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.
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.
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.
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.
The field of cold molecules has become an important source of new insight in fundamental chemistry and molecular physics. High-resolution spectroscopy benefits from translationally and internally cold molecules by increased interaction times and reduced spectral congestion. Completely new effects in scattering dynamics become accessible with cold and controlled molecules. Many of these experiments use molecular beams as a starting point for the generation of molecular samples. This review gives an overview of methods to produce beams of cold molecules, starting from supersonic expansions or effusive sources, and provides examples of applications in spectroscopy and molecular dynamics studies.
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.
Different from atoms, molecules have unique properties, and play an important role in the research of atomic, molecular and optical physics. Cold molecules have important applications in science and have been studied for more than 20 years. But traditional methods, such as the Stark decelerator, have hit a bottleneck: it is hard to increase the phase space density of molecules. Extending the direct laser-cooling technique to new molecular species has recently been a hot topic and also a big challenge. In this review paper, on one hand, we make a brief review to recent progresses on the direct laser cooling of polar molecules. On the other hand, a demonstration on the feasibility of laser cooling BaF molecule has been experimentally illustrated, including the analysis on the molecular energy levels, measurements of the high-resolution spectroscopy, efficient pre-cooling and state preparation via buffer-gas cooling and detailed investigations on the molcule-light interactions. All these results not only pave the way for future laser-cooling and -trapping experiments, but also serve as a reference for the laser-cooling explorations on new molecular species.
A diversity of experimental techniques has been developed over the last 25 years to create samples of molecular gases at temperatures close to the Absolute Zero—here we consider samples in the range from 10s of Kelvin (cold) down to 10s of nanoKelvin (ultracold). In these exotic physical environments, a range of novel experiments can be conducted which bring high levels of control over the properties of these ‘almost stationary’ molecules, in some cases with control over single trapped molecules achievable. In this article, recent advances in this field since 2020, both in terms of the ability to produce and manipulate the molecules and understand their properties, and also in the development of new applications of these technologies, are highlighted. Applications include observing explicit ‘quantum effects’ in chemical reactivity, developing an understanding of chemistry in cold astrophysical media, the creation of exotic phases of matter, the use of trapped molecules in quantum computation and simulations systems, and the use of very high-precision spectroscopic measurements to answer fundamental physics questions beyond the standard model of particle physics.