Atomic and Molecular Beam Methods

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Atomic and Molecular Beam Methods VOLUME 1 Edited by GIACINTO SCOLES Donner Professor of Science Princeton University Associate Editors DAVIDE BASSI Professor of Electronics University of Trento

1 Atomic and Molecular Beam Methods VOLUME 1 Edited by GIACINTO SCOLES Donner Professor of Science Princeton University Associate Editors DAVIDE BASSI Professor of Electronics University of Trento UDO BUCK Max-Planck-lnstitut für Strömungforschung and Professor of Physics University of Göttingen DEREK LAINE Reader in Physics University of Keele New York Oxford OXFORD UNIVERSITY PRESS 1988

Contents PART I BASIC TECHNIQUES 1. Introduction, 3 G. Scoles 1.1 A Microscopic Approach to the Study of Matter, 3 1.1.1 The Importance of Scattering in Molecular Physics, 4 1.1.2 Beams and the Mechanism of Chemical Reactions, 5 1.

2 Contents PART I BASIC TECHNIQUES 1. Introduction, 3 G. Scoles 1.1 A Microscopic Approach to the Study of Matter, The Importance of Scattering in Molecular Physics, Beams and the Mechanism of Chemical Reactions, Jet Cooling and Its Impact on Spectroscopy, The Study of Surfaces and Overlayers with Beams, Beams as Sources of Unstable Species and as Clean Sample Injection and Extraction Tools, A Few "Standard" Molecular Beam Experiments, Experiments with One Beam Collimation, Experiments with More than One Beam Collimation, The Molecular Beam Laboratory, Universality versus Specialization in a Molecular Beam Apparatus: The Case for Modular Construction and Standarization, The Layout, Organization, and Economics of a Molecular Beam Laboratory, Free Jet Sources, 14 David R. Miller 2.1 Introduction and Background, Structure of the Free Jet, Ideal Thermodynamic Analysis of the Expansion, Continuum Properties of the Free Jet Expansion, The Subsonic Expansion, The Supersonic Expansion and Method of Characteristics, Calculated Mow Field Properties, Nonequilibrium Kinetic Effects, Collision Rates, Noncontinuum Flow and Kinetic Theory, Measurements and Correlations for the Terminal Speed Ratio, 29

2.3.4 The Perpendicular Temperature and the "Quitting Surface"Model, 31 2.3.5 Free Jet Intensity Background Gas Scattering and Skimmer Losses, 32 2.3.6 Internal Energy Relaxation, 35 2.3.7 Clustering and Condensation, 38 2.

3 2.3.4 The Perpendicular Temperature and the "Quitting Surface"Model, Free Jet Intensity Background Gas Scattering and Skimmer Losses, Internal Energy Relaxation, Clustering and Condensation, Binary Mixtures Separation, Velocity, and Temperature Slip, Free Jet Design, Component Fabrication and Pump Selection, Design Calculation Examples, Low-Energy Pulsed Beam Sources, 54 W. Ronald Gentry 3.1 General Considerations, Definition and Characteristics of Molecular Beam Pulses, Comparison of Pulsed and Steady-State Beams, Methods for Producing Pulsed Beams, Fundamental Limitations, Pulsed Source Designs, Auxiliary Techniques, Design of Pulsed Beam Experiments, Single-beam Experiments, Crossed-beam Experiments, Gas-surface Scattering Experiments, Other Low-energy Beam Sources, 83 H. Pauly 4.1 Introduction, Theory of Effusive Sources, Thin-walled Orifice, Channels, Comparison of Effusive Sources, Applications of Effusive Beams, Description of Sources, Associated Equipment, and Experimental Techniques, General Remarks, Sources Using Gases (Temperature Range 4-300K), Sources for Operation to about 1000 K, Atomic Beam Sources for Operation at High Temperatures (to 2800 K), Sources for Highly Refractory Materials (Operation Temperatures above 2800 K), Recirculating Sources, Sources with Internal Shutter, and Gated Sources, 104

4.4 Sources for Beams of Radicals, 107 CONTENTS xiii 4.4.1 Radical Production by Pyrolysis, 107 4.4.2 Gas Discharge Sources, Microwave Arcs, and Flow Tubes, 109 4.4.3 Flow Tube Sources, 111 4.4.4 Radical Production by Photolysis, 112 4.

