Research on molecular collision dynamics at low energies, and with control of nearly all degrees of freedom, has made tremendous progress in recent years, opening many new avenues to study the most fundamental aspects of molecular interactions in general and of chemical reaction dynamics in particular. Generally, cold chemistry refers to dynamical processes between atoms and molecules, taking place in the gas phase at temperatures or—more precisely—energies that are low enough for the dynamics to be dominated by the long-range interaction between particles, and by quantum effects like resonances or tunneling which also implies a low number of angular momentum partial waves that contribute to the collision. A quantitative assessment of the relevant temperature ranges is difficult and would require system-specific definitions, since the interaction potentials change, and the dynamics are mass-dependent. Nevertheless, a rough classification can be made by defining 1 kelvin as the limit to the so-called cold regime while temperatures below 1microkelvin are considered ultracold.
With the present book we intend to provide readers with the foundations needed to understand this exciting research field, by introducing and reviewing both the theoretical and experimental aspects of research in cold and ultracold molecular collisions. The book features 13 chapters written by world-leading scientists in the field. We deliberately avoided arranging the chapters into traditional blocks of “theory” and “experiment”. Instead, we propose to the reader an itinerary exploring various ambitious experimental techniques that have been developed in the past years specifically for the study of cold chemistry, intermingled with the description of the necessary basic, and still developing, theoretical concepts.
We open with a presentation of the very first experimental approach that enabled the study of chemical kinetics at temperatures in the 10 K range, namely the CRESU technique (the name is the French acronym for “reaction kinetics in uniform supersonic flow”) that makes use of uniform flows in molecular beams and of the low relative molecular velocities therein. Fournier, Le Picard, and Sims (Chapter 1) demonstrate the immense interest of this approach to investigate the behavior of gas-phase chemical reactions at temperatures as low as 7 K, both for fundamental interest and for their relevance in atmospheric and interstellar chemistry. Molecular beams are also at the basis of the merged- and low-angle crossed-beam techniques presented by Naulin and Bergeat (Chapter 3), nowadays allowing dynamical studies at energies as low as 10 mK. The development of accurate models for cold elastic and inelastic scattering is a highly challenging task on its own, as discussed by Kłos and Lique (Chapter 2) who present the relevant theoretical and numerical approaches that start from the Born–Oppenheimer approximation.
The accurate description of van der Waals interactions between gas-phase particles is critical for an understanding of their dynamics. This is particularly true at low temperatures where these attractive interactions begin to dominate the collision dynamics. This motivates the theory work presented by Lepers and Dulieu (Chapter 4) who describe in detail the general theory of long-range interactions between atoms and molecules, applied here to ultracold collisions well below 1 K. Hapka and Żuchowski (Chapter 5) demonstrate how to use the theory of intermolecular interactions in the context of cold and ultracold chemistry, in particular how to apply the formalism of symmetry-adapted perturbation theory (SAPT), which provides partitioning of the interaction energy into physically meaningful components such as electrostatic, dispersion, induction and exchange interactions. Tscherbul (Chapter 6) discusses the modification of long-range interactions induced by the presence of external electromagnetic fields. This chapter gives an overview of the mechanisms for controlling molecular collisions, both elastic and inelastic. It reveals the central role of Feshbach resonances, which is one of the topics at the focus of Chapter 7 by Côté in the last of these four theoretical chapters. Starting from the basic concepts of resonances in quantum systems, possibilities of modifying and controlling the behavior of ultracold atomic and molecular gases by using resonances are investigated.
A highly interesting environment for the study of cold molecular dynamics is provided by helium nanodroplets, i.e., ensembles of thousands of helium atoms with a temperature around 0.4 K. Tanyag, Jones, Bernando, O’Connell, Verma, and Vilesov (Chapter 8) give a detailed introduction to the thermodynamics of such systems, as well as to experimental techniques for their production and investigation. Lemeshko and Schmidt (Chapter 9) present a very new approach to the description of impurities in helium nanodroplets by introducing a new quasiparticle, the “angulon,” that can also be used to describe the dynamics of particles embedded in the droplet.
Chapters 10 and 11 report on two main current experimental pillars of ultracold chemistry. Cold ion–molecule chemistry is introduced by Zhang and Willitsch (Chapter 10) who describe theoretical concepts required for the description of low-energy ion–molecule reactions and present key experimental methods for laboratory studies of such reactions. Covey, Moses, Ye and Jin (Chapter 11) review how ultracold polar molecules, prepared at temperatures in the nanokelvin range, open a new world of precision quantum chemistry in which quantum statistics, quantum partial waves, and even many-body correlations can play important roles. These contributions are complemented by Quéméner (Chapter 12) who presents the theoretical approaches required to describe a molecular collision event at ultralow temperatures, based on the evaluation of their long-range interactions in the strongly anisotropic environment induced by an external field. Koch (Chapter 13) concludes this panorama with a presentation of the coherent control of cold collisions, consisting in the shaping of electromagnetic fields to control the evolution of a system in the desired way by suppressing or enhancing the propensity of certain scattering channels.
All these chapters provide a complete coverage of relevant aspects of cold chemistry. On several occasions the authors present complementary aspects of a theoretical formalism, retaining the aspects which are the most relevant for their purpose. Wherever possible, notations are kept consistent throughout the book so that the reader should not have difficulty in identifying similarities in these various treatments. Moreover, all contributions present basic concepts in a scholarly fashion and combine them with a review of state-of-the-art experimental developments and very recent results. Such a multifaceted ensemble thus provides a stimulating view of the new field of ultracold chemistry, for which spectacular achievements are undoubtedly still to come.
We are highly impressed by the quality of each of the chapters and we express our thanks to all the contributing authors. We also gratefully thank Professor Dudley Herschbach for writing the Foreword. His visionary statement perfectly summarizes our key intent with this book:
The mission of the book is evangelical: to provide a collection of tutorials on key properties of cold chemistry, designed to introduce the new field and its tools to students at the advanced undergraduate level and beyond, by augmenting curricular topics as well as preparing for research projects.
Olivier Dulieu and Andreas Osterwalder
Orsay and Lausanne