The underlying terahertz vibrational spectrum of explosives solids

Abstract

Using waveguide terahertz time-domain spectroscopy, we demonstrate the measurement of the underlying terahertz vibrational spectrum of the explosive solids hexahydro-1,3,5-trinitro-1,3,5-triazine, and 2,4,6-trinitrotoluene. Each explosive is cast as a polycrystalline thin film with planar ordering on the inner surface of metal parallel plate waveguide. For measurements near 10 K, each explosive reveals a complex spectrum of approximately 20 vibrational modes between 0.5 and 3.5 THz. The explosive films are of sufficient quality to produce vibrational linewidths as much as an order of magnitude sharper compared to conventional terahertz measurements of corresponding pellet samples of explosives. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2949068͔One of the most promising and discussed applications of terahertz ͑THz͒ spectroscopy is the detection and identification of explosive materials.1-3 Previous work has measured the THz absorption of explosive solids, revealing a number of broad features associated with the signature vibrational modes of the material.1,2,4-9 However, the majority of this work has been performed on disordered polycrystalline pellet samples, 1,2,4-8 resulting in inhomogeneous broadening of the absorption features, making their identification more difficult. We have addressed this problem by developing the technique of waveguide terahertz time-domain spectroscopy ͑THz-TDS͒, 10,11 whereby an ordered polycrystalline film is formed on a metal surface and its THz vibrational response is characterized via a parallel plate waveguide ͑PPWG͒. 12,13The planar order of the sample together with the formation of microcrystals of high crystalline quality reduces the inhomogeneous broadening, resulting in considerably sharper and much more identifiable features.11,14-18 Here, we present the application of waveguide THz-TDS to the explosive solids hexahydro-1,3,5-trinitro-1,3,5-triazine ͑RDX͒ and 2,4,6-trinitrotoluene ͑TNT͒, resulting in a dramatic sharpening of the vibrational absorption lines as the films are cooled to near 10 K. The linewidths measured at their full width at half maximum ͑FWHM͒ by waveguide THz-TDS are in some cases more than an order of magnitude sharper than corresponding THz measurements with standard pellets of explosives, and reveal a rich spectrum of approximately 20 narrow vibrational absorption lines between 0.5 and 3.5 THz. This information can provide an extraordinary precise "spectral fingerprint" capable of identification by a series of unique vibrational absorption lines. Such narrow vibrational lines can also provide rigorous input to help guide theoretical modeling 19 and advance the understanding of the vibrational properties of explosive solids.The THz measurement apparatus, method for the recovery of absorbance spectra for the waveguide films, and details of the PPWG have been described previously in Refs. 11 and 14. Prior to film formation, the metal waveguide plates were thoroughly washed with solvent and then plasma cleaned. Thin films of RDX and TNT were fo...

Terahertz Time-Domain and Low-Frequency Raman Spectroscopy of Organic Materials

“…144 The data indicate that the terahertz vibrational modes broaden with increasing disorder in the intermolecular interaction compared at the same temperature (i.e., independent of the effect of kT compared to hm). Crystalline disorder forms a continuum from well-defined relatively narrow peaks that are detected for the most-ordered structures such as single crystals 171 or thin films [172][173][174][175][176][177] all the way to the complete breakdown of the vibrational modes into the VDOS (as discussed in the section on Mesoscopic Structure and Dynamics for amorphous samples) or as a result of polymerization. 178 For example, using crystalline and amorphous poly(3-hydroxybutyrate), Hoshina and colleagues [179][180][181][182] and Yamamoto et al 183 studied this effect on a polymer system by combining far-IR spectra, low-frequency Raman spectra, and computational methods.…”

Polarization orientation dependence of the far infrared spectra of oriented single crystals of 1,3,5-trinitro-S-triazine (RDX) using terahertz time–domain spectroscopy

“…Common energetic materials such as RDX, HMX, PETN, and TNT have unique spectral signatures in the <100 cm-l range. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. At visible wavelengths, many explosives look remarkably similar to common baking ingredients such as sugar or modeling clay but have unique signatures in the THz range.…”

Non-invasive terahertz field imaging inside parallel plate waveguides

“…[2][3][4][5] Recently, a tapered parallel plate waveguide (TPPWG) has been proposed. [6][7][8] THz waves can be confined inside a TPPWG on subwavelength scale in both transverse dimensions, 9 thus enabling THz near-field spectroscopy with subwavelength resolution.…”