Ultra-violet and visible absorption characterization of explosives by differential reflectometry.

This study presents some optical properties of TNT (2,4,6-trinitrotoluene), RDX, HMX and tetryl, specifically their absorption spectra as a function of concentration in various solvents in the ultraviolet and visible portion of the electromagnetic spectrum. We utilize a standoff explosives detection method, called differential reflectometry (DR). TNT was diluted in six different solvents (acetone, acetonitrile, ethanol, ethyl acetate, methanol, and toluene), which allowed for a direct comparison of absorption features over a wide range of concentrations. A line-shape analysis was adopted with great accuracy (R(2)>0.99) to model the absorption features of TNT in differential reflectivity spectra. We observed a blue shift in the pertinent absorption band with decreasing TNT concentration for all solvents. Moreover, using this technique, it was found that for all utilized solvents the concentration of TNT as well as of RDX, HMX, and tetryl, measured as a function of the transition wavelength of the ultra-violet absorption edge in differential reflectivity spectra shows three distinct regions. A model is presented to explain this behavior which is based on intermolecular hydrogen bonding of explosives molecules with themselves (or lack thereof) at different concentrations. Other intermolecular forces such as dipole-dipole interactions, London dispersion forces and π-stacking contribute to slight variations in the resulting spectra, which were determined to be rather insignificant in comparison to hydrogen bonding. The results are aimed towards a better understanding of the DR spectra of explosives energetic materials.

Developments on standoff detection of explosive materials by differential reflectometry.

Differential reflectometry (DR) is an effective tool to supplement existing explosives detection systems thus making the combined unit more effective than one tool alone. It is an optical technique in which the light beam (UV) emanates from an extended distance onto the substance under investigation, thus rendering it to be a standoff method. DR allows the measurement of the energies that electrons absorb from photons as they are raised into higher, allowed energy states. These electron transitions serve as a "fingerprint" for identifying substances. The device can be made portable; it is fast, safe for the public, does not require human involvement, is cost effective, and most of all, does not require ingestion of a suspicious substance into an instrument. Various embodiments are presented.