Temperature-dependent far-infrared spectra of explosives and drugs measured by terahertz time-domain spectroscopy

Using terahertz time-domain spectroscopy, we have measured the temperature dependence of the far-infrared vibrational modes of a number of illegal drugs and high explosives that are of interest to the forensic and security services. Well-resolved low-frequency absorption peaks were observed in the frequency range 0.3 - 3.0 THz and over the temperature range 4 - 296K, some of which were found to become more intense and shift to higher frequencies as the temperature was reduced. Our results confirm that the low-frequency collective modes are highly sensitive to the structural and spatial arrangement of molecules.