Terahertz (THz) spectra of two TNT-related compounds (4-NT and 2,6-DNT) are investigated using Fourier transform infrared spectroscopy (FTIR) and THz time-domain spectroscopy (THz-TDS) between 0.1 to 20 THz. The Density Functional Theory (DFT) is applied to calculate the THz spectra of these two compounds. The transmission, diffuse reflection, and calculated spectra are in good agreement. The measured THz resonance lines from the transmission and diffuse reflectance spectra are assigned based on DFT simulation. The observed THz signatures imply that the THz spectrum has potential for standoff detection of explosive and related compounds (ERCs) in the THz range.
We report the sensing of explosive materials by using terahertz (THz-TDS) time domain spectroscopy at standoff distance. The 0.82 THz absorption peak of RDX is observed at a distance up to 30 m away from the emitter and receiver. This result supports the application of THz-TDS technique in remote sensing and detection of explosives.
Terahertz spectroscopy can be used to detect explosives and related compounds by the unique absorption lines in the far infrared region. We will present results in both transmission and reflection geometries using a conventional time-domain terahertz spectroscopy system. We illustrate the importance of a uniform reference, and uniform sample preparation. Some problems can be avoided by generalizing this technique to detection with narrow linewidth terahertz illumination and a square law (intensity) detector.
Diffuse reflectance spectrum (DRS) technique is extensively used in UV/visible and middle/near infrared for characterizing/analyzing powders and samples with rough surface. In this paper, we report on the DRS investigation of explosives & related compounds in the THz region, which is more difficult because of the limitations of far infrared sources, beam splitters and detectors. We also discussed the penetration depth and the sensitivity for this technique in the THz range.
THz diffuse reflectance spectra (50-680 cm<sup>-1</sup>) were taken on a Bruker 66V/S FTIR spectrometer with a Specac diffuse reflectance accessory. A number of explosive and related compounds were investigated in both transmission and diffuse reflectance modes, and a good agreement between transmission and diffuse reflection spectra was demonstrated. Our experimental results show that DRS technique has advantages over transmission spectrum technique, such as better sensitivity and easier sample preparation. Therefore, the THz DRS has the potential for the standoff detection of explosives and related compounds in the real world applications.
Far infrared spectra of 14 commonly used explosive samples have been measured by using Fourier Transform Infrared Spectroscopy (FTIR) and THz Time-Domain Spectroscopy (THz TDS). New absorption resonances between 20 cm<sup>-1</sup> and 650 cm<sup>-1</sup> are reported. Below 20 cm<sup>-1</sup>, no clear absorption resonances are observed in all the explosives. There is a good consistency of far-IR spectrum measured by Far-FTIR and by THz TDS in explosives 3,5-DNA and 2,4-DNT. Observed far-IR spectrum of TNT is compared with a previously reported theoretical calculation.
Gas sensing and identification in far infrared or THz band is useful because many polar molecules have unique spectral fingerprints in this range, which are from the rotational transitions of the molecules. We have investigated the potential of THz time-domain spectroscopy (THz TDS) as a quantitative analysis technique for gas sensing. Ammonia vapor has been chosen as a sample gas. The absorption cross section at 0.572 THz of ammonia in the pressure range of 0.2-20 Torr was extracted to be (5.7±0.3)×10<sup>-20</sup> cm<sup>2</sup>/molecule. In addition, a pressure calibration curve based on pure ammonia measurements was obtained. Using this calibration curve, we made quantitative analysis on the mixture of ammonia and air at 100 Torr. The result shows that THz TDS is an appropriate technique for quantitative analysis of polar gas and gas mixture. We measured the THz spectra of ammonia at different partial pressures in ~590 Torr nitrogen (78% nitrogen in atmosphere), and obtained a pressure calibration curve. THz spectra of ammonia at different partial pressures in 760 Torr atmosphere were measured. Based on the principle of differential optical absorption spectroscopy (DOAS) and the pressure calibration curve got in ~590 Torr nitrogen, we obtained the ammonia partial pressures. The result is compared with the value measured by vacuum gauge and the maximum error is 30%. This indicates that THz TDS based on principle of DOAS is an applicable quantitative technique for sensing ammonia or other polar gases in atmosphere.