XUV and VUV Photoabsorption and Emission Studies in Thorium and other High-Z Laser Plasmas

314 Pages·2013·9.28 MB·English

XUV and VUV Photoabsorption and
Emission Studies in Thorium and
other High-Z Laser Plasmas
A thesis submitted for the degree of
Doctor of Philosophy
by
Oonagh Elizabeth Meighan B.Sc.
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UNIVERSITY
Ollscoi! Chalhair Bhaile Atha Cliath
School of Physical Sciences
Research Supervisor
Dr. John T. Costello
August 2000
Mam, !Dad and Vincent
Déclaration
I hereby certify that this material, which I now submit for assessment on the programme
of study leading to the award of Doctor of Philosophy is entirely my own work and has
not been taken from the work of others save and to the extent that such work has been
cited and acknowledged within the text of my work.
Signed: w NOm. 9-//0g .
Candidate
Date: ^ //o ^ o a o
Contents
Dedication i
Declaration ii
Abstract 1
Chapter 1 Introduction and Theory
2
1.1 The Actinide Series and Wavefunction Collapse 3
1.1.1 Background 3
1.1.2 Trends in Actinide Research - Nuclear Physics and Technology 3
1.1.3 Actinide Physics and Chemistry 7
1.1.4 Actinide Atomic Research 8
1.1.5 Recent Advances in Actinide Research at Large-Scale Synchrotron
Facilities 11
1.1.6 XUV and VUV Photoabsorption Studies of Thorium and Uranium 12
1.1.7 5f and 6d Orbital/Wavefunction Collapse in Ground State and Low-Lying
Configurations along the Actinide Series 14
1.1.8 Investigation of Orbital Collapse along the Thorium Isonuclear Sequence 26
1.2 The Atomic Giant Dipole Resonance 29
1.2.1 Background 29
1.2.2 Properties of an Atomic Giant Dipole Resonance 30
1.2.3 The Origin of an Atomic Giant Dipole Resonance 33
1.2.4 Atomic Excitation and Decay Processes Contributing to the formation of
a Giant Dipole Resonance 39
1.2.4.1 Excitation Processes: Photoionisation and Photoexcitation 40
1.2.4.2 Decay Processes: The Auger Effect and Autoionisation 42
1.2.5 Observation of the Collapse of a Giant Dipole Resonance along an
Isonuclear Sequence 46
Chapter 2 The Laser Plasma Light Source and the Dual Laser
Plasma Technique
52
iii
2.1 The Laser Produced Plasma (LPP) 53
2.1.1 Introduction 5 3
2.1.2 Fundamental Physical Properties of a Laser Produced Plasma 53
2.1.3 Generation and Evolution of Laser Produced Plasma 55
2.1.4 Atomic Processes occurring within a Laser Produced Plasma 58
2.1.4.1 Bound - Bound Transitions 59
2.1.4.2 Bound - Free Transitions 60
2.1.4.3 Free - Free Transitions 61
2.1.4.4 Standard Plasma Models 62
2.1.5 Origins and Characteristics of VUV/XUV Continuum Emission from a
Laser Produced Plasma 66
2.1.5.1 Spectral Characteristics of Tungsten XUV and VUV
Continuum Emission 72
2.1.5.2 Temporal Characteristics of XUV and VUV Continuum Emission 74
2.1.6 Recent Advances in Ultra-Short, Ultra-Intense Laser-Matter Interactions 75
2.1.7 Physical Characteristics of Laser Plasmas Produced using Short Pulse
(< Ins) Lasers 77
2.1.8 Advantages of Laser Produced Plasma Sources 78
2.1.9 Primary Applications of the Laser Produced Plasma 81
2.2 The Dual Laser Plasma (DLP) Technique 84
2.2.1 Implementation of the Dual Laser Plasma Technique 84
2.2.2 Origins and Development of the Dual Laser Plasma Technique 86
Chapter 3 VUV and XUV Experimental Spectroscopic Regimes
89
3.1 Nanosecond Experimental Facility 90
3.1.1 Introduction 90
3.1.2 Nd:YAG Nanosecond Laser Systems 90
3.1.3 1 m Normal Incidence VUV Experimental Regime 92
3.1.3.1 System Overview 92
3.1.3.2 Acton Research Corporation™ 1 m Normal Incidence
Spectrometer 96
3.1.3.2.1 Bausch & Lomb™ Diffraction Grating 98
iv
3.1.3.2.2 Off-Rowland Circle Concave Diffraction Grating
Mount 98
3.1.3.3 lm Target Chamber 102
3.1.3.3.1 lm Target Holders 103
3.1.3.3.2 Preparation and Mounting of Target Materials 106
3.1.3.3.3 Handling Thorium - Precautions Required 107