4 4.4 Sources for Beams of Radicals, 107 CONTENTS xiii Radical Production by Pyrolysis, Gas Discharge Sources, Microwave Arcs, and Flow Tubes, Flow Tube Sources, Radical Production by Photolysis, Sources of Atoms in Excited States, Electron Impact Excitation, Gas Discharge Sources, Optical Excitation, High-energy Beam Sources, 124 H. Pauly 5.1 Introduction, Sources with Resonant or Near-resonant Charge Exchange, Beams of Ground State Atoms, Beams of Metastable Atoms, Sources Utilizing Electron Detachment Processes, Photodetachment, Auto detachment, Stripping and Collisional Detachment, Aerodynamic Acceleration, Seeded Beams, Supersonic Plasma Jets, Shock Tube Sources, Sputtering Sources, Mechanical Acceleration, Discharge Sources, Hollow Cathode Discharge Source for Metastable Atoms, Corona Discharges, Laser-generated Pulsed Atomic Beams, Evaporation of Accelerated Micropellets, Comparison of High-energy Beam Sources and Concluding Remarks, Detection Principles, 153 D. Bassi 6.1 Introduction, The Response Function of Beam Detectors, Counting detectors, Noise in Beam Detectors, Power Spectral Density Function, Shot Noise, 157

MV CONTENTS 6.3.3 Johnson Noise, 159 6.3.4 Flicker Noise, 159 6.3.5 Interference Noise, 160 6.4 Electronic Methods for Signal Analysis, 161 6.4.1 Filters, 161 6.4.2 Lock-in Amplifiers, 162 6.4.3 Impulse Response Measurements, 164 7.

5 MV CONTENTS Johnson Noise, Flicker Noise, Interference Noise, Electronic Methods for Signal Analysis, Filters, Lock-in Amplifiers, Impulse Response Measurements, Ionization Detectors I: Ion Production, 168 D. Bassi 7.1 Introduction, Surface Ionizers, Electron Impact Ion Sources, General Considerations, Design and Operation of Electron Impact Sources, Thermoionic Cathodes, Field Emission and Field Ionization Sources, Ionization Detectors II: Mass Selection and Ion Detection, 180 D. Bassi 8.1 Quadrupole Mass Spectrometer, 180 & 1.1 Practical Quadrupoles, Electrical Alignment of Quadrupoles, Magnetic Sector Mass Spectrometer, Time-of-flight Mass Spectrometer, Ion Detection, Faraday Cup, Electron Multipliers, The Scintillation Detector, Sensitivity of Ionization Detectors, Spectroscopic Detection Methods, 193 U. Hefter and K. Bergmann 9.1 Theoretical Background and Methodology, Absorption and Emission of Light, Kinetics of Excitation, Detection of Flux and Density, Detection of Internal State Populations, Detection of Alignment, Detection of Velocities, Experimental Components, Propagation of Laser Beams, Optical Fibers, Collection of Laser-induced Fluorescence, Special Problems Related to the Detection of Fluorescence with Photomultipliers, 244

9.3 Detection by Photoionization, 245 CONTENTS xv 9.3.1 Brief Survey of Ionization Pathways, 245 9.3.2 Qualitative Features of Photoionization Detection, 246 9.3.3 Quantitative Analysis, 247 9.3.4 Line Shape Problems, 249 10.

6 9.3 Detection by Photoionization, 245 CONTENTS xv Brief Survey of Ionization Pathways, Qualitative Features of Photoionization Detection, Quantitative Analysis, Line Shape Problems, Accommodation, Accumulation, and Other Detection Methods, 254 M. Zen 10.1 Cryogenic Bolometers, Semiconductor Bolometers, Superconducting Bolometers, Noise Sources, Construction Details and Typical Performance of Practical Bolometers, Pyroelectric Detectors, Metastable Particles Detection, Adsorption Detectors, Accumulation Detectors, State Selection by Nonoptical Methods, 276 J. Reuss 11.1 History, Magnetic Deflection and Magnetic Focusing, Electric Deflection and Electric Focusing, Principles and Typical Applications, The Basics of Selection by Deflection (SBD), Applications of SBD, Z3 The Basics of Selection by Focusing (SBF), Applications of SBF, Technical Aspects, Conventional Focusing Devices and Design Problems, Beam Stops and Wobbling, Majorana Flops, Unconventional Selectors, Alternating Gradient Focusers, Obtainable Number Densities, State Selection by Optical Methods, 293 K. Bergmann 12.1 Introduction and Selection Principles, Molecular State Selection by Population Depletion, Rotational and Vibrational State Selection, Molecular Alignment and m State Selection by Optical Pumping, 301

