and direct techniques of producing cold and slow molecules (such as buffer‐gas cooling and magnetic trapping, see picture) are described. Advanced techniques of manipulating cold or slow molecules are reviewed and ongoing work with cold molecules is outlined. magnified image Herein, we aim to show why the work on translationally (and otherwise) cold molecules has sparked so much inspiration—and anticipation—in both the physics and chemistry communities. We begin by discussing the basic features of cold molecules as implied by their de Broglie wavelengths, large compared with molecular dimensions. We juxtapose cold molecule and cold atom research and recount the challenges that had to be met if molecules were to be cooled and trapped. Subsequently, both the indirect and direct techniques of producing cold and slow molecules are described in some detail, and their applicability to various classes of molecules is discussed. Advanced techniques of manipulating cold or slow molecules are illustrated by the examples of DC and AC trapping and storage. Finally, ongoing and future work with cold and/or trapped molecules is outlined. This includes precision spectroscopy, chemical reaction dynamics, simulations of few‐ and many‐body physics, quantum computing, and tests of fundamental physics.

DOI
10.1002/cphc.200800577

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The development of techniques to cool/slow and trap atoms and molecules brought about a revolution in atomic and molecular physics, with an impact on physics in large. When slowed/cooled down, atoms and molecules become matter waves whose properties differ vastly from those of thermal matter. In particular, close to absolute zero temperature, the interaction range of atoms or molecules increases far beyond their dimensions, imparting to them highly desirable collective and collisional properties. The latter have been predicted to give rise to a novel ultracold chemistry. The article describes the main techniques of producing cold molecules, both the indirect ones (photoassociation of cold atoms, collisional association of cold atoms controlled via Feshbach resonances) and the direct techniques (buffer-gas cooling, Stark deceleration and others). Ongoing and anticipated future experiments with cold molecules are reviewed, including pioneering ventures aimed at radiative lifetime measurements, reduction of transit time broadening, measurements of ultracold collisional cross-sections, and testing of fundamental symmetries in nature, such as the time-reversal symmetry and parity.

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Abstract Brrrr! Cold molecule and cold atom research are juxtaposed and the challenges in cooling and trapping molecules are recounted. Both indirect and direct techniques of producing cold and slow molecules (such as buffer‐gas cooling and magnetic trapping, see picture) are described. Advanced techniques of manipulating cold or slow molecules are reviewed and ongoing work with cold molecules is outlined. magnified image Herein, we aim to show why the work on translationally (and otherwise) cold molecules has sparked so much inspiration—and anticipation—in both the physics and chemistry communities. We begin by discussing the basic features of cold molecules as implied by their de Broglie wavelengths, large compared with molecular dimensions. We juxtapose cold molecule and cold atom research and recount the challenges that had to be met if molecules were to be cooled and trapped. Subsequently, both the indirect and direct techniques of producing cold and slow molecules are described in some detail, and their applicability to various classes of molecules is discussed. Advanced techniques of manipulating cold or slow molecules are illustrated by the examples of DC and AC trapping and storage. Finally, ongoing and future work with cold and/or trapped molecules is outlined. This includes precision spectroscopy, chemical reaction dynamics, simulations of few‐ and many‐body physics, quantum computing, and tests of fundamental physics.
The development of techniques to cool/slow and trap atoms and molecules brought about a revolution in atomic and molecular physics, with an impact on physics in large. When slowed/cooled down, atoms and molecules become matter waves whose properties differ vastly from those of thermal matter. In particular, close to absolute zero temperature, the interaction range of atoms or molecules increases far beyond their dimensions, imparting to them highly desirable collective and collisional properties. The latter have been predicted to give rise to a novel ultracold chemistry. The article describes the main techniques of producing cold molecules, both the indirect ones (photoassociation of cold atoms, collisional association of cold atoms controlled via Feshbach resonances) and the direct techniques (buffer-gas cooling, Stark deceleration and others). Ongoing and anticipated future experiments with cold molecules are reviewed, including pioneering ventures aimed at radiative lifetime measurements, reduction of transit time broadening, measurements of ultracold collisional cross-sections, and testing of fundamental symmetries in nature, such as the time-reversal symmetry and parity.
