Terahertz absorption spectra of highly energetic chemicals E. J. Slingerland a , M. K. Vallon a , E. G. E. Jahngen b , R. H. Giles a and T. M. Goyette a a Submillimeter-Wave Technology Laboratory, University of Massachusetts Lowell Lowell, MA 01854 b Department of Chemistry, University of Massachusetts Lowell Lowell, MA 01854 ABSTRACT Research into absorption spectra is useful for detecting chemicals in the field. Each molecule absorbs a set of specific frequencies, which are dependent on the molecule’s structure. While theoretical models are available for predicting the absorption frequencies of a particular molecule, experimental measurements are a more reliable method of determining a molecule’s actual absorption behavior. The goal of this research is to explore chem- ical markers (absorption frequencies) that can be used to identify highly energetic molecules of interest to the remote sensing community. Particular attention was paid to the frequency ranges located within the terahertz transmission windows of the atmosphere. In addition, theoretical derivations, with the purpose of calculating the detection limits of such chemicals, will also be presented. Keywords: Terahertz, Spectroscopy 1. INTRODUCTION Researchers have been investigating the detection of solid and liquid forms of highly energetic chemicals in recent years 1-5 . Wilkinson et al. measured the absorption spectrum of solid triacetone triperoxide (TATP) in the tera- hertz region for the first time this past year 1 . This project was initiated to determine the practicality of detecting vapor-phase highly energetic chemicals in the terahertz (THz) region. A series of absorption measurements have been performed under controlled conditions, allowing detection limits to be established. The detection limits will vary with stand-off distance to the sample and gaseous sample concentration. Both highly energetic chemicals and their constituents were investigated between 20 and 80 wavenumbers (cm -1 ), 0.6 THz to 2.4 THz, with particular attention to atmospheric transmission windows in the THz region, as these frequencies would be the most useful for remote sensing under field conditions. 2. EXPERIMENT 2.1 Set-up The data was collected using a Bruker IFS 66v Fourier transform (FTIR) spectrometer with a variable pathlength gas cell from Pike Technologies mounted in the sample compartment. A cryogenically cooled IR Labs silicon bolometer was used as the detector. Figure 1 shows the experimental set-up. Several highly energetic chemicals were identified for investigation in this experiment. Those samples that could not be obtained through standard channel suppliers, such as Fisher Scientific, were synthesized in the laboratory. All synthesized chemicals were verified and characterized via standard IR and mid-IR chemical spectroscopy measurements and verified against known IR spectra. The samples are kept in glass sample tubes which are evacuated to remove any air. The sample then sublimates or evaporates inside the glass sample tube until reaching equilibrium pressure. The optics and source for the FTIR spectrometer used in the measurements required optimization to operate in the long wavelength region. The FTIR was equipped with a 125 Watt mercury arc lamp and 23μm thick mylar beam splitter, providing good spectral range from 20 cm -1 to 80 cm -1 . The variable pathlength gas cell has Further author information: (Send correspondence to E.J.S.) E.J.S.: E-mail: Elizabeth Slingerland@student.uml.edu