Infrared Absorption (IR) spectroscopy, has demonstrated to be a very sensitive and selective technology and its capability to be used as a standoff technology. However, it has not been used up to now for standoff detection of explosives at distances of more than 6 metres.

Mid-infrared (MIR) absorption spectroscopy is known to be a very sensitive and selective method for detection and analysis of species. But low vapour pressure of most explosives prevents direct measurement in the mid-infrared, therefore, the substance to be investigated by this technology would need to be evaporated/fragmented first and subsequently analyzed in the mid-infrared.  

The approach to be used for the IR analysis in OPTIX will be PLF (Pulsed Laser Fragmentation) and subsequent detection within the infrared spectral region. Up to now, there has not been presented any stand-off system for the detection of explosives with low vapour pressure realized based on infrared spectroscopy. The reason for this is the very low vapour pressure of most explosives, i.e. only very scarce amounts of explosives are present in the gas phase.  

However, measurements in absorption cells with long optical path length or with increased concentrations due to heating of the sample show that substances posses distinctive spectra in the mid-infrared. A first pilot survey in Technical University of Clausthal (beneficiary in OPTIX) has shown that the ratio of NO/NO2 produced by the fragmentation laser is different for energetic and non-energetic material . Within OPTIX, this will be analyzed further and a spectral database for fragmentation products of different explosives will be generated. For standoff detection, these concepts are not applicable since absorption spectroscopy e.g. in white cells calls for sampling and feeding a gas flow into the cell. Within the project OFDEX from the Fraunhofer Gesellschaft it is planned to use absorption spectroscopy for remote detection. However, first results show only measurable contrast when the explosive, diluted in acetone, is coated on a highly reflective surface.  

On the other hand, long-range applications are feasible in the mid-infrared, and absorption measurements of e.g. methane and ethane have been performed in distances longer than 500 metres. Different laser systems have been used as sources, ranging from CO2-lasers to optical parametric oscillators (OPO). Since the scattering efficiency is rapidly decaying with higher wavelength, high output powers are desirable. 

With distances suitable for stand-off detection of explosives, quantum cascade lasers are well applicable sources in terms of output power and mechanical size. In the Technical University of Clausthal, some results for remote detection of HMX samples via NO analysis by PLF and MIR laser at 6 m distance in the laboratory have been achieved. Following with this experience, OPTIX will extend the results of this last experience from 6m to 20m, which is an important step forward in the state of the art of IR technology for standoff detection of explosives.

Another fact to take into account is that when addressing IR technology, OPTIX will generate global parameters to distinguish between energetic and non-energetic materials based on the photofragmentation products. Based on this a database on fragmentation products for different incident energies on the samples of different explosives will be generated, which is another important step beyond the state of the art. Finally, as stated before, the combination of IR with other technologies for standoff optical detection of explosives is nowadays far beyond the state of the art.