Cold and ultracold molecules are the next wave of ultracold physics, giving rise to an exciting array of scientific opportunities, including many body physics for novel quantum phase transitions, new states of matter, and quantum information processing. Precision tests of fundamental physical laws benefit from the existence of molecular internal structure with exquisite control. The study of novel collision and reaction dynamics will open a new chapter of quantum chemistry. Cold molecules bring together researchers from a variety of fields, including atomic, molecular, and optical physics, chemistry and chemical physics, quantum information science and quantum simulations, condensed matter physics, nuclear physics, and astrophysics, a truly remarkable synergy of scientific explorations. For the past decade there have been steady advances in direct cooling techniques, from buffer-gas cooling to cold molecular beams to electro- and magneto-molecular decelerators. These techniques have allowed a large variety of molecules to be cooled for pioneering studies. Recent amazing advances in experimental techniques combining the ultracold and the ultraprecise have furthermore brought molecules to the point of quantum degeneracy. These latter indirect cooling techniques magnetically associate atoms from a Bose–Einstein condensate and/or a quantum degenerate Fermi gas, transferring at 90% efficiency highly excited Fano–Feshbach molecules, which are on the order of 10 000 Bohr radii in size, to absolute ground state molecules just a few Bohr across. It was this latter advance, together with significant breakthroughs in internal state manipulations, which inspired us to coordinate this focus issue now, and is the reason why we say the next wave of ultracold physics has now arrived. 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Therefore, although this focus issue contains a few articles on homonuclear molecules, more complex molecules such as benzene, and even a contribution on atomic chromium, which has a significant magnetic dipole moment, our main focus is on the heteronuclear polar case. This focus issue explores both direct and indirect cooling of mainly polar molecules, and the theory to support and inspire these advances. Thirty-eight research groups have contributed original work, and there are two review articles to complement these advances: the first covers cold and ultracold molecules broadly from few body to many body physics, including foundational theory, the technology to make them, and their scientific applications. The second is on the search for time variation of fundamental constants. The former review, which is comprehensive in nature, concludes with a list of open questions. This sets the tone for the focus issue, namely, openness, innovation, and possibility, an emphasis for which New Journal of Physics , an open-access journal of the highest quality, is especially fitted. Focus on Cold and Ultracold Molecules Contents Cold and ultracold molecules: science, technology and applications Lincoln D Carr, David DeMille, Roman V Krems and Jun Ye Ultracold molecules: new probes on the variation of fundamental constants Cheng Chin, V V Flambaum and M G Kozlov Probing the unitarity limit at low laser intensities Philippe Pellegrini and Robin Côté Single-photon molecular cooling Edvardas Narevicius, S Travis Bannerman and Mark G Raizen Quantum simulations of extended Hubbard models with dipolar crystals M Ortner, A Micheli, G Pupillo and P Zoller Collisional and molecular spectroscopy in an ultracold Bose–Bose mixture G Thalhammer, G Barontini, J Catani, F Rabatti, C Weber, A Simoni, F Minardi and M Inguscio Multi-channel modelling of the formation of vibrationally cold polar KRb molecules Svetlana Kotochigova, Eite Tiesinga and Paul S Julienne Formation of ultracold, highly polar X 1 Σ + NaCs molecules C Haimberger, J Kleinert, P Zabawa, A Wakim and N P Bigelow 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resonance on the photoassociation of LiCs J Deiglmayr, P Pellegrini, A Grochola, M Repp, R Côté, O Dulieu, R Wester and M Weidemüller The kinematic cooling of molecules with laser-cooled atoms Ken Takase, Larry A Rahn, David W Chandler and Kevin E Strecker Coherent collapses of dipolar Bose–Einstein condensates for different trap geometries J Metz, T Lahaye, B Fröhlich, A Griesmaier, T Pfau, H Saito, Y Kawaguchi and M Ueda High-energy-resolution molecular beams for cold collision studies L P Parazzoli, N Fitch, D S Lobser and H J Lewandowski Collisional effects in the formation of cold guided beams of polar molecules M Motsch, C Sommer, M Zeppenfeld, L D van Buuren, P W H Pinkse and G Rempe Towards sympathetic cooling of large molecules: cold collisions between benzene and rare gas atoms P Barletta, J Tennyson and P F Barker Efficient formation of ground-state ultracold molecules via STIRAP from the continuum at a Feshbach resonance Elena Kuznetsova, Marko Gacesa, Philippe Pellegrini, 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formed by photoassociation J T Kim, D Wang, E E Eyler, P L Gould and W C Stwalley Pumping vortex into a Bose–Einstein condensate of heteronuclear molecules Z F Xu, R Q Wang and L You Intense atomic and molecular beams via neon buffer-gas cooling David Patterson, Julia Rasmussen and John M Doyle Dynamical properties of dipolar Fermi gases T Sogo, L He, T Miyakawa, S Yi, H Lu and H Pu Collisions of bosonic ultracold polar molecules in microwave traps Alexander V Avdeenkov Cold TiO(X 3 Δ)–He collisions Mei-Ju Lu and Jonathan D Weinstein Investigation of dephasing rates in an interacting Rydberg gas U Raitzsch, R Heidemann, H Weimer, B Butscher, P Kollmann, R Löw, H P Büchler and T Pfau Impact of electric fields on highly excited rovibrational states of polar dimers Rosario González-Férez and Peter Schmelcher Phase transition from straight into twisted vortex lines in dipolar Bose–Einstein condensates M Klawunn and L Santos Stimulating the production of deeply bound RbCs molecules with laser pulses: the role of spin–orbit coupling in forming ultracold molecules Subhas Ghosal, Richard J Doyle, Christiane P Koch and Jeremy M Hutson Sensitive measurement of m p / m e variance using vibrational transition frequencies of cold molecules Masatoshi Kajita
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.
