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Applications to Laser-Induced Breakdown Spectroscopy for Analysis of Aerosols and Single Particles

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Applications to Laser-Induced Breakdown Spectroscopy for Analysis of Aerosols and Single Particles INVESTIGATION OF PHYSICAL AND SPECTRAL CHARACTERISTICS OF LASER-INDUCED PLASMAS: APPLICATIONS TO LASER-INDUCED BREAKDOWN SPECTROSCOPY FOR ANALYSIS OF AEROSOLS AND SINGLE PARTICLES By VINCENT PAUL HOHREITER A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2005 Copyright 2005 by Vincent Paul Hohreiter ACKNOWLEDGMENTS I would like to briefly and generally acknowledge everyone—family, friends, labmates, colleagues—who helped me make this dissertation happen. Specifically I would like to thank my parents, Vaughn and Christine Hohreiter, who—beyond their constant love and support—always accepted, fostered, and encouraged my academic progress and misgivings alike. Additionally I owe a debt of gratitude to my advisor, Dr. David Hahn, without whose rigorous academic lead and unwavering personal support I most likely would have abandoned doctoral study years ago. Finally, I would like to thank my wife, Dr. Aline Gubrium, who has inspired me with both love and a tenaciously organized and diligent work ethic. iii TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................................................................................. iii LIST OF TABLES............................................................................................................ vii LIST OF FIGURES ......................................................................................................... viii ABSTRACT....................................................................................................................... xi CHAPTER 1 INTRODUCTION ........................................................................................................1 The Laser-Induced Plasma ...........................................................................................1 Plasma Formation..................................................................................................2 Spatial and Temporal Evolution............................................................................3 The Laser-Induced Breakdown Spectroscopy (LIBS) Technique................................4 Temporal Properties of Laser-Induced Plasmas for LIBS Analysis.............................6 Stability of Laser-Induced Plasmas ..............................................................................8 LIBS and Calibration..................................................................................................10 Spatial Considerations in Plasma Emission................................................................13 2 EXPERIMENTAL METHODS .................................................................................16 The LIBS Experimental Apparatus ............................................................................16 Delivery Optics....................................................................................................16 Extraction Optics.................................................................................................17 System Synthesis.................................................................................................18 Plasma Transmission Study........................................................................................19 Transmission Measurements ...............................................................................20 Electron Density and Plasma Continuum Measurements ...................................21 Laser Shot Stability Study ..........................................................................................22 Carbon Calibration Study ...........................................................................................24 Solid Phase Carbon Experiments ........................................................................25 Gas Phase Carbon Experiments...........................................................................27 LIBS Data Collection ..........................................................................................28 Plasma-Aerosol Interaction Study ..............................................................................29 The Imaging System............................................................................................30 The Spectroscopy System....................................................................................32 iv 3 PLASMA TRANSMISSION PROPERTIES.............................................................34 Time-Resolved Plasma Transmission.........................................................................34 Time-Resolved Continuum Measurements ................................................................36 Electron Density Measurements.................................................................................38 Plasma Absorption Cross-Section ..............................................................................40 Modeling Plasma Transmission ..........................................................................40 Plasma Absorption...............................................................................................42 Extrapolation for Small Time Scales...................................................................43 4 LIBS PLASMA STABILITY.....................................................................................46 Effects of Laser Cavity Seeding .................................................................................46 Effects on Shot-to-Shot Precision .......................................................................48 Temporal Effects on the Plasma Formation Process...........................................52 Effects of Aerosol Loading.........................................................................................56 Effects on Shot-to-Shot Precision .......................................................................57 Temporal Effects on the Plasma Formation Process...........................................59 5 LIBS CALIBRATION................................................................................................61 Raw and Baseline-Subtracted Spectra........................................................................61 Calibration Using Atomic Spectra..............................................................................63 Calibration Using Molecular Bands ...........................................................................65 Analysis of Matrix Effects..........................................................................................67 Aerosol Loading and Size Considerations..................................................................69 Proposed Mechanism for Particle-Plasma Interaction................................................70 6 SPATIAL CONSIDERATIONS IN PLASMA EMISSION......................................74 Background Emission.................................................................................................74 Spatial Distribution of Atomic Emission....................................................................78 Correlation of Image and Spectral Data .....................................................................81 Rate of Aerosol Dissociation......................................................................................84 Implications for LIBS.................................................................................................90 7 CONCLUSIONS AND FUTURE WORK.................................................................91 Conclusions.................................................................................................................91 Proposal of Future Work ............................................................................................92 APPENDIX A SUMMARY OF ATOMIC AND MOLECULAR LINES USED .............................94 B VERIFICATION OF IMAGING SYSTEM...............................................................95 v C TABLE OF α1/2 VALUES FOR FREE-ELECTRON DENSITY CALCULATIONS......................................................................................................97 D IMAGES OF NON-HOMOGENEOUS SPATIAL EMISSION................................98 LIST OF REFERENCES.................................................................................................102 BIOGRAPHICAL SKETCH ...........................................................................................105 vi LIST OF TABLES Table page 4-1 Ratios of the area-integrated spectral peaks (peak-to-base and peak-to-peak) for seeded and unseeded laser operation........................................................................48 5-1 Summary of the carbon calibration response and plasma conditions. .....................65 A-1 Summary of atomic lines used in spectral analysis..................................................94 A-2 Summary of molecular lines used in spectral analysis.............................................94 C-1 Values of α1/2 for calculation of electron density.....................................................97 vii LIST OF FIGURES Figure page 2-1 Optical setup for delivery of laser energy to the sample volume resulting in the creation of a plasma..................................................................................................16 2-2 Optical setup for extraction of plasma emission from sample volume. ...................17 2-3 Basic LIBS system. ..................................................................................................18
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