HIGH RESOLUTIONLASERPHOTOIONIZATIONANDPHOTOELECTRONSTUDIESWiley Series in Ion Chemistryand Physics

Edited by

Ivan PowisDepartment of Chemistry, University of Nottingham, Nottingham, England, UK

Tomas BaerDepartment of Chemistry, The University of North Carolina at Chapel Hill,Chapel Hill, North Carolina, USA

Cheuk-Yiu NgAmes Laboratory, USDOE, and Department of Chemistry, Iowa State University, Ames ,Iowa, USA

List of Contributors

xi i i

Preface

xv

CHAPTER 1 An Historical Introduction to Threshold Photoionization

1T. Baer and P-M Guyon

1 .1

Introduction

11 .2 Threshold Photoelectron Spectroscopy (TPES)

3

1 .2 .1 The Suppression of Energetic Electrons by TOF

8

1 .2 .2 The Role of Autoionization in TPES

91 .2 .3 The Mechanism for Threshold Electron Production

1 1

1 .3 Threshold Ionization with Pulsed Laser Sources

1 31 .3 .1

The Third Surprise

1 51 .3 .2 A Comparison of PFI-TPES and TPES

1 6

1 .4 Conclusions

1 7

CHAPTER 2 High Resolution Spectroscopy with Photoelectrons :ZEKE Spectroscopy of Molecular Systems

2 1K. Müller-Dethlefs

2 .1 Introduction

2 22 .1 .1 Development of Photoionization Experiments

2 82 .2 From Photoelectron to ZEKE Spectroscopy

3 12 .2 .1 Principle of ZEKE Detection

3 12 .2 .2 The Delayed Pulsed Field Ionization of Rydberg States

3 42 .3 Details of the ZEKE Experiment : Toward Highest Resolution

3 72 .4 Rotationally Resolved ZEKE Spectroscopy of Molecules

4 32 .4.1 NO

4 32 .4.2 Benzene

4 72 .4.3 H2O and H2 S

5 82 .5 Vibrationally Resolved ZEKE Spectroscopy of Molecular Clusters

602 .5 .1 Ar-NO

602 .5 .2 The NO Dimer

602 .5 .3 Hydrogen-bonded Complexes

6 1

2 .5 .4 Comparison between the Different Hydrogen-bondedPhenol-X Complexes

692 .6 Perspectives and Conclusions

70

CHAPTER 3 State-resolved Photoionization Dynamics of SmallMolecules Using Coherent VUV Radiation

7 9R.T. Redman and MG. White

3 .1 Introduction

80

3 .2 Photoionization Dynamics

84

3 .3 Experiment

87

3 .3 .1 Coherent VUV Harmonic Generation

893 .3 .2 ZEKE-PFI Techniques

9 23 .4 Rotationally Resolved Photoionization of Molecules: Selected

Examples

9 6

3 .4.1 Diatomic Molecules

9 6

3 .4 .2 Nonlinear Polyatomic Molecules

10 4

3 .5 Summary

11 2

CHAPTER 4 VUV-ZEKE Photoelectron Spectroscopy : Final-stateInteractions in Small Molecular Systems

119F. Merkt and T.P. Sofiley

4.1 Experimental Techniques

1204 .1 .1 Introduction : The Advantages of Single-photon Ionization

12 04 .1 .2 Vacuum Ultraviolet and Extreme Ultraviolet Radiation Sources 12 14 .1 .3 Experimental ZEKE Photoelectron Spectroscopy

