Resonance enhanced Raman spectroscopy for explosives detection. April 2008 - March 2010, Final report

The Resonant Raman cross sections for nitromethane, DNT and TNT in vapor phase have been measured in the wavelength range 210-300 nm in the laboratory, in order to determine how large resonance enhancement factors can be achieved for explosives in the UV, and to investigate whether Resonance Raman Spectroscopy has potential for stand-off detection of explosives vapors or not. IED:s as well as other explosive devices emit explosives vapor (to different extent depending on the explosive material and its containment), and if this vapor can be detected and identified it will be a giant step towards detecting also hidden IED:s. The nitromethane cross section was measured at room temperature at a concentration of 32.500 ppm, whereas the DNT and TNT cross sections were measured in a concentration range of 26 ppm to 1445 ppm at elevated temperatures (354 K-401 K). The Resonance Raman cross sections and enhancement factors show that the signal is greatly enhanced, up to 100,000 times for 2,4-DNT and 50.000 times for 2,4,6-TNT compared to the non-resonant signal at 532 nm. This is very good when compared to the literature estimation of 100-106 times enhancement. Also for nitromethane there is a clear enhancement, enabling realistic outdoors measurements at near-resonance wavelengths where the capacity to detect nitromethane vapor in free air on 13 m distance under warm conditions (328 K) has been shown. The measurements were limited not only by laser parameters and geometrical overlap, but also by absorption and self-absorption, and the measured enhancement factors are thus believed not to be the maximal enhancement factors achievable. This all indicates a great potential for resonance Raman spectroscopy as a stand-off technique for detection of vapor phase explosives.