THz time-domain spectroscopic characterization of surface roughness scattering and DFT analysis of Ammonium Nitrate...

We report the terahertz time-domain spectroscopy (THz-TDS) study of opto-mechanical properties of secondary high energy materials ammonium nitrate (NH₄NO₃) and urea (CH₄N₂O), in reflection mode. These compounds are used as principal ingredient in improvised explosive devices (IEDs). The measured values of complex reflection coefficients lie between 0.4 and 3.5 THz range. The corresponding values of the complex refractive index of ammonium nitrate and urea lie between 2.6-1.7 and 1.5 range, respectively. In addition, the absorption coefficients of ammonium nitrate and urea are laying between 1000 cm^-1 and 1500 cm^-1 range, respectively. The surface roughness measurements were interpreted using a Kirchhoff approximation that also help to decouple intrinsic dielectric losses from surface-induced scattering to estimates of the root-mean-square (RMS). Finally, density functional theory (DFT) was performed to validate the lattice vibrational modes and intrinsic dielectric response, which show a good agreement with the experimentally extracted optical constants. The reflected data of THz spectra were used for ascertaining the surface roughness of the materials, which is the order of micrometer (μm) range. It was evident from the results that the surface roughness of the samples lies between 0.05 and 0.15 μm range which is smaller than the wavelength of the incident THz radiation. Consequently, the scattering mechanism covers under the Rayleigh regime and responsible for the modulation of both the amplitude and phase of the reflected THz signal. These results establish a rigorous correlation between crystalline structure, optical constants, dielectric dispersion, and surface morphology and standoff detection in the terahertz domain.