Recently, terahertz spectroscopy has attracted increasing interest as a probe of organic molecular crystals such as
pharmaceutical compounds and explosives. Terahertz time-domain spectroscopy (THz-TDS) focuses on low-frequency
(10-120 cm<sup>-1</sup>) intermolecular modes that are characteristic of the crystal structure. The prospect of material-specific
identification, combined with the ability of THz light to penetrate many common materials, has lead to an emphasis on
the prospect of a THz-based, standoff explosives detection device. In this presentation, THz spectra of materials
ranging from military-grade explosives to home-made explosive components will be discussed. In addition to the
experimental results, THz spectral assignments will be presented based on solid-state density functional theory
simulations. Finally, the effect of particle size on the observed spectra will also be considered.
Solid-state organic compounds such as &agr;-lactose-monohydrate and biotin have been shown to have narrow and intense THz absorption features at room temperature. Interest in lineshapes in the THz region is justified not only for practical reasons, since they are of crucial importance to spectroscopy-based identification of materials, but also because of the information the line-widths contain about the solid-state physics of the materials. The line-width of THz absorption features (generally from lattice vibrations) in solids is excepted to be inversely proportional to the scattering time of optical phonons. The line-width of absorption features might thus have implications on the solid-state physics of the material, in particular, the interaction of phonons and the phonon density of states. We use a continuous wave THz photomixing system to obtain a high resolution spectrum of &agr;-lactose-mohohydrate and analyze two of its lowest-frequency absorption lines. For comparison we measure the transmission spectra of 5 chemically related saccharides: melecitose, trehalose, maltose, cellobiose, and raffinose. Since &agr;-lactose-monohydrate has a stronger and narrower absorption feature than any of its related saccharides, this comparison study is an important step in understanding the mechanism of THz radiation absorption by organic solids and what line-widths to expect in THz spectroscopy.
This paper focuses on the establishment of methodologies for the successful application of first principles theoretical analyses in the understanding of the terahertz spectroscopy of molecular solids, particularly high-energy materials. A solid-state density functional theoretical study has been performed on the high explosive pentaerythritol tetranitrate (PETN) in order to address the relationships between the choice of computational parameters and the
predictions of molecular and solid-state properties, such as molecular geometries, intermolecular interactions within the crystal cell, charge distributions, and solid-state vibrations in the terahertz (3 to 200 cm<sup>-1</sup>) region. This investigation has lead to the conclusion that the BP functional has the best overall performance and the choice of basis set is the most critical theoretical variable. Varying other parameters such as grid size, orbital cut-off, and convergence criteria lead to negligible differences among the calculations.