3.1.3.3.4 Target Chamber Laser Focusing Optics 108
3.1.3.4 Photoelectric Detectors 109
3.1.3.4.1 Photometries™ CCD 110
3.1.3.5 lm Normal Incidence Spectrometer System: Performance 112
3.1.3.5.1 Spectral Resolution with CCD Detector 115
3.1.4 2.2m Grazing Incidence XUV Experimental Regime 117
3.1.4.1 System Overview 117
3.1.4.2 McPherson™ 2.2m Grazing Incidence Spectrometer 120
3.1.4.2.1 McPherson™ Spherical Concave Diffraction Grating 120
3.1.4.2.2 Toroidal Coupling/Focusing Optic 121
3.1.4.3 Galileo™ Channel Electron Multiplier Array (CEMA) and
EG&G™ PhotoDiode Array (PDA) 124
3.1.4.4 Performance and Resolution Capabilities of the McPherson™
2.2m Grazing Incidence Spectrometer System 127
3.1.4.4.1 Spectral Resolution of CEMA/PDA Detector System 129
3.1.5 Laser Synchronisation and Procedure for Spectral Data Acquisition 130
Picosecond Experimental Facility 134
3.2.1 System Overview 134
3.2.2 Picosecond Laser System 135
3.2.2.1 Laser Synchronisation and Timing 137
3.2.3 5.6m Grazing Incidence (XUV) Flat Field Spectrometer Experiment 138
3.2.3.1 System Overview 138
3.2.3.1.1 Picosecond XUV Emission Studies 138
3.2.3.1.2 Picosecond XUV-DLP Photoabsorption Studies 141
3.2.3.2 5.6m Grazing Incidence Flat Field Spectrometer 144
3.2.3.2.1 Hitachi™ Diffraction Grating 145
3.2.3.3 Performance of the 5.6m Grazing Incidence Flat Field
Spectrometer System 146
3.2.3.3.1 Resolution with Front- and Back-Illuminated CCD
Detector Systems 147
3.2.4 Picosecond XUV Streak Camera 150
3.2.4.1 System Overview 150
3.2.4.2 Kentech™ XUV Streak Camera System 153
3.2.4.2.1 Operating Principles and System Specifications 153
3.2.4.2.2 Photonic Science™ Image Intensified CCD Detector 154
3.2.4.2.3 Performance and Temporal Resolution of Combined
Streak Camera and Image Intensifier Detector System 154
Chapter 4 Development and Characterisation of a Picosecond XUV
Continuum Laser Plasma Light Source
158
4.1 Introduction and Key Experimental Objectives 159
4.2 Time Integrated, XUV Spectroscopy of a Picosecond Laser Plasma Light
Source 162
4.2.1 XUV Transmission Bandwidth of Flat Field Spectrometer System -
Emission Studies 162
4.2.2 Transition from Line to Continuum Emission in the XUV Spectral
Region 164
4.2.3 XUV Continuum Emission from High-Z Metal Plasmas 168
4.2.4 Single-Shot Sensitivity of Picosecond XUV Continuum Light Source 170
4.2.5 Spectral Reproducibility and Flux Reproducibility of Picosecond XUV
Continuum Light Source 171
4.2.6 XUV Transmission Bandwidth of Flat Field Spectrometer System - DLP
Photoabsorption Studies 173
4.3 Time Resolved, Broadband XUV Emission Measurements of Picosecond
Laser Plasma Light Source 176
4.3.1 XUV Transmission Bandwidth of Streak Camera System 176
4.3.2 Picosecond XUV Emission from High-Z Metal Plasmas 177
4.3.2.1 Deconvolution of Measured Streak Profiles with Static System
Instrument F unction 183
4.3.3 Influence of Amplified Spontaneous Emission (ASE) on XUV
Continuum Pulse Length 188
4.3.4 Influence of Picosecond Laser Pulse Energy on XUV Continuum Pulse
Length 192
4.4 Conclusions and Future Development/Application of the Picosecond XUV
Continuum Laser Plasma Light Source 195
Chapter 5 6p and 5d Inner Shell Photoabsorption and Emission Studies
of ThI to Th VII
197
5.1 Introduction and Key Experimental Objectives 198
5.2 VUV Absorption Spectrum of Atomic Thorium 200
5.2.1 DLP Experimental Set-up and Procedure for VUV Photoabsorption
Measurements in Atomic Thorium 200
5.2.2 6p—>6d Giant Dipole Resonance in the VUV Photoabsorption Spectrum
of Atomic Thorium 202
5.2.3 Atomic Processes Contributing to the Observed 6p-»6d Giant Dipole
Resonance Profile in Atomic Thorium 207
5.2.4 Calculation of the Temporal Evolution of Electron Temperature in the
Expanding Thorium Plasma Plume 209
5.2.5 Term Dependent Hartree-Fock Calculations of the 6p Photoabsorption
