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Cold Chemistry

Cold Chemistry

Olivier Dulieu | Andreas Osterwalder

(2017)

Additional Information

Abstract

Recent years have seen tremendous progress in research on cold and controlled molecular collisions, both in theory and in experiment. The advent of techniques to prepare cold and ultracold molecules and ions, to store them in optical lattices or in charged quasicristalline structures, and to use them in crossed or merged beam experiments have opened many new possibilities to study the most fundamental aspects of molecular interactions. At the same time, theoretical work has made progress in tackling these problems and accurately describing quantum effects in complex systems, and in proposing viable options to control chemical reactions at ultralow energies. Through tutorials on both the theoretical and experimental aspects of research in cold and ultracold molecular collisions, this book provides advanced undergraduate students, graduate students and researchers with the foundations needed to understand this exciting field.
Andreas Osterwalder is a Senior Scientist at Ecole Polytechnique Fédérale de Lausanne, Switzerland, where he leads a team working on developing a merged beam technique to study neutral chemical reactions below 1K.
Olivier Dulieu is Research Director at Laboratoire Aimé Cotton, CNRS, France. His research interests include the theory of structure and dynamics of cold and ultracold neutral and ionic molecules, quantum chemistry, molecular spectroscopy and field-assisted dynamics.

Table of Contents

Section Title Page Action Price
Cover Cover
Cold Chemistry: Molecular Scattering and Reactivity Near Absolute Zero i
Foreword v
Preface ix
Contents xiii
Chapter 1 Low-temperature Chemistry in Uniform Supersonic Flows 1
1.1 Introduction 1
1.2 The CRESU Technique 6
1.2.1 Uniform Supersonic Flows 6
1.2.2 Kinetics Techniques in Combination with Uniform Supersonic Flows 12
1.2.3 Limitations of the CRESU Technique 15
1.3 Case Studies 16
1.3.1 Radical–Radical Reactions: O + OH O2 + H 17
1.3.2 Radical–Molecule Reactions—O(3P) + Alkenes 18
1.3.3 Reactions with Barriers and the Roleof Tunnelling—The F + H2 Reaction 22
1.3.4 Thermodynamics—The HO3 Radical 25
1.3.5 Kinetics of Cluster Formation: Water Clusters (H2O)n 28
1.3.6 Product Branching Studies at Low Temperatures—C2H + Butenes 34
1.3.7 Product Branching Studies at Low Temperatures—The CPUF Technique 36
1.4 Conclusion and Perspectives 39
References 41
Chapter 2 Cold Molecular Collisions: Quantum Scattering Calculations and Their Relevance in Astrophysical Applications 46
2.1 Introduction 46
2.2 Quantum Scattering Calculations 48
2.2.1 Elastic and Inelastic Processes 48
2.2.2 Born–Oppenheimer Approximation 49
2.2.3 Center-of-mass Frame of Coordinates 51
2.2.3.1 Classical Mechanics Framework 51
2.2.3.2 Quantum Mechanics Framework 52
2.2.4 Structure of the Hamiltonian, Scattering Equation, and Boundary Conditions 53
2.2.5 Spectrum of the Monomer Hamiltonian 55
2.2.6 Partial Wave Expansion and Scattering Matrix 56
2.2.7 Solving Scattering Equations 59
2.2.8 Decoupling Approximations 62
2.2.9 Collisional Excitation of Non-linear and Open-shell Molecules 63
2.2.9.1 Collisions of Rare-gas Atoms with 2 Electronic State Molecule 64
2.2.9.2 Hund's Case (a) Wavefunction and Potential Matrix Elements for an Atom–2 Molecule Complex 67
