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Standoff Laser Interferometric Photoacoustic Spectroscopy for the Detection of Explosives View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by eGrove (Univ. of Mississippi) University of Mississippi eGrove Electronic Theses and Dissertations Graduate School 1-1-2012 Standoff Laser Interferometric Photoacoustic Spectroscopy for the Detection of Explosives Logan Marcus University of Mississippi Follow this and additional works at: https://egrove.olemiss.edu/etd Part of the Physics Commons Recommended Citation Marcus, Logan, "Standoff Laser Interferometric Photoacoustic Spectroscopy for the Detection of Explosives" (2012). Electronic Theses and Dissertations. 1512. https://egrove.olemiss.edu/etd/1512 This Dissertation is brought to you for free and open access by the Graduate School at eGrove. It has been accepted for inclusion in Electronic Theses and Dissertations by an authorized administrator of eGrove. For more information, please contact [email protected]. STANDOFF LASER INTERFEROMETRIC PHOTOACOUSTIC SPECTROSCOPY FOR THE DETECTION OF EXPLOSIVES A Dissertation Presented for the degree of Doctor of Philosophy In the Physics and Astronomy Department The University of Mississippi by LOGAN MARCUS AUGUST 2012 Copyright Logan Marcus 2012 ALL RIGHTS RESERVED ABSTRACT There will unfortunately always be a pressing need to detect and identify explosive materials. Performing detection at a standoff distance is much safer and this is the impetus for this work. Photoacoustic spectroscopy is well suited to the task of explosive detection at a standoff distance. A detailed theory of the standoff measurement of the physical response of a system to photoacoustic excitation with an interferometric sensor has been constructed and tested. The use of this methodology to measure the standoff photoacoustic spectrum of TNT with an interferometric sensor has clearly been demonstrated. This methodology and the tested theory can both be used to examine pressing problems in explosive detection and characterization. ii DEDICATION I would like to dedicate this work to my friends and family. Without decades of your love and support this would not have been possible. iii ACKNOWLEDGEMENTS To reach this point required the help and dedication of many people. I would first like to thank and acknowledge my advisor Dr. Richard Raspet for the years of constant help and for the determination to see every student improve. I would also like to acknowledge Dr. Vyacheslav Aranchuk for his infinite patience with me in the laboratory. Thank you to Dr. James Sabatier for taking in a graduate student and making sure they always ended up in the right place. I would also like to acknowledge the U.S. Army Armament Research, Development, and Engineering Center for funding this research under contract #W15 QKN-09-C-0094. The views and conclusions contained in this document are those of the author and should not be interpreted as representing the official policies, either expressed or implied, of the U.S. Army Armament Research, Development, and Engineering Center or the U.S. Government. iv LIST OF FIGURES Figure 1.1: Schematic of the Rosencwaig and Gersho photoacoustic cell. ................................... 5 Figure 1.2: Diagram of rotational degrees of freedom in molecules. .......................................... 10 Figure 1.3: Diagram of vibrational degrees of freedom in molecules. ........................................ 11 Figure 1.4: Plot of the absorbance of infrared light by 2,4,6-trinitrotoluene (TNT) versus wavelength. ................................................................................................................ 12 Figure 1.5: Diagram of 2,4,6-trinitrotoluene (TNT). .................................................................. 13 Figure 1.6: The transmission spectrum of TNT is plotted versus inverse wavelength in cm-1. 14 Figure 2.1: Diagram of the modeled system. .............................................................................. 20 Figure 3.1: Induced surface temperature vs. radial distance from excitation beam center. ........ 67 Figure 3.2: Plot of surface deformation in the negative z direction as a function of radial distance from the excitation beam center. ................................................................. 70 Figure 3.3: Source velocity as a function of radial distance from excitation beam center. ........ 71 Figure 3.4: Plot of the change in pressure versus negative z distance from the sample surface. 72 Figure 3.5: Plot of calculated signal versus excitation power. .................................................... 76 Figure 3.6: Plot of change in optical path length versus frequency of modulation. .................... 77 Figure 3.7: Semi logarithmic plot of normalized change in optical path length versus frequency of modulation for the four signal components. ......................................................... 78 Figure 4.1: Picture of the ideal gold mirror sample. ................................................................... 81 Figure 4.2: Optical schematic for the excitation optics. .............................................................. 82 Figure 4.3: Diagram of an Acousto-Optic Modulator (AOM). ................................................... 83 Figure 4.4: Schematic of the Target Optics. ................................................................................ 84 Figure 4.5: Diagram of the Polytec PDV100 LDV. .................................................................... 85 Figure 4.6: Plot of measured excitation laser power versus the displayed value at the laser. ..... 89 Figure 4.7: Diagram of the experimental setup used to measure the transmitted power of the excitation laser. .......................................................................................................... 90 Figure 4.8: Diagram of the changes made to the experimental setup to perform beam width measurements. ........................................................................................................... 92 v Figure 4.9: Plot of experimentally measured and calculated change in optical path length versus excitation frequency for the idealized system. .......................................................... 95 Figure 4.10: Plot of measured and calculated change in optical path length versus excitation laser power. ............................................................................................................... 97 Figure 4.11: Optical setup for the measurement of TNT photoacoustic spectrum. .................. 100 Figure 4.12: TNT IR absorption spectrum from the literature. ................................................. 101 Figure 4.13: Experimentally measured photoacoustic IR spectrum of TNT. ........................... 102 Figure 4.14: Experimental (Blue) and Literature (Black) TNT IR absorption spectra overlaid to show agreement. ...................................................................................................... 103 vi LIST OF TABLES Table 1.1: Table of TNT spectral lines and their corresponding vibrational modes. .................. 15 Table 4.1: Table of beam width measurements. .......................................................................... 93 vii TABLE OF CONTENTS ABSTRACT .................................................................................................................................. ii DEDICATION ............................................................................................................................. iii ACKNOWLEDGEMENTS ......................................................................................................... iv LIST OF FIGURES ....................................................................................................................... v LIST OF TABLES ...................................................................................................................... vii CHAPTER 1: INTRODUCTION ................................................................................................ 1 Section 1.1: Overview ................................................................................................................. 1 Section 1.2: Review of Literature ............................................................................................... 2 Section 1.3: Spectroscopy ........................................................................................................... 9 Section 1.4: Project Description ................................................................................................ 17 CHAPTER 2: THEORY ............................................................................................................. 20 Section 2.1: Temperature Profile .............................................................................................. 20 Section 2.2: Surface Deformation ............................................................................................. 30 Section 2.3: Entropic Temperature............................................................................................ 55 Section 2.4: Acoustic Profile ..................................................................................................... 58 Section 2.5: Index of Refraction ............................................................................................... 61 CHAPTER 3: THE NUMERICAL MODEL .............................................................................. 64 Section 3.1: Motivation ............................................................................................................
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