12 64.2 Intensity Perturbations in Rotationally Resolved Spectra

12 84 .2 .1 Intensities in Conventional, Threshold and ZEKE

Photoelectron Spectroscopy

12 84 .2 .2 The Hydrogen Molecule

13 14 .2 .3 The Nitrogen Molecule

13 44.2 .4 The Oxygen Molecule

13 64 .2 .5 The Carbon Dioxide Molecule

13 84 .2 .6 Other Examples of Final-state Interactions

14 0

4 .2 .7 Conclusions

14 1

4 .3 Review of Factors Affecting High Rydberg State Lifetimes

14 34 .3 .1 Lifetimes of the States Detected in ZEKE-PFI Experiments

14 34 .3 .2 The Nature and the Formation of the ZEKE States

14 5

4 .3 .3 Effects of Electric Fields

1474.4 Theoretical Models

1494 .4 .1 Rotational Autoionization

1494 .4 .2 Electric Field Effects

15 54 .4 .3 Complex Resonances in Vibrational and Electronic

Autoionization

164

CHAPTER 5 Rotationally Resolved Autoionization of MolecularRydberg States

17 1H. Lefebvre-Brion

5 .1

Introduction

17 25 .2 Theoretical Treatment of High Rydberg States

17 25 .2 .1 Generalities on Different Hund's Coupling Cases

17 25 .2 .2 Rydberg States Having a E Ion Core

17 35 .2 .3 Rydberg States Having a 2n Ion Core

17 45 .2 .4 Examples of Rydberg Complexes

17 55 .3 Rotational Autoionization

18 45 .3 .1 Rotational Autoionization for a E Ion Core

18 45 .3 .2 Rotational Autoionization for a 2II Ion Core

18 85 .4 Conclusion

19 0

CHAPTER 6 Rotationally Resolved Resonance-enhanced MultiphotonIonization Photoelectron Spectroscopy of Diatomi cHydrides

19 5C.A. de Lange

6 .1 Introduction

19 66 .2 Experimental Techniques

19 96.2 .1

`Magnetic Bottle' Electron Spectrometer

19 96.2 .2 Time-of-flight to Energy Conversion and Calibration

2026 .2 .3 Wavelength Spectra and Photoelectron Spectra

2046 .2 .4 Mass-resolved Ion Detection

20 56 .2.5 ZEKE-PFI

20 56 .2 .6 Nanosecond Laser System

20 66.3 Theory

2076.4 Diatomic Hydrides

21 16 .4.1 The OH Radical

21 16 .4.2 The NH Radical

2206 .4.3 The HC1 Molecule

2326.5 Conclusions

242

CHAPTER 7 Exploiting Polarization in the Study of Molecula rPhotoionization Dynamics

247K.L. Reid and D.J. Leahy

7 .1 Introduction

2487.2 Two-photon Ionization

2547 .2.1 Advantages

2547 .2.2 Formalism

2597.3 Applications

265

7 .3 .1 The First `Complete' Molecular PhotoionizationExperiment : NO(A ZE+)

26 57 .3 .2 Further Experiments

2747 .3 .3 Determination of Alignment

27 57 .3 .4 Extension to Other Systems

2767.4 Conclusions

277

CHAPTER 8 Rotationally Resolved Photoelectron Spectra a tNear-threshold Kinetic Energies

28 1K. Wang and V. McKoy

8 .1 Introduction

2828 .2 Theory and Formulation

2848 .2 .1 (n + 1') REMPI for Linear Molecules

2848 .2 .2 (n + 1') REMPI for Asymmetric Tops

29 18 .2 .3 (n + 1') REMPI for Symmetric Tops

29 58 .2 .4 Computational Procedures

2968.3 Results and Discussion

2978 .3 .1 REMPI via Rydberg States of Linear Molecules

2978 .3 .2 ZEKE-PFI of Linear Molecules

31 28 .3 .3 ZEKE-PFI of Nonlinear Molecules

31 9

CHAPTER 9 Low-frequency Torsional and Vibrational Motions i nToluene +, Phenylsilane+, and Benzyl +: Experimentsand Ab Initio Calculations