2.3 Astrophysical Applications 69
2.3.1 Context 69
2.3.2 Collisional Rate Coefficients and Their Use in Radiative Transfer Calculations 71
2.3.3 Available Data 73
2.3.4 The Importance of Accurate Rate Coefficients 74
2.4 Examples 75
2.4.1 CO(X1 + ) 75
2.4.2 OH(2) 79
2.5 Conclusion 85
References 88
Chapter 3 Low-energy Scattering in Crossed Molecular Beams 92
3.1 Introduction 92
3.2 Basic Notions on Molecular Collisions 93
3.2.1 System Description 93
3.2.1.1 Laboratory Frame Versus Centre-of-mass Frames 93
3.2.1.2 Centrifugal Barrier 96
3.2.2 Collision Cross-section 97
3.2.2.1 Opacity Function 97
3.2.2.2 Integral and Differential Cross-section 98
3.2.2.3 Influence of Collision Energy: Classical ExcitationFunctions for Endo/Exoergic Processes 100
3.2.2.4 Microreversibility and Yield Function 101
3.2.2.5 Microcanonic to Canonic Transform: From Cross-section to Rate Coefficient 103
3.3 Molecular Beams and Detection Techniques 104
3.3.1 Crossed Supersonic Beams 105
3.3.1.1 Molecular Supersonic Beam 106
3.3.1.2 Crossed Molecular Beams 113
3.3.1.2.1 Principle. 113
3.3.1.2.2 Beam Velocity Control. 114
3.3.1.2.3 Experimental Setup. 115
3.3.1.2.4 Relative Translational Energy: Energy Spread. 116
3.3.1.2.5 Merged Beams. 118
3.3.2 Detection Techniques 121
3.3.2.1 ``Universal'' Detection (MS + TOF) and TOF MS Detection 121
3.3.2.2 Spectroscopic Methods and Doppler Profiles 123
3.3.2.3 Velocity-map Imaging (VMI) 123
3.3.3 Signal Acquisition and Treatment 125
3.3.3.1 Noise Reduction 125
3.3.3.2 Uncertainty Propagation 126
3.4 Relation Between Measured Data and Observable Quantities 126
3.4.1 Production Flux and Cross-section 126
3.4.2 Interaction Time 131
3.4.3 Flux-to-density Conversion 134
3.5 Selected Examples 137
3.5.1 Reactive Collisions 137
3.5.1.1 C+ CxHy Reactions 137
3.5.1.2 S + HD Reaction Dynamics 140
3.5.2 Inelastic Collisions: O2 +H2 143
3.6 Conclusion 146
References 146
Chapter 4 Long-range Interactions Between Ultracold Atoms and Molecules 150
4.1 Introduction 150
4.2 Multipolar Expansion of the Potential Energy Between Two Distant Charge Distributions 157
4.2.1 The Interaction Energy in Cartesian Coordinates 158
4.2.1.1 Electrostatic Potential Created by the Distribution A at a Distant Point 158
4.2.1.2 Interaction Energy Between Distribution B and an External Weakly Varying Electrostatic Potential 161
4.2.1.3 Interaction Energy Between the Distributions A and B 162
4.2.2 Calculation in Spherical Coordinates 164
4.2.2.1 Calculation in the Body-fixed Frame 165
4.2.2.2 Calculation in the Space-fixed Frame 166
4.3 Perturbative Calculation of Long-range Interactions 168
4.3.1 Matrix Elements of Multipole-moment Operators 169
4.3.2 Potential Energy in an External Electric Field 171
4.3.3 First-order Energy Correction from Long-range Interactions 174
4.3.4 Second-order Energy Correction from Long-range Interactions 175
4.3.4.1 Induction Energy 176
4.3.4.2 Dispersion Energy 177
4.3.4.3 Second-order Energy Correction and Irreducible Tensors 178
4.3.5 Example: Long-range Interaction Between Two Alkali-metal Atoms 182
4.3.5.1 Interaction Between Two Ground-state Atoms 182
4.3.5.2 Interaction Between One Ground-state Atom and One Atom of a Different Species in the First Excited State 184
4.3.5.3 Interaction Between One Ground-state Atom and One Atom of the Same Species in the First Excited State 185
