We have investigated the absorption spectra of seventeen explosives and related compounds (ERCs) by using terahertz time-domain spectroscopy in the 0.1–2.8 THz region. Most of these substances show characteristic absorption features in this frequency range. The measured absorption coefficients of these ERCs form a database, which is of great importance for biochemical, defense and security related applications.
©2007 Optical Society of America
Detection of explosives and energetic materials has been the focus of many investigations over the past ten years as they are closely related to safety screening and pollution monitoring. Many commonly used solid-state explosives and related compounds (ERCs), including TNT (2,4,6-Trinitrotoluene), HMX (Cyclotetramethylene tetranitramine), RDX (Cyclotrimethylenetrinitramine) and PETN (Pentaerythritol tetranitrate), have spectral fingerprints in the THz range [1–6]. These fingerprints arise from the intramolecular and intermolecular vibrational modes or phonon modes of these explosive materials. THz Time-Domain Spectroscopy (THz-TDS) has been considered a competitive method for inspection and detection of explosives because of its unique advantages over other spectroscopic techniques. THz waves can penetrate through many daily-used materials, such as clothing, paper, plastic, leather, wood and ceramic; THz radiation has low photon energies (4 meV for 1 THz, one million times weaker than an X-ray photon) and will not cause harmful photoionization in biological tissues. THz-TDS is capable of exploring the signatures of targets by using coherent emission and detection. Recently, the results of using THz-TDS for explosive detection in transmission mode have been reported [1, 3–6]. One major advantage of THz-TDS is that both the amplitude and phase of the THz pulse are measured, allowing for each spectral component of the THz pulse to be determined. The amplitude and phase can be used to calculate the THz complex dielectric properties (i.e., absorption coefficient and refractive index) of the sample without carrying out a Kramers-Kronig analysis, which is usually used in mid-infrared or other spectroscopic technologies. The THz dielectric properties of these ERCs are of great significance for defense and security related applications, especially for standoff detection of explosives. In this paper, we present the measured spectroscopic characterizations and absorption coefficients of seventeen commonly used explosives and related compounds in the range of 0.1–2.8 THz by using THz-TDS. The ample amount of absorption features within the THz range forms a database of these organic and inorganic compounds. In addition, we investigate the absorption spectra of explosives under covering materials to demonstrate that a THz system can be used for the detection of hidden explosives.
2. Sample preparation
Sample preparation is critical in gathering reliable data. All ERCs used (listed in Table 1) were powdered samples. When the particle size of ERCs is comparable to the THz wavelength (300 µm), the scattering causes a significant loss in THz amplitude and yields a slanted baseline. In order to minimize the influence of scattering, the samples were gently ground using a mortar and pestle to reduce the particle size to 20–50 µm. For safety consideration, these samples were divided into two kinds: sensitive and insensitive to high pressure. The samples insensitive to high pressure were compressed into pellets (~0.5–1 mm in thickness and 13 mm in diameter) directly using 5 tons of pressure with a hydraulic press. For those that are unsafe under high pressure, the samples were mixed with polyethylene (PE) powder, which is almost transparent in the THz range. The mixing ratios were between 5% and 20% (weight of ERCs vs. total weight of mixed samples). For safety consideration, the weight of each pure ERC did not exceed 30 mg when used to make ERC/PE samples. The mixed samples were compressed into pellets under the same conditions as the pure samples. The thickness of the sample pellets was measured using a micrometer with a precision of 0.001 mm.
THz pulses were generated by an 800 nm femtosecond laser (MaiTai Ti:Sapphire, Spectra Physics) with a pulse duration of 80 fs, repetition rate of 80 MHz, and average power of 800 mW. The laser beam was split into a pump and probe. The chopped pump beam was focused on a p-type InAs crystal to generate THz pulses through a surface effect . The emitted THz pulses were collimated and focused by a pair of off-axis parabolic mirrors. The samples were placed right at the THz focus point, perpendicular to the incident THz beam. The transmitted THz beam was collected and focused using another pair of off-axis parabolic mirrors onto a 1 mm thick <110> ZnTe crystal, in which the probe beam detected the THz field by electro-optic sampling . The THz spectrum, ranging between 0.1 and 3 THz was obtained by applying a fast Fourier transform to the THz waveform. Fig. 1 presents the obtained THz spectrum. This THz-TDS system had a dynamic range of ~80 dB and a signal-to-noise ratio of ~72 dB. The achieved spectral resolution was 0.05 THz. The system was purged with dry nitrogen gas to eliminate the absorption of water vapor. The THz spectrum without samples in dry nitrogen is used as reference for explosive measurement.
4. Theoretical background
THz-TDS is capable of obtaining the refractive index and absorption coefficient without using Kramers-Kronig relation [6, 7]. The field of the transmitted THz pulse is changed by both dispersion and absorption of the samples. The THz amplitude transmittance T can be expressed as:
where E 0 and Etrans are the incident and transmitted THz amplitude respectively; φ is the phase difference between the sample and reference waveforms; ñ=n+iκ is the complex refractive index of the sample; d is the sample thickness; ν is the frequency and c is the speed of light in the vacuum. Thus we can obtain the extinction coefficient and index:
The power absorption coefficient α is related to κ by
Therefore, providing E 0 and Etrans, and the thickness of the sample yields the calculated refractive index and the absorption coefficient of the sample material.
