| Information detection technology is an important branch of Control Science and Engineering. Currently, using terahertz spectroscopy to detect information, especially that of organic ingredients, is a hot research field. Among the applications of terahertz spectroscopy, detection of biological molecules by means of terahertz spectroscopy is extremely valuable basic research, because it not only deepens our understanding of the interaction mechanism of terahertz wave and biomolecules, but also provides a prerequisite for the development of medical, pharmaceutical, security and so on.Processing and analysis of the terahertz spectra include the quantum-chemical calculation and assignment of vibration modes, qualitative and quantitative analysis of material using the spectra. This paper selects glutamine, threonine, and histidine as the research object, and the transmission-type teraherz time-domain spectroscopy (THz-TDS) was utilized to obtain their absorption coefficient, refractive index and dielectric spectra in the frequency band of 0.3-2.6 THz. Regarding the quantum-chemical calculation of vibration modes and quantitative analysis, the main results of this study are as follows:1) The terahertz vibration modes of amino acids were accurately simulated based on the solid-state unit cell model, and using the generalized gradient approximation (GGA) density functional theory (DFT) with the plane wave basis set and norm-conserving pseudopotentials. Using the WC density functional and the density functional perturbation theory (DFPT), the lattice-dynamics calculations on glutamine and threonine were performed. For glutamine, the simulated vibration frequencies were closer to the observed than that in the previous report, in which the B3LYP density functional was utilized for the lattice-dynamics calculations. Similarly, the quantum-chemical calculation of the vibration modes of threonine achieved a high accuracy. The results show that the vibration modes of the two amino acids originated from the intermolecular translation, the hydrogen-bond interation and the collective bending vibration of functional groups.2) A weighted iterative polynomial curve-fitting algorithm was put forward to eliminate the scattering baseline in the THz absorption spectra. Regarding the baseline in the Raman spectra, Lieber et.al. proposed an iterative polynomial curve-fitting algorithm to simulate the baseline and correct the spectra. The method could remove the baseline in the Raman and infrared spectra, but they did not discuss how to determine the polynomial order, and their data showed that a high-order polynomial is not necessary. To address this problem, a creterion for determination of the order was proposed by estimating the convergence of the weight of corrected spectra. If the weight converged to a given value for the two adjacent polynomial orders, the order number was no longer increased and the optimal polynomial order was obtained. Using this algorithm, the absorption spectra of glutamine and histidine were corrected. The linear relationsip between the characteristic absorption and the concentrationwas significantly improved.3) The qualitative and quantitative analysis of materials in terahertz band mainly depends on the absorption spectra, however, the refractive index and dielectric spectra were underutilized. Through the analysis of CRI (Complex Refractive Index) model, which is an effective medium theory, the linear relations between the refractive index and the volume concentration of glutamine and histidine were obtained, as well as the quadratic relations between the complex permittivity and the volume fraction at feature peaks. Based on the linear relation between the refractive index and the concentration, the concentrations of mixed samples composed of glutamine and histidine were determined by the partial least squares method. The results proved the refractive index can be used for quantitative analysis of solid samples. |
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