4.4 Long-range Interactions Between Two Heteronuclear Alkali-metal Diatomic Molecules in an External Electric Field 186
4.4.1 Calculation in the Body-fixed Frame Without an External Field 187
4.4.1.1 Calculation for Each Individual Rotational Level 187
4.4.1.2 Calculation for Coupled Rotational Levels 190
4.4.2 Calculation in the Space-fixed Frame 192
4.4.2.1 Choice of Basis for the Unperturbed Space 193
4.4.2.2 Dipole–Dipole Interaction Without Electric Field 195
4.4.2.3 Application of an External Electric Field 196
4.5 Conclusion 197
References 198
Chapter 5 Interactions of Atoms and Molecules in Cold Chemistry 203
5.1 Introduction 203
5.1.1 Preface 203
5.1.2 Relevance 205
5.1.3 State of the Art 206
5.2 Atomic and Molecular Interactions—A GeneralTheory 207
5.2.1 The Born–Oppenheimer Approximation 207
5.2.2 Definition of the Interaction Energy 210
5.2.3 Components of the Interaction Energy 211
5.2.4 Electrostatic Energy 213
5.2.5 Induction Energy 215
5.2.6 Dispersion Energy 216
5.2.7 Exchange Energy 217
5.2.8 Many-electron SAPT 218
5.2.9 Problem of Charge Transfer 219
5.3 State-of-the-art Quantum Chemistry Methodsfor Calculations of Potential Energy Surfaces 219
5.3.1 Hartree–Fock Approximation and Electron Correlation 220
5.3.2 Hartree–Fock Interaction Energy 222
5.3.3 Correlated Methods: Interaction Energies in MBPTand CC frameworks 223
5.3.4 Open-shell Systems 226
5.3.5 Multireference Methods 227
5.3.6 DFT Methods 230
5.3.7 Basis Set Issues and Explicitly Correlated Wavefunctions 232
5.4 Example Systems 236
5.4.1 He–NH and N–NH Systems 236
5.4.2 He–OH(2) 241
5.4.3 NH–NH and O2–O2 244
5.4.4 Alkali-metal and Alkaline-earth Atoms Interacting with Molecules 248
5.4.5 Interactions Involving He(3S) and Ne(3P) Atoms 253
5.5 Remarks on Accuracy in Calculations forCold Systems 255
5.5.1 Scattering Length Dependence on theInteraction Potential 255
5.5.2 Highly Accurate Long-range Potential 260
5.6 Summary and Outlook for the Future 262
References 263
Chapter 6 Effects of External Magnetic Fields on Cold Molecular Collisions 276
6.1 Introduction 276
6.2 Molecular Collisions 278
6.3 Molecular Energy Levels in External Fields 280
6.3.1 1+ Molecules: KRb and RbCs 280
6.3.2 2+ Molecules: YbF, CaF, SrF 284
6.3.3 3 Molecules: NH 287
6.3.4 2 Molecules: OH 289
6.4 Coupled-channel Equations for Molecules in External Fields 292
6.4.1 Fully Uncoupled Basis 293
6.4.2 Total Angular Momentum Basis 294
6.5 Elastic Collisions: Controlling the Scattering Length 297
6.6 Inelastic Collisions: Controlling Molecular Spin Relaxation 301
6.6.1 2+ Molecules 302
6.6.2 3 Molecules 304
6.7 Summary and Outlook 308
References 309
Chapter 7 Role of Resonances at Ultracold Temperatures 313
7.1 Introduction 313
7.2 Scattering Theory: An Overview 314
7.2.1 General Results 315
7.2.1.1 Channels 315
7.2.1.2 Single-channel Case 316
7.2.1.3 Multichannel Case 321
7.2.1.4 The Close-coupling Method 324
7.2.2 Jost Function 328
7.2.2.1 Single-channel Case 328
7.2.2.2 Multichannel Case 329
7.3 Resonances 331
7.3.1 Resonances in the Single-channel Case 331
7.3.1.1 Breit–Wigner Formula 332
7.3.1.2 Time Delay 334
7.3.1.3 Effect of Background Scattering 335
7.3.2 Multichannel Case 336
7.3.3 Feshbach Resonances 339
7.3.3.1 General Results 340
7.3.3.2 A Simple Example 345
7.4 Simple Reactions: Photoassisted Reactions 350
7.4.1 Photoassociation and Dissociation: Theory 350