xvi CONTENTS 12.2.3 Alignment of Ground State Molecules by Photodissociation, 307 12.2.4 Combined Internal State and Velocity State Selection, 311 12.

7 xvi CONTENTS Alignment of Ground State Molecules by Photodissociation, Combined Internal State and Velocity State Selection, Molecular State Preparation by Selective Population: Electronic Ground State Molecules and Molecular Ions, Techniques Involving One Laser Photon, Techniques Involving Two Laser Photons, Vibrational State Selection in Molecular Ions, State Preparation by Selective Population or Depletion of Atomic Ground and Electronically Excited States, Analytical Solution of the Rate Equations, Experiments Involving Atomic Ground States, Experiments Involving Electronically Excited States, Two-step Excitation of Higher-lying Levels, Experimental Problems, Other Selection Principles, Dissociation of van der Waals Molecules, "Selection" of Impact Parameters, Photodeflection, Concluding Remarks, Velocity Selection by Mechanical Methods, 345 С ). N. van den Meijdenberg 13.1 Introduction, The Slotted Disk Velocity Selector, General Design Considerations, Transmission Function and Resolution for a Parallel Beam, Transmission Function and Resolution for a Divergent Beam, Velocity Sideband Elimination, Velocity Calibration, Technical and Practical Comments, Other Designs, Slotted Cylinder Velocity Selectors, Slotted Plate Velocity Selectors, Slotted Ring Velocity Selectors, Compact Velocity Selectors, Velocity Measurements by Time-of-flight Methods, 362 Daniel J. Auerbach 14.1 Introduction, Basic Ideas of Time-of-flight Methods, Classification of Methods, 363

8 14.2 Mathematical Preliminaries, 365 CONTENTS xvii Transformations from Velocity to Time Distributions, Resolution Considerations, Deconvolution, Cross-correlation Method, Experimental Apparatus, Mechanical Choppers, Other Choppers, Detectors, TOF Electronics, Electronics for Cross-Correlation Methods, Ъ1А 14.5 Calibration, Illustrative Examples, Molecular Beams of Clusters, 380 M. Kappes and S. Leutwyler 15.1 Research with Atomic and Molecular Clusters, Cluster Formation and Growth in Expanding Gas Flows, Theoretical Models, Experimental Studies, Effusive and Coalescence Growth Sources Types I and II, Effusive Sources, Aggregation and Aerosol Sources, Supersonic Cluster Beams Types HI and IV, Free Jets, Constrained Expansions, Pulsed Versus Continuous Cluster Sources, Mixed Expansions, Clusters oflnvolatile Materials, Beams of Charged Clusters, Manipulated Expansion Zones, Photons, Electrons, Atoms and Molecules, Cluster Size and Size Distribution Measurements in Molecular Beams, Electron and Light Scattering Measurements, Mass Spectrometry Techniques, Cluster Structures and Dynamics, Electric Deflection, High-energy Electron Diffraction, Cluster Temperatures and Phase Transitions, 408

xviii CONTENTS 16. Molecular Beam Epitaxy, 416 K. Ploog 16.1 Introduction, 416 16.2 Technology and Film Growth Processes, 417 16.2.1 General Considerations, 417 16.2.2 MBE Growth Apparatus, 419 16.2.3 Effusion Cells for Molecular Beam Generation, 420 16.

