Resonance-Enhanced Raman Spectroscopy on Explosives Vapor at Standoff Distances

Resonance-enhanced Raman spectroscopy has been used to perform standoff measurements on nitromethane (NM), 2,4-DNT, and 2,4,6-TNT in vapor phase. The Raman cross sections for NM, DNT, and TNT in vapor phase have been measured in the wavelength range 210–300？nm under laboratory conditions, in order to estimate how large resonance enhancement factors can be achieved for these explosives. The results show that the signal is enhanced up to 250,000 times for 2,4-DNT and up to 60,000 times for 2,4,6-TNT compared to the nonresonant signal at 532？nm. Realistic outdoor measurements on NM in vapor phase at 13？m distance were also performed, which indicate a potential for resonance Raman spectroscopy as a standoff technique for detection of vapor phase explosives. In addition, the Raman spectra of acetone, ethanol, and methanol were measured at the same wavelengths, and their influence on the spectrum from NM was investigated. 1. Introduction Improvised explosive devices (IEDs) are a common and a growing threat to the civilian society as well as to peace-keeping operations. Today, detection and identification of explosive charges and IEDs in real environments (indoors or outdoors) is a very challenging task and can in many cases also be a very hazardous operation. IEDs are in a way unique compared to regular ordnances, since the IED manufacturer has to improvise the design and use whatever materials are available. For this reason, the threat varies over time and in different parts of the world; it also evolves as countermeasures are taken. The diverse and varying nature of IEDs poses great challenges to the methods for their detection since they can contain any explosive which is available; this means commercial, military, or homemade explosives (HMEs). An overview of HME properties is given by Menning and Ostmark [1]. From the perspective of the user of the detection equipment, much would be gained if detection and identification could be reliably performed from a safe distance. This drives the research of possible standoff detection methods, where the developed methods will need to be both sensitive and selective in order to detect an explosive material in vapor phase and at low concentrations, or as trace amounts of particles on a surface, among an unknown number and composition of possible interfering substances. In addition, the method should be applicable at a distance of some tenths to a few hundreds of meters. Techniques that are anticipated to fulfill these requirements are laser-based spectroscopy methods. A number of different laser-based methods for