The absorption spectra of two kinds of important functional oligosaccharides were firstly acquired based on Fourier transform infrared spectroscopy in the range of 0.15-10THz. The simulation results of their infrared spectra were given based on Gaussian software, which were in good agreement with the experiment results. The rotation spectra and some perssad vibration spectra of these molecules were analyzed, and their absorption peaks were exactly identified. The components information was obtained by comparing the simulation results of different molecules.
As a powerful tool for the research of molecular structure, infrared absorption spectrum has been extensively studied in the field of biomedical photonics. The absorption spectrum of anhydrous glucose in terahertz region has been measured by Fourier transform infrared spectrometer (FTIR). The experimental results show that there are many characteristic absorption peaks. The origins of characteristic absorption are generally attributed to intermolecular vibrations and intramolecular torsions. CASTEP quantum chemical calculation software package was utilized to simulate the infrared spectroscopy of glucose crystal structure based on periodic boundary condition and plane wave pseudopotential method. Also, linear response approach and norm conserving pseudopotentials are essential. Besides, the performance of the generalized gradient approximation (GGA) functional has been commendably examined. The theoretical results show that the standard Perdew-Burke-Ernzerhof (PBE) approach along with its line Broyden-Fletcher-Goldfarb-Shanno (L-BFGS) algorithm tends to be superior. We analyzed the vibration mode corresponding to each characteristic absorption peak with DFT theory. The agreement between theory and experiment indicates that the crystal simulation calculation based on solid-state density functional theory can identify absorption peaks of substance and vibration attribution accurately in terahertz region.
The water vapor noise will affect the accuracy of the extracted optical parameters based on terahertz time domain spectroscopy technology. Because vapor noise has the characteristics of wide distribution and high intensity, the existing denoising methods cannot be effectively applied to the THz signal with vapor noise. In this paper, a numerical denoising method is presented. First, based on Van Vleck-Weisskopf lineshape function and the linear absorption spectrum of water molecules in the HITRAN database, we have simulated the water vapor absorption spectrum with line width, and the continuum effect of water vapor molecules are considered in the simulation. Then, the transfer function of different humidity is constructed by the calculation of the water vapor absorption coefficient and the real refractive index; Finally, based on the propagation factor formula of the mutual effects of THz wave and water vapor, the THz signal of the Lacidipine sample containing vapor noise in the continuous frequency domain of 0.3-1.8THz is denoised by using the constructed transfer function of the water vapor; the optical parameters of the sample signal before and after denoising can be extracted. It can be seen that the optical parameters extracted from the denoised signal are close to the optical parameters in the nitrogen environment, which proves the effectiveness of denoising. Under low humidity, this method can still accurately extract the optical parameters of samples without nitrogen filling, which saves the cost, enhances the convenience of the application of terahertz time domain spectroscopy in pharmaceutical production, safety inspection, imaging etc.