Font Size: a A A

Research On Measurement Technology Of Bio-molecules And Pesticides Molecules On Terahertz Time-domain Spectroscopy

Posted on:2009-02-05Degree:DoctorType:Dissertation
Country:ChinaCandidate:Z G YanