Review pubs.acs.org/CR
Ultracold Molecules Formed by Photoassociation: Heteronuclear Dimers, Inelastic Collisions, and Interactions with Ultrashort Laser Pulses Juris Ulmanis,† Johannes Deiglmayr,‡ Marc Repp,† Roland Wester,§ and Matthias Weidemüller*,† †
Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Philosophenweg 12, 69120 Heidelberg, Germany Laboratorium für Physikalische Chemie, ETH Zürich, Wolfgang-Pauli-Strasse 10, 8093 Zürich, Switzerland § Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, Technikerstrasse 25/3, 6020 Innsbruck, Austria ‡
5. Ultracold Molecules and Ultrashort Laser Pulses 5.1. Photoassociation with Ultrashort Laser Pulses: Concepts 5.1.1. General Considerations 5.1.2. Experimental Considerations 5.1.3. Theoretical Considerations 5.2. Photoassociation Experiments with Shaped Laser Pulses 5.2.1. Frequency-Chirped Light-Induced Dynamics in Ultracold Collisions 5.2.2. Destruction of Ground State Molecules with Shaped Femtosecond Pulses 5.2.3. Coherent Transients in Femtosecond Pump−Probe Experiments with Ultracold Gases 5.3. Population Redistribution with Pulse Trains 5.3.1. Piecewise Population Transfer 5.3.2. Vibrational Cooling of Molecules by Optical Pumping 6. Perspectives Author Information Corresponding Author Notes Biographies References
CONTENTS 1. Introduction 2. Photoassociation of Ultracold Heteronuclear Molecules 2.1. Theoretical Concepts 2.1.1. Photoassociation Process 2.1.2. Molecular Structure 2.1.3. Symmetries 2.1.4. Population Distribution of Ground State Levels 2.2. Experimental Concepts 2.2.1. Molecule Formation and Detection 2.2.2. Photoassociation Line Shapes and Motional Temperature 2.2.3. Excited State Spectroscopy 2.2.4. Ground State Spectroscopy 2.2.5. Coherent Population Transfer 3. Photoassociation of Ultracold Heteronuclear Molecules: Experiments 3.1. 6Li7Li 3.2. 6Li40K 3.3. 39K85Rb and 41K87Rb 3.4. 7Li133Cs 3.5. 23Na133Cs 3.6. 85Rb133Cs 3.7. 176Yb87Rb 3.8. 170Yb174Yb and 174Yb176Yb 4. Inelastic Molecule−Atom Scattering at Ultracold Temperatures 4.1. Inelastic and Reactive Collisions 4.2. Ultracold Molecule−Atom Collision Experiments 4.3. Universal Collision Model 4.4. Ultracold Collisions Influenced by the Electric Dipole Moment 4.5. Dimer−Atom Collisions Close to a Feshbach Resonance © 2012 American Chemical Society
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1. INTRODUCTION Photoassociation of molecules out of an ultracold gas was the first successful approach to create a gas of molecules in the electronic ground state at translational temperatures in the micro-Kelvin range.1 The major challenge in synthesizing a molecule from a pair of free atoms consists in overcoming the large gap between the initial interatomic pair distance in an ultracold gas (typically in the micrometer range) and the final binding length of a diatomic molecule in the ground state (typically in the subnanometer range). As shown in Figure 1a, photoassociation thus proceeds via a weakly bound excited molecular state, which can be excited at long interatomic distances, and subsequent stabilization of the molecule by spontaneous emission at shorter range. In this way, photoassociation explores the interatomic
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Special Issue: 2012 Ultracold Molecules
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Received: May 30, 2012 Published: August 29, 2012
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dx.doi.org/10.1021/cr300215h | Chem. Rev. 2012, 112, 4890−4927
Chemical Reviews
Review
Figure 1. Schematic depiction of cold ground state molecule formation by (a) photoassociation and (b) magnetoassociation followed by stimulated adiabatic rapid passage. In photoassociation, a continuum state of a free pair of atoms in an ultracold gas is coupled to a weakly bound excited molecular state at large interatomic distances (solid arrow) followed by stabilization via spontaneous decay at short range (dashed arrow). In magnetoassociation, the molecule is first created in the highest vibrational state close to the dissociation threshold by adiabatically sweeping through a magnetically tuned Feshbach resonance in the ultracold gas of atoms. Population is then transferred coherently into a lower bound state (solid arrows) by stimulated adiabatic Raman passage via coupling to an excited state.
condensed matter phases and complex quantum dynamics.27−31 The experimental capabilities that allow for precise quantum control over the rich internal molecular degrees of freedom have created a unique playground for cold and ultracold chemistry.32 At ultralow temperatures of the center-of-mass motion, the dynamics of the molecular gas dramatically changes. Elastic and inelastic collision processes are governed by only few partial waves, giving rise to scattering resonances and extreme sensitivity to external fields. Emerging from early research performed on ultracold atom collisions,33,34 more complex reactions are now being tailored in the quantum regime, where quantum statistics, quantum threshold laws, and single partial wave scattering dominate the collision processes.35,36 These phenomena are rendered accessible with the possibilities of additional control via external electric and magnetic fields37,38 and recent technological breakthroughs in sample preparation in well-defined internal and external degrees of freedom.4,8−10,39,40 There are several approaches toward physics with cold (1 mK to 1 K) and ultracold (
