Semi-Empirical model to retrieve finite temperature terahertz absorption spectra using Morse potential

Abstract

Terahertz (THz) absorption is a fingerprint property of materials, due to the underlying low-frequency vibration/phonon modes being strongly dependent on the chemical constitutions and microscopic structures. The low excitation energies (0.414−41.4 meV) are related to two intrinsic properties of THz vibrations: the potential energy surfaces (PESs) are shallow, and the vibrationally excited states are usually populated via thermal fluctuations. The shallow PESs make the vibrations usually anharmonic, leading to redshifted vibrational excited state absorption; combined with considerable vibrational excited states population, characteristic THz signals are usually redshifted and congested with varying degrees at different temperatures. Combining existing experimental THz spectra at low temperatures, first principles vibration analysis, and the Morse potential, we developed a semi-empirical model to evaluate the anharmonicity of the low-frequency modes. The model was benchmarked with purine molecular crystal to generate THz spectra at different temperatures, the results were consistent with experiments. The good agreement suggests this model would facilitate the application of THz spectroscopy in molecular crystal characterization.

Efficient Identification of Crude Oil via Combined Terahertz Time-Domain Spectroscopy and Machine Learning

“…Due to the high transmittance of THz waves in crude oil, it can measure the amplitude attenuation and time delay of the incident terahertz pulse without sample pretreatment, making it operationally simple. Additionally, the THz-TDS can be applied to analyze the compound structures and intermolecular interaction among organic molecules (such as hydrogen bonding, Van der Waals forces, dipole rotation, and vibrational transitions) [28][29][30][31]. It has been demonstrated that THz-TDS is applicable in qualitative and quantitative analysis of the crude oil properties [32,33], but its application in crude oil classification has not yet been explored.…”

First principles terahertz spectroscopy of molecular crystals: the crucial role of periodic boundary conditions benchmarked with experimental l-ascorbic acid spectra

“…Terahertz (THz) vibrational spectroscopy is a powerful technique for detecting low frequency vibration modes 1–3 and intermolecular interactions in the condensed phase. 4,5 The very low photon energy of THz waves (∼4.1 meV for 1 THz), which is much lower than the bonding or ionization energies of conventional molecules, enables the non-destructive detection and characterization. 6,7 Moreover, because of the high sensitivity to intra- and inter-molecular structures, THz spectroscopy has a fingerprint characteristic, which is effective in the detection of isomers and other similar substances.…”