ABSTRACT
The ability to detect explosives both at close range and from a distance is important for providing a capability to warn potential victims. A wide variety of optical techniques are being explored in order to obtain this capability, but the problem is proving to be a signicant technical and scientic challenge. These challenges were described in detail in a review by Steinfeld and Wormhoudt, who categorized them into a number of physical constraints, including low vapor pressures, limited sample size, deliberate concealment, and interferences.1 Each of these constraints impacts the ability of optical techniques to detect and identify explosive material, especially in the vapor phase. Optical techniques are attractive, however, because they can potentially provide a noncontact and standoff detection capability requiring no sample preparation. Because of the aforementioned vapor pressure and concealment
5.1 Introduction ....................................................................................................99 5.2 Raman Spectroscopy .................................................................................... 100
5.2.1 Classical Description ........................................................................ 100 5.2.2 Raman Cross Sections ...................................................................... 102
5.3 Raman Spectra of Explosives ....................................................................... 103 5.3.1 Raman Cross Sections of Explosives ................................................ 103 5.3.2 Solution versus Solid Phase .............................................................. 104
5.4 Raman Detection Systems ............................................................................ 108 5.4.1 Raman Signal .................................................................................... 109 5.4.2 Excitation Wavelength Dependence ................................................. 109 5.4.3 Handheld Raman Instruments .......................................................... 112 5.4.4 Standoff Raman Detection ............................................................... 112
5.4.4.1 Visible Raman .................................................................... 113 5.4.4.2 UV Raman ......................................................................... 114
5.4.5 Coherent Raman Detection............................................................... 117 5.5 Conclusions ................................................................................................... 117 Acknowledgments .................................................................................................. 118 References .............................................................................................................. 118
constraints, the application most suited to detection and identication of bulk or trace material on surfaces is optical spectroscopy. Raman spectroscopy is one such optical technique that has shown promise for explosives detection.2-16
As a vibrational spectroscopy, Raman spectra contain information on the specic arrangement and interaction of the atoms constituting the molecule.17,18 This information is highly molecule-specic and allows for differentiation between types of explosives. Whereas infrared (IR) spectroscopy is also a vibrational spectroscopy and provides the same ability to differentiate molecular species, IR spectra are much more dependent on the particle size and nature of the background surface than Raman spectra. Sample thickness and surface reectivity and roughness affect the IR peak positions, as well as their peak intensities.