9 xviii CONTENTS 16. Molecular Beam Epitaxy, 416 K. Ploog 16.1 Introduction, Technology and Film Growth Processes, General Considerations, MBE Growth Apparatus, Effusion Cells for Molecular Beam Generation, Substrate Processing, In Situ Growth Monitoring by RHEED, Kinetic and Mechanism of Growth Processes, Dopant Incorporation, Initiation of Growth, Application of MBE to Band-structure Engineering in Semiconductors, Concluding Remarks, Molecular Beams in High-energy Physics: Beam Targets, 438 U. Valbusa 17.1 Introduction, Design of a Jet Target, Luminosity, Vacuum in the Accelerator, Target Source, Polarized Beam Targets, 444 PART II MOLECULAR SCATTERING 18. General Principles and Methods, 449 U. Buck 18.1 Introduction, The Basic Scattering Problem, Design Criteria for the Experimental Setup, The Center-of-mass-Laboratory Transformation, Experimental Methods, Elastic Scattering, Inelastic Scattering, Reactive Scattering, Elastic Scattering I: Integral Cross Sections, 472 J. J. H. van den Biesen 19.1 Introduction, Experimental Setup, The Leiden Apparatus, Some General Experimental Remarks, 476

19.3 Data Reduction, 477 CONTENTS xix 19.3.1 General Formulas, All 19.3.2 The Velocity Resolution Correction, 480 19.3.3 The Angular Resolution Correction, 483 19.3.4 Concluding Remarks, 485 19.

10 19.3 Data Reduction, 477 CONTENTS xix General Formulas, All The Velocity Resolution Correction, The Angular Resolution Correction, Concluding Remarks, Determination of /,(t>,)> f k (v k ), P(a), and W(G), The Primary Beam Speed Distribution, fi(vj, The Secondary Beam Velocity Distribution, f k (v k ), P(a), The Angular Resolution Function W(0), Absolute Calibration of (ra*l) e E, Introduction, Relative Measurements with Respect to a Known Standard, Combination with Small Angle Differential Cross Sections, Calibration of a Supersonic Expansion, Calibration by Means of a Scattering Cell, Measurements of Glory Undulations, Measurements of Glory Undulations for Atommolecule Systems, Measurements of Symmetry Oscillations, Measurements of Orbiting Resonances, Measurements at High Energies, Elastic Scattering II: Differential Cross Sections, 499 U. Buck 20.1 Introduction, Experimental Geometries and Components, Data Analysis and Extraction of Cross Sections, Cross-section Features and Results, Rainbow Scattering, Diffractive Scattering, Symmetry and g-u Oscillations, Other Features, Molecular Systems, Interplay Between Theory and Experiment, Inelastic Scattering I: Energy Loss Methods, 525 U. Buck 21.1 Introduction, The Basic Experimental Setup, Initial State Preparation, 527

21.2.2 Velocity Analysis, 528 21.2.3 Beam Machines and Time-of-flight Spectra, 530 21.3 Data Analysis and the Extraction of Cross Sections, 535 21.3.1 Simple Corrections, 535 21.3.2 Simulation of Final Velocity Distributions, 535 21.

11 Velocity Analysis, Beam Machines and Time-of-flight Spectra, Data Analysis and the Extraction of Cross Sections, Simple Corrections, Simulation of Final Velocity Distributions, Determination of Cross Sections, Inelastic Molecular Collision Cross Sections, State-resolved Cross Section for Rotational Excitation, Energy-loss Spectra: Rotational Rainbows, Vibrational Excitation, Electronic Excitation, Interplay Between Theory and Experiment, State-resolved Data, Unresolved Energy-loss Spectra, Reactive Scattering I: Nonoptical Methods, 553 Y. T. Lee 22.1 General Considerations of Reactive Scattering, Feasibility of Reactive Scattering Experiments Measuring Angular and Velocity Distributions of Products, Signal-to-noise Ratio and the Differential Pumping of Detector Chambers, Kinematic Considerations, Experimental Arrangement for Reactive Scattering, Different Arrangements for Reactive Scattering, Some Typical Examples of Reactive Scattering Arrangements, Problems Associated with Product Identification, Fragmentation of Products during Electron Impact Ionization, Elucidation of Reaction Mechanism, Laboratory to Center-of-mass Transformation, Inelastic Scattering II: Optical Methods, 569 P. J. Dagdigian 23.1 Detection of Final States, Laser Fluorescence Detection, Infrared Laser Excitation and Bolometric Detection, Incident State Preparation, 572

23.3 Scattering Geometries and Determination of CONTENTS xxi Inelastic Cross-sections, 575 23.3.1 Angle-resolved Experiments, 575 23.3.2 Doppler Shift Experiments, 579 23.3.3 Total Cross-section Measurements, 582 23.