7.4.2 Photoassociation and Dissociation: Examples 354
7.4.3 Feshbach Optimized Photo-Association (FOPA) 357
7.5 Simple Chemical Reactions 363
7.5.1 General Theory 363
7.5.2 Systems with Reaction Barrier 365
7.5.3 Higher Partial Waves and Spin Symmetry Effects 374
7.5.4 Tuning Resonances 381
7.6 Conclusions 384
References 385
Chapter 8 Experiments with Large Superfluid Helium Nanodroplets 389
8.1 Introduction 389
8.2 The State of Large Helium Droplets 391
8.2.1 Excitations in a Helium Droplet 391
8.2.2 Evaporative Cooling of Helium Droplets 398
8.3 Production of Large Droplets 401
8.4 Characterisation of the Helium Droplet Beam 409
8.4.1 Determining the Sizes of Large Helium Droplets and Embedded Clusters 409
8.4.2 Size Distribution of Helium Droplets 412
8.4.3 Determination of Droplet Velocity 415
8.4.4 Imaging Droplets by Microscopy 416
8.5 Formation Kinetics of Large Clustersin He Droplets 418
8.6 Spectroscopy in Helium Droplets 421
8.6.1 Optical and Infrared Spectroscopy 421
8.6.2 Chemical Reactions in Helium Droplets 425
8.7 Deposition Experiments 426
8.8 X-ray Coherent Diffractive Imaging Experiments 429
8.9 Conclusions 435
References 435
Chapter 9 Molecular Impurities Interacting with a Many-particle Environment: From Ultracold Gases to Helium Nanodroplets 444
9.1 Introduction 444
9.2 Superfluidity and Bose–Einstein Condensation 448
9.3 Molecular Impurities Trapped Inside Superfluids 451
9.4 Theoretical Description of the Molecular Impurity Problem 456
9.4.1 Molecular Hamiltonian 456
9.4.1.1 Angular Momentum Operators in the Molecular and Laboratory Frames 457
9.4.1.2 Linear Molecules 459
9.4.1.3 Symmetric-top Molecules 459
9.4.1.4 Asymmetric-top Molecules 461
9.4.2 Boson Hamiltonian 461
9.4.2.1 Introduction to Second Quantization 461
9.4.2.2 A System of Interacting Bosons 462
9.4.2.3 Bogoliubov Approximation and Transformation 463
9.4.2.4 Angular-momentum Representation of the Boson Operators 466
9.4.3 Molecule–Boson Interaction 467
9.5 The Angulon Quasiparticle 471
9.5.1 Second-order Perturbation Theory 471
9.5.2 Non-perturbative Analysis in the Weak Coupling Regime 473
9.5.3 The Canonical Transformation 481
9.5.4 The Limit of a Slowly Rotating Impurity 484
9.6 Conclusions and Outlook 486
References 489
Chapter 10 Cold Ion Chemistry 496
10.1 Introduction 496
10.2 Theory of Ion–molecule Collisions and Reactions 497
10.2.1 Elastic Collisions 497
10.2.2 Inelastic Collisions 499
10.2.3 Capture Models 501
10.2.4 Radiative Processes 506
10.3 Experimental Methods 507
10.3.1 Ions in Flows and Beams 507
10.3.2 Ion Trapping 510
10.3.3 Buffer-gas Cooling 512
10.3.4 Laser and Sympathetic Cooling 514
10.3.5 State-preparation of Neutral Molecules and Ions 517
10.4 Review of Selected Results 520
10.4.1 Temperature Dependence of Reaction Rates 520
10.4.2 Combined Cold-molecules–Cold-ions Experiments 522
10.4.3 Ion–Atom Hybrid Traps 524
10.5 Conclusions and Outlook 527
References 528
Chapter 11 Controlling a Quantum Gas of Polar Molecules in an Optical Lattice 537
11.1 Introduction 537
11.2 Creation of Ultracold Molecules 538
11.2.1 Magneto-association 540
11.2.2 Coherent Optical Transfer 542
11.3 Quantum-state Controlled Chemical Reactions and Dipolar Collisions 545
11.3.1 Quantum State Control of the Molecule 545
11.3.2 Inducing the Dipole Moment in a Laboratory Frame with DC and AC Electric Fields 546