Cross Section of Atomic Thorium 212
5.2.6 Evolution in the Profile of the 6p Photoabsorption Cross Section along
the Thorium Isonuclear Sequence 216
5.2.7 Comparison of the 6p-»6d Giant Dipole Resonance Profile in the Atomic
and Solid State Environments of Thorium 220
5.2.8 Comparison of the 6p—>6d Giant Dipole Resonance Profile in Atomic
Thorium and Uranium 222
5.3 XUV Absorption Spectrum of Atomic Thorium 224
5.3.1 DLP Experimental Set-up and Procedure for XUV Grazing Incidence
Photoabsorption Measurements in Atomic Thorium 224
5.3.2 5d —> 5,sf Giant Dipole Resonance in the XUV Photoabsorption
Spectrum of Atomic Thorium 226
vii
5.3.3 Term Dependent Hartree-Fock Calculations and RTDLDA Calculations
of the 5d Photoabsorption Cross Section of Atomic Thorium 229
5.3.4 Comparison of the 5d -» 5,£f Giant Dipole Resonance Profile in Atomic,
Molecular and Solid State Thorium 236
5.3.5 Comparison of the 5d —» 5,ei Giant Dipole Resonance Profiles in
Atomic Thorium and Uranium 238
5.4 XUV Absorption Spectra of Ionic Thorium (ThI to ThVTI) 239
5.4.1 DLP Experimental Set-up and Procedure for XUV Photoabsorption
Measurements in Ionic Thorium 239
5.4.2 Collapse of the 5d -» 5,£f Giant Dipole Resonance along the Thorium
Isonuclear Sequence 240
5.4.2.1 Application of Picosecond XUV Continuum Laser Plasma Light
Source to the Study of 5f Wavefunction Collapse 244
5.4.3 Configuration Interaction Hartree-Fock Calculations of ThV to ThVII:
5d Photoabsorption Cross Sections 247
5.4.4 RTDLDA Calculations of ThI to ThVII: 5d Photoabsorption Cross
Sections 254
5.5 Conclusions and Future Work on Thorium and other Actinide Atoms and
Ions 256
References 259
List of Figures and Photographs 282
List of Tables 289
Appendix I AI-1
Appendix II AII-1
Acknowledgements
Abstract
Fundamental investigations into the atomic structure of actinide atoms and ions provide
a wealth of information on the physical and chemical behaviour of these elements. In
this work, thorium, the second actinide element is investigated.
Inner shell photoabsorption features in the 6p and 5d spectra of the free thorium atom
and thorium ions produced in a laser plasma plume have been recorded using the space
and time resolved Dual Laser Plasma (DLP) spectroscopic technique. Studies focus, in
particular, on the investigation of the 6p —» 6d and the 5d —> 5, si' giant dipole resonance
features residing in the VUV (15 to 40eV) and XUV (80 to 150eV) inner shell
photoabsorption spectra of atomic and ionic thorium. The evolution of both giant dipole
resonance profiles with increasing ionisation along the thorium isonuclear sequence Th
to Th6+ has been studied. Results indicate that the 6d wavefunction is collapsed in
neutral thorium, whereas the 5f wavefunction is to a large extent un-collapsed in neutral
thorium but collapses with increasing ionisation along the thorium isonuclear sequence.
A picosecond XUV continuum laser plasma light source has been developed in this
work and is characterised both spectrally and temporally using time integrated flat field
spectroscopy and time resolved streak camera detection. XUV continuum emission
pulsewidths of less than 200ps in the 100 to 200eV energy region were recorded from
the high-Z metal plasmas of tungsten, gold, lead and samarium. When incorporated into
the DLP technique, the picosecond XUV continuum source permits probing of the
absorbing plasma plume with superior temporal resolution than previously achieved
using XUV laser plasma sources produced using nanosecond laser pulses. The newly
developed source was used to the study the collapse of the broad asymmetric 5d -»
5, rf giant dipole resonance feature to discrete Rydberg-like structure along the thorium
isonuclear sequence.
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