12 23.3 Scattering Geometries and Determination of CONTENTS xxi Inelastic Cross-sections, Angle-resolved Experiments, Doppler Shift Experiments, Total Cross-section Measurements, Beam-Gas Geometry, Measurement of m-dependent Cross Sections, Reactive Scattering II: Optical Methods, 596 P. J. Dagdigian 24.1 Introduction, Analyzing Laser Fluorescence Spectra, Spectroscopic Data Required, Rotational Line-strength Factors, Determination of Moments of the Product m-state Distribution, Vibrational Populations and Deconvolution of Unresolved Spectra, Electronic State Branching Ratios, Extraction of Cross Sections, Total Cross Sections, Product Angular Distributions, Doppler Velocity Analysis, Cross Sections as a Function of Collision Energy, Chemiluminescence Experiments, Optical Preparation of Reagents for Reactive Scattering, Vibrationally Excited Reactants, Radiating Excited States, Optical Pumping State Selection, Promising New Optical Techniques, Scattering Experiments with State Selectors, 631 S. Stolte 25.1 Introduction, Elastic Scattering, Measurement of Total Collision Cross Sections, Measurement of Differential Cross Sections, Inelastic Scattering, Measurements of State Distributions after Inelastic Collisions, State Selection before and after an Inelastic Collision, 641

xxü CONTENTS 25.4 Reactive Scattering, 643 25.4.1 Measurement of Reaction Products Using State Selection, 643 25.4.2 Reactive Scattering with State-selected Reactants, 644 25.5 Outlook, 648 26.

13 xxü CONTENTS 25.4 Reactive Scattering, Measurement of Reaction Products Using State Selection, Reactive Scattering with State-selected Reactants, Outlook, Scattering Experiments with Laser-excited Atomic Beams, 653 R. Düren 26.1 Introduction, Experiments with Initial State Preparation, Experiments with Final State Analysis, Data Analysis and Interpretation Methods, Definitions, Measured Quantities, Data Analysis: Averaging and Lab-cm Transformation, Data Analysis: Frame Transformation z E * z s, Excitation of the Atomic Beam, Laser Requirements, System Requirements, Scattering, Excitation Schemes, Model Calculation, Monitoring the Excitation, Theoretical Background, Examples, Structureless Targets, StructuredNonreactive Targets, Reactive Targets, Experiments with Spin-Polarized Beams, 683 S. lannotta 27.1 Introduction, General Considerations, Brief Atomic Species Characterization and Chapter Plan, Magnetic Selectors of Atomic Spin States, Stern-Gerlach, Quadrupolar, and Hexapolar Magnets, Criteria for the Best Magnet Choice for Beams of Different Atomic Species, Inversion of Population for Spin States, Introduction, Adiabatic Radiofrequency Transitions in a Weak Magnetic Field, The Majorana Method: Diabatic Transitions, 696

27.3.4 RF Transitions in Strong Fields: Use of Several CONTENTS xxiii Different Units to Produce an Arbitrary Spin Polarization, 698 27.3.5 Other Methods in the Production of Fast Polarized Beams, 699 27.

14 RF Transitions in Strong Fields: Use of Several CONTENTS xxiii Different Units to Produce an Arbitrary Spin Polarization, Other Methods in the Production of Fast Polarized Beams, Further Design Considerations, Comparison of Methods for Different Atoms and Field of Application, Experiments with Spin-polarized Beams, Gas-phase Experiments, The Stored-beam Spectroscopy Method, Spin-polarized Experiments in Surface Physics, Outlook on Possible Developments, 706 Contributors, 709 Index,715

Vibrational Spectroscopy of Molecules on Surfaces

Vibrational Spectroscopy of Molecules on Surfaces Edited by John T. Yates, Jr. University of Pittsburgh Pittsburgh, Pennsylvania and Theodore E. Madey National Bureau of Standards Gaithersburg, Maryland