PROCEEDINGS ARTICLE | October 25, 2006
Proc. SPIE. 6378, Chemical and Biological Sensors for Industrial and Environmental Monitoring II
KEYWORDS: Explosives detection, Reflection, Spectroscopy, Ultraviolet radiation, Molecules, Crystals, Reflectometry, Explosives, Reflectance spectroscopy, Absorption
It is shown that 2, 4, 6-Trinitrotoluene (TNT) displays strong and distinct structures in differential reflectograms, near
420 nm and 250 nm. These characteristic peaks are not observed from approximately two dozen organic and inorganic
substances which we tested and which may be in or on a suitcase. This exclusivity infers an ideal technique for
explosives detection in mass transit and similar locations. The described technique for detection of explosives is fast,
inexpensive, reliable, portable, and is applicable from some distance, that is, it does not require contact with the
surveyed substance. Moreover, we have developed a curve discrimination program for field applications of the
technique. Other explosives such as 1, 3, 5-trinitro-1, 3, 5 triazacyclohexane (RDX), 1, 3, 5, 7-Tetranitro-1, 3, 5, 7-
tetraazacyclooctane (HMX), 2, 4, 6, N-Tetranitro-N-methylaniline (Tetryl), Pentaerythritol tetranitrate (PETN), and
nitroglycerin have also been investigated and demonstrate similar, but unique, characteristic spectra. The technique
utilizes near-ultraviolet to visible light reflected from two spots on the same sample surface yielding a differential
reflectogram corresponding to the absorption of the sample. The origin of the spectra is attributed to the highest
occupied molecular orbital to lowest unoccupied molecular orbital (HOMO-LUMO) transitions of the respective
explosive molecule. Experiments using transmission spectrophotometry have also been performed to compliment and
confirm the specific transitions. The results are supported by computer modeling of the molecular orbitals that yield
UV and visible transitions.