Slowing, cooling, and trapping molecules One of the major achievements in physics during the last two decades of the twentieth century was the ability to cool and trap atoms [1]. Laser cooling can produce previously inconceivable atomic temperatures of less than 1 μK. Atoms with sufficiently low kinetic energy can be trapped in magnetic and optical traps and thus studied for a prolonged time, thereby improving the resolution of spectroscopy by many orders of magnitude as well as reducing Doppler broadening. Furthermore, besides the external degrees of freedom, the internal degrees of freedom of the atoms can also be manipulated, leading to full control of the atoms. One of the holy grails of physics, the achievement of Bose–Einstein condensation in a dilute gas, was achieved mainly because of laser cooling and trapping of atoms. One of the natural extensions of laser cooling and trapping of atoms is the application of the techniques to molecules. Molecules offer a much richer internal structure than atoms, and thus provide extended possibilities to experimentalists. However, atomic laser cooling is based on the repetitive scattering of light and requires a closed, or at least nearly closed, two-level system. Because of the complicated internal structure of molecules, a closed two-level system cannot usually be found because they have many different electronic, vibrational, and rotational states and in general can decay to many different states after absorption of light (see Chap. 16). This requires a complete rethinking of the schemes that are used to cool atoms. Still, because of the extended possibilities cold molecules can offer with respect to cold atoms, there have been a large number of techniques developed to slow, cool, and trap them. These techniques can be divided into two classes, namely direct and indirect cooling. In the first case, molecules at cryogenic or room temperature are directly slowed and cooled. Although many different slowing techniques are used, this chapter focuses on Stark slowing of molecules since it was the first technique developed. Apart from the Stark shift of the molecules, the Zeeman and light shift of molecules can also be used to slow them. The idea behind Stark slowing is to subject the moving molecules to a continuously increasing electric field so that their kinetic energy is converted into potential energy.
One of the current themes in quantum physics is the creation of cold ( 1 K) molecules to study molecule-molecule, molecule-atom, and molecule-radiation interactions under these novel low-temperature conditions. This book highlights the field of cold trapped molecular ions. In particular, the development of a novel, universal cooling method for charged particles, sympathetic cooling using laser-cooled atoms, together with the experimental and computational techniques required are extensively reviewed. The book also reviews first spectacular applications of the cold ion ensembles produced: the study of the properties of cold non-neutral plasmas, the study of ion-neutral chemical reactions and of photodissociation processes, and high-resolution rotational overtone spectroscopy of cold hydrogen molecular ions with unprecedented spectral resolution. The large potential of this field of research and its possible impact to other fields of physics, chemistry, and biology are highlighted. The book summarizes the main lines of research in cold trapped molecular ion physics, thus, making it useful as an introduction for advanced students and experienced researchers.
Research with cold molecules has developed rapidly in recent years. There is now a variety of established methods for cooling molecules into the millikelvin range. Nevertheless, a focal point of current research is directed toward finding new ways to bring the temperature of molecules even closer to absolute zero. Samples of cold molecules offer not only important applications for high-resolution spectroscopy, which benefit from the increased interaction time of slow molecules with electromagnetic radiation; they also promise access to an exotic regime of chemical reactivity, in which phenomena such as quantum tunneling and quantum resonances predominate. This review begins with an introduction to the methods by which cold molecules can be prepared, with special emphasis on Stark deceleration and traps. In addition to applications of cold molecules that have already been partially achieved, an important focus of the review concentrates on possible future applications, and both aspects are illustrated with selected examples.