33 1K.-T. Lu, G.C Eiden, J.K. Badenhoop, F.A . Weinhold,and JC Weisshaar

9 .1

Introduction

33 29.2 Experimental Technique

33 39 .3 , Internal Rotation in Toluene and Phenylsilane Neutral s

and Cations

3349 .3 .1 Introduction to Internal Rotation Barriers

3349 .3 .2 Rotor States

33 69 .3 .3

S 1 -So Selection Rules

3379 .3 .4 PFI Selection Rules

33 89 .3.5 S1 -So Spectra and Assignments

3409 .3 .6 Cation Rotor States : PFI Spectra of Toluene+ and

Phenylsilane +

34 19 .3 .7 Bond Energies from Adiabatic Ionization Potentials

3479 .3 .8 Ab Initio Equilibrium Geometries and Torsional Barriers

3489 .3 .9 Vibronic Coupling in Phenylsilane + and Toluene+

35 19 .3 .10 Donor-Acceptor Model of CH3 and SiH3 Torsional

Potentials

354

9 .4 R2PI-PFI Studies of the Benzyl Radical

3599 .4 .1 R2PI and PFI Spectra

3599 .4 .2 Assignment of Benzyl+ Vibrational Bands

3609 .4 .3 Vibronic Coupling Mechanism

3649 .4 .4 Nature of Exocyclic C-C Bond in Benzyl and Benzyl+

364

CHAPTER 10 ZEKE Studies with Picosecond Lasers

369J.L. Knee

10 .1

Introduction

37010 .2 Background

37 110 .2 .1 Formulation of the Molecular Problem

37 110 .2 .2 Available Experimental Techniques

37 510 .3 Photoelectron Probing of Molecular Dynamics

37 910 .3 .1 Laser Time-of-flight Photoelectron Spectroscopy

38010 .3 .2 Methodology for Dynamics Studies Using ZEKE PES

38310 .4 Applications of Time-resolved ZEKE

38 910 .4 .1 ZEKE Studies of IVR

38910 .4 .2 Van der Waals Molecule IVR and Vibrational

Predissociation

39 310 .4 .3 Electronic State Nonradiative Transitions

39 810 .5 Future Directions

400

CHAPTER 11 On the Control of Molecular Photoionization : Effectsof Mode Specificity in Discrete-Continuum Couplingon the Dynamics of Near-threshold Electron Ejectio nin NO2

407E.R. Gran t

11 .1

Introduction

40811 .1 .1

Vibrational Autoionization

40811 .1 .2 Normal Coordinate Dependence of Vibrationa l

Autoionization

40911 .1 .3 Electronic Structure of the Higher Excited States of NO 2

41011 .2 Photoselection and the Preparation of Excited States of NO 2

41 111 .3 Triple Resonance and the Mode-selected Vibrational

Autoionization of NO 2

41 311 .4 Triple-resonant Threshold Photoionization of NO 2 : The Effect

of Mode Selectivity in Discrete-Continuum Interactions onThreshold Photoionization Intensities

41911 .4 .1 Method for High-resolution Threshold Photoionizatio n

Spectroscopy

41911 .4.2 Vibrational Structure

420

11 .4 .3 Rotational Structure in the Vibrational Ground State o fNO2+

42 311 .4 .4 Rotational Intensities for Threshold Photoionization to

Form Vibrationally Excited States of NO 2 +

42 911 .5 Conclusions

43 2

CHAPTER 12 Physics of Near-threshold States in Molecula rHydrogen

437E.E. Eyler

12 .1

Introduction

43 812 .2 General Behavior of Near-threshold Levels

43912 .2 .1 Long-range Potentials : Rydberg States in the Coulomb

Potential

43 912 .2 .2 Short-range Potentials

44 1

12 .3 Atom-like Molecules: Nonpenetrating Rydberg States

44 512 .3 .1 Overall Structure of Rydberg States with High n and 1

44 512 .3 .2 Autoionization and Predissociation

44812 .3 .3 Effects of External Electric Fields

44812 .4 Rydberg np States of Molecular Hydrogen

45 012 .4 .1

Understanding Strong Interchannel Perturbations :Two-channel Quantum-defect Theory

45 112 .4 .2 High-precision Energy Level Measurements

45 312 .4 .3 Determination of the Ionization Potential

45 512 .4 .4 Dynamics of the np States

45912 .5 High Vibrational Levels near the Second Dissociation Limit

46 112 .5 .1

General Structure

46 112 .5 .2 Perturbations, Including Gerade-Ungerade Mixing

46412 .5 .3 Relation to Long-range Atomic Collisions and Trap

Physics

46 712 .6 Near-threshold Dissociation to H(ls) + H(2s or 2p)

46 912 .6 .1 General Observations and Calculated Cross-sections

46 912 .6 .2 High-resolution Measurements of Continuum Structure ,

Including Atomic 2s : 2p Branching Ratios

47 012 .6 .3 Determination of the Dissociation Limit

47412 .6 .4 Shape Resonances above the Threshold

47 512 .6 .5 Prospects for Observing Vibrational Rydberg States of a n

Ion Pair

47712 .7 Summary and Conclusions

47 8

Author Index

48 3

Subject Index

503