11.3.3 Role of Quantum Statistics in Collisions 548
11.3.4 Reduced Dimensions: Quantum Stereodynamics of Chemical Reactions in a Two-dimensional gas 550
11.3.5 Sticky Collisions: Three-molecule Collision Channels 554
11.4 Suppression of Chemical Reactions in a Three-dimensional Lattice 555
11.4.1 The Continuous Quantum Zeno Mechanism: Stability from Strong Dissipation 555
11.4.2 Long-lived Molecules in a Three-dimensional Optical Lattice 557
11.4.3 Quantum Magnetism with Polar Molecules in a Three-dimensional Optical Lattice 559
11.4.4 A Low-entropy Quantum Gas of Polar Molecules in a Three-dimensional Optical Lattice 563
11.5 Outlook: New Directions 568
11.5.1 Evaporative Cooling of Reactive Dipolar Molecules in Quasi-Two-dimensional Geometry 568
11.5.2 Putting Molecules Under the Microscope: High Resolution Imaging 569
11.5.3 Large, Stable Electric Fields Provided with In vacuo Electrodes 569
11.5.4 Detection of Cold Chemical Reaction Products 572
11.5.5 Molecules with both Electric and Magnetic Dipoles 572
References 573
Chapter 12 Ultracold Collisions of Molecules 579
12.1 Introduction 579
12.2 The Schrödinger Equation 581
12.2.1 The Schrödinger Equation for One Particle 581
12.2.2 The Schrödinger Equation for Two Colliding Particles 582
12.2.2.1 Coordinate Systems 582
12.2.2.2 Types of Collisions 585
12.3 In the Region Far from Collision 587
12.3.1 Asymptotic Form of the Wavefunction 587
12.3.2 Observables 588
12.4 In the Region of Collision 590
12.4.1 Partial Wave Expansion 590
12.4.2 Coupled Equations 594
12.4.3 Long-range Interactions Described by an Electrostatic Multipole–Multipole Expansion 596
12.4.4 Propagation: Log-derivative Z Matrix 597
12.4.5 Symmetry Considerations 598
12.4.5.1 Inversion Symmetry 598
12.4.5.2 Permutation Symmetry 599
12.4.5.3 Collisions in External Fields 600
12.5 Matching the Two Regions 601
12.5.1 Reactance Matrix K: Relation with Z 601
12.5.2 Scattering Matrix S: Relation with K 602
12.5.3 Transition Matrix T: Relation with Observables 604
12.5.4 Link to Scattering of Structureless Particles: The Central Potential Problem 605
12.6 Behaviour at Ultralow Energy: Scattering Length and Threshold Laws 607
12.7 Application to Ultracold Collisions of Dipolar Molecules in Electric Fields 609
12.7.1 A Simplified Problem 610
12.7.1.1 Long-range Interaction 610
12.7.1.2 A Short-range Tunable Condition 611
12.7.2 Molecules in an Electric Field 613
12.7.3 Collisions of Molecules in an Electric Field 614
12.7.3.1 Molecules in the Ground Rotational State: Enhancement of the Loss Rates 617
12.7.3.2 A Quantum Threshold Model 619
12.7.3.3 Molecules in the First Rotational Excited State:Suppression of the Loss Rates 621
12.8 Conclusion and Perspectives 624
References 627
Chapter 13 Coherent Control of Cold Collisions 633
13.1 Introduction 633
13.2 The Concept of Coherent Control 634
13.3 The Difficulty of Coherently Controlling Collisions 639
13.4 Coherent Control of Photoassociation 642
13.4.1 Pump–Dump Photoassociation with Short Pulses: Optimizing the Photoassociation Efficiency 643
13.4.2 Enhancing the Initial Pair Density for Photoassociation 646
13.4.3 Pump–Dump Photoassociation with Short Pulses: Optimizing the Dump Step 649
13.4.4 Short-pulse Photoassociation Experiments 652
13.5 Further Prospects for the Coherent Control of Cold Collisions 654
13.6 Summary 657
References 658
Subject Index 663