Study On Organic Molecules By Terahertz Time-domain Spectroscopy And Theoretical Calculations

Terahertz Time-Domain Spectroscopy (THz-TDS) is a novel coherent techniqueproviding a new insight into the structural characteristics and mechanism of thespectral responses in molecular level. Absorption features in THz region are relevantto intramolecular modes and intermolecular modes arising from hydrogen-bondinginteraction, van der Waals force and other weak interactions. It is evident THzvibrational spectra represent a characteristic “fingerprint” of a molecular substance.Investigation on THz spectra of molecules is significantly important in substanceidentification, biomedical field, security detection and quality assurance.THz spectra of benzoic acid (BA) and its salt sodium benzoate (SB), biomoleculecytosine, high explosive RDX are investigated in this work by THz-TDS andtheoretical simulations. The experimental results clearly show these important organicmolecules exhibit distinctive features in THz range. Furthermore, the absorption peakof BA at0.63THz is reported for the first time, and the characteristic “fingerprint” ofSB in THz region is also reported for the first time, to the best of our knowledge.Moreover, theoretical calculations have been performed. For monomer andgas-phase dimer, full geometry optimizations and normal mode analyses areperformed by Hartree-Fock method, second-order M ller-Plesset Perturbation Theory(MP2), and Density Functional Theory (DFT) with Becke-3-Lee-Yang-Parr (B3LYP)hybrid functional. Geometry optimizations and lattice dynamic calculations on solidcrystal are performed with the pseudo-potential plane wave (PPW) method of DFT bytaking periodic boundary conditions into account. All measured terahertz absorptionfeatures of BA and cytosine are successfully assigned, respectively.