5. Experimental results and discussion
The absorption spectra of all seventeen ERCs are presented in Table 1. These measurements are considered accurate between 0.1–2.8 THz. The value of absorption coefficients above 2.8 THz may not be accurate because 2.8–3 THz is close to the bandwidth limit of our experimental system. For each ERC, multiple samples with different thicknesses were prepared and measured. According to Beer-Lambert’s Law, the absorbance is proportional to the thickness of the sample; therefore, if the amplitude of peaks in the absorption spectra increases with the thickness, we identify these peaks as resonant absorption features. For those which have identified spectral features in the THz range, the typical absorption feature locations and corresponding absorption coefficients are also listed in Table 1. For RDX, HMX, PETN, TNT, all identified features agree well with those reported in the literatures [1, 3–5, 11, 12]. For 2,4-DNT, 2,6-DNT, 2A-DNT, 4A-DNT, 4-NT, and 1,3-DNB, some features agree well with the results in the literature , and some are observed for the first time. These discrepancies are mainly caused by the different sample preparation and different measuring systems. Standard pellets are measured using THz-TDS in our experiment while ERC solutions coated on PE film are measured using FTIR in the literature . For Tetryl, NG, DMNB, KClO4 and Black powder, the identified absorption features in 0.1–2.8 THz are observed for the first time. For NH4NO3, no absorption features are found in this frequency range.
The chemical origin of these absorption features is complicated. It is easy to think that these absorptions are caused by the intramolecular vibrational modes of these materials because their frequencies are located in the far infrared region. Some materials have been studied theoretically using density function theory (DFT), for example, 2,4-DNT , RDX , HMX and PETN. The isolated-molecular simulation results indicate several intramolecular vibrational modes in the THz range. However, compared with experimental results, the simulation results are only partially in agreement. For 2,4-DNT, the simulation result shows there is no vibrational mode below 3 THz while the measured spectrum shows four (4) resonant absorptions below 3 THz (0.45 THz, 0.67 THz, 1.08 THz and 2.52 THz); for RDX, six (6) resonant peaks (0.82 THz, 1.05 THz, 1.36 THz, 1.54 THz, 1.97 THz and 2.21 THz) are found experimentally while only three (3) peaks (1.32 THz, 1.8 THz and 1.89 THz) exist theoretically below 3 THz. Those features which do not correspond to inherent molecular vibrational modes arise from the intermolecular vibrational modes or phonon modes of these explosive materials. Therefore, the THz structure of these ERCs below 3 THz is a combination of both intramolecular and intermolecular vibrational motions [11, 12].
Besides these organic compounds, some inorganic compounds present THz fingerprints, such as KClO4 and black powder (KNO3, sulfur and charcoal). For KClO4 and KClO3, the measurement results show these two compounds have different absorption properties in 0.1–3 THz even though their chemical compositions are somewhat similar. These results indicate that THz can also be used to identify structure-similar molecules.
6. Characterization of explosives under covering materials
As we mentioned above, commonly used non-polar dielectric materials are almost transparent in the THz range. This advantage, combined with THz fingerprints of ERCs, makes THz technology a competitive technique for detecting hidden explosives. In order to demonstrate this capability, we investigate the THz spectra of explosives under covering materials. Both RDX and HMX samples are covered with three different barrier materials, plastic, cotton and leather (thickness ~1 mm), which are all opaque in visible light. The measurements are conducted in the atmosphere (with a relative humidity of ~20% at 25 °C). The obtained spectra are presented in Fig. 2. For comparison, the transmission spectra of each pure sample and covering material are also plotted in the same graph. The results indicate that covering materials are almost transparent in the 0.1–3 THz range. For the samples with covers, their typical features are identified clearly: 0.82 THz, 1.05 THz, 1.36 THz and 1.54 THz for RDX; 1.78 THz for HMX. The spectra of covered samples are also in good agreement with the sum of the spectra of pure samples and covering materials, which indicates that the barrier materials do not present too much distortion and shift of the absorption peak locations. In the atmosphere, water vapor absorptions affect the measurements. The strong water vapor absorptions reduce the signal-to-ratio and lead to band distortions or artificial spikes in the absorption spectra of the samples. The big spikes above 2 THz in the absorption spectra of RDX and HMX result mainly from water vapor absorption.
We measured the absorption properties of seventeen ERCs compounds using THz-TDS system. The obtained absorption spectra in the range of 0.1–3 THz show that most of the ERCs have THz fingerprints which are caused by both the intramolecular and intermolecular vibrational modes of these materials. The absorption coefficients extracted from the absorption spectra form a fingerprint database of the ERCs in the 0.1–2.8 THz range, which is of great importance for biochemical, defense and security related applications. In addition, the explosives under covering materials can also be positively identified by THz-TDS, which makes THz technology applicable for the detection of hidden explosives.
This work is supported by the U.S. Army Research Office MURI program, HSARPA SBIR contact through Intelligent Optical Systems, Inc., and U.S. Office of Naval Research CIED basic research program.
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