Physics and Chemistry of Cold Molecules

Ultracold chemistry is emerging as a new frontier of physical chemistry as revealed by the recent explosion of experimental and theoretical studies on chemical applications of cold and ultracold molecules.^1–3 The terms “cold” and “ultracold” refer to the translational motion of the molecules, characterized by a temperature T < 1 Kelvin or T < 1 milliKelvin, respectively. So far, the interest in cold and ultracold molecules has been largely stimulated by a variety of prospective applications in different areas of physics, ranging from quantum computing and quantum simulation of complex many-body systems to precision metrology aiming at tests of fundamental physical theories. As a new direction, the research on cold molecules has recently started to explore crossroads with chemistry, in particular through studies of inelastic and reactive scattering in the cold and ultracold regimes. These perspectives have driven the development of new experimental approaches for the creation of dense ensembles of cold molecules, as well as intense activity towards a better conceptual understanding of chemistry at ultralow temperatures. The traditionally close synergies between theoretical and experimental studies have now advanced the field to a point at which cold chemistry is becoming an exciting reality, promising unanticipated discoveries and breakthroughs.

This special issue presents a snapshot of the current research in the field of cold molecules. We decided to divide it into themed sections each of which reflects a major area of activity in the field. Ensembles of cold molecules, with the prospect of reaching high phase-space densities close to quantum degeneracy, are created following methods relying on Photo- and magneto-association of ultracold atoms or by Slowing and controlling molecular beams. Such ensembles are suitable for Spectroscopy of cold molecules and for new control and trapping schemes of molecules relying on their Interaction with external fields. This issue also presents a range of articles focusing on Cold chemistry through the observation and modeling of elementary collision or reactive processes between cold molecules.

Compared with previous special issues on Cold molecules,^4–7 the present collection of papers highlights several new trends. First, the focus of the field is clearly broadening to include cold and ultracold chemistry. Articles in this special issue demonstrate that current technology allows for the production of slow, dense molecular beams. The translational motion and the internal degrees of freedom of molecules in these beams can be manipulated by external electric and magnetic fields, which elevates traditional molecular beam experiments to a new level of precision and control. Second, studies of cold molecular collisions in general require both the experimental measurements of intermolecular interactions at low temperatures and corresponding theoretical calculations for their interpretation. Third, there is an increased interest in the creation of ensembles of cold molecular ions, opening up new avenues for the study of cold ion-neutral chemistry and the spectroscopy of cold ions.

Our hope is that the articles in this volume will transfer to the reader our excitement about this promising research field. Even more we hope that this special issue will stimulate the reader to think about questions that remain open. We identify the following major milestones that need to be reached for the continued progress of this field:

Bridging cold and ultracold temperature regimes: While a variety of experimental methods now exist for the creation of molecules at temperatures near 1 milliKelvin, it is still very challenging to create molecules at temperatures below 1 microKelvin. It is therefore necessary to develop experimental methods that would bridge the cold and ultracold temperature regimes.

The creation of ensembles of molecules in a single internal and motional state: The achievement of Bose–Einstein condensates and of Fermi degenerate gases of molecules would open a variety of new research opportunities for both quantum simulators of condensed matter and for state-dependent chemistry driven, for instance, by resonances.

Increasing the chemical variety of ultracold molecules: Although an arsenal of different methods to produce various kinds of cold molecules exists by now, techniques to cool a wider range of species, in particular large polyatomic molecules, still remain to be established.

The detection of product states: Most of the measurements of chemical reactions and collisions of cold molecules are based on detecting the loss of molecules from trapped ensembles. The experimental techniques to measure the products of molecular collisions departing from the traps would be invaluable for unravelling the complexity of molecular dynamics at ultralow temperatures.

Photoassociation beyond alkali metal dimers: The powerful technique of photoassociation is still largely applicable to alkali dimer molecules, and its extension to other species must be developed to enrich the field of cold and ultracold chemistry.

Our utmost hope is that this volume will provide a comprehensive introduction into the field and will inspire the reader to work on one of these, or related, research problems to further push the frontiers of chemistry and physics at ultralow temperatures.

References

1. I. D. M. Smith (ed.), Low Temperatures and Cold Molecules, Imperial College Press, London, 2008.
2. M. Weidemüller and C. Zimmermann (eds.), Cold Atoms and Molecules: Concepts, Experiments and Applications to Fundamental Physics, Wiley-VCH, Weinheim, 2009.
3. R. V. Krems, W. C. Stwalley, and B. Friedrich (eds.), Cold Molecules: Theory, Experiment, Applications, CRC Press, Boca Raton, USA, 2009.
4. Special issue on ultracold polar molecules: formation and collisions, Eur. Phys. J. D, 2004, 31.
5. Special issue on cold molecules, J. Phys. B, 2006, 39.
6. Special issue on cold and ultracold molecules, New J. Phys., 2009, 11.
7. Faraday Discuss., 2009, 142 10.1039/b914241j.

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