Temperature dependent vibrational modes of Ammonium Nitrate in terahertz regime

Ammonium Nitrate (AN), often used as an ingredient for making improvised explosives, does not show any spectral feature at room temperature. We employed terahertz time domain spectroscopy (THz-TDS), a non-invasive technique, in the range of 0.30 THz to 2.50 THz to measure the effective dielectric properties of AN embedded in a polytetrafluoroethylene (PTFE) binder in the temperature range 5K to 300K. We extracted the dielectric constants of AN using effective medium theories and fitted them using Lorentz dispersion model combined with Huang equation. We identified six low frequency vibrational modes of pure AN at low temperature (<240 K). These lattice vibrations in the low frequency range agree reasonably well against our first principles Density Functional Theory (DFT) simulation of molecular AN, that reveals four low frequency vibrational modes within 3THz.