Temperature Dependent Characterization of Terahertz Vibrations of Explosives and Related Threat Materials
NAVAL RESEARCH LAB WASHINGTON DC ELECTRONICS SCIENCE AND TECHNOLOGY DIV
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Waveguide terahertz time-domain spectroscopy THz-TDS is used to characterize the temperature dependent vibrational properties of three threat-related materials 4-amino-dinitrotoluene 4A-DNT, pentaerythritol tetranitrate PETN, and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine HMX. These materials are characterized as thin polycrystalline layers deposited in the 50 micron gap of a metal parallel plate waveguide. For each material waveguide THz-TDS at least partially resolves the underlying vibrational spectrum and reveals new features that have not been observed in previous free space measurements of these materials. Strong experimental evidence for a phase transformation is observed for 4A-DNT as the polycrystalline layer on the waveguide surface is cooled to near 200 K. For PETN a highly resolved spectrum containing eleven vibrational lines is observed at 11 K with full-width at half maximum linewidths ranging from 7 GHz to 40 GHz. Based on comparison to measurements in the literature, our PETN measurement suggests that it is possible to produce narrow linewidths from a polycrystalline layer that approach those from a single crystal. Finally, for HMX, a highly resolved vibrational spectrum is measured that is assigned to the metastable gamma polymorph.
- Atomic and Molecular Physics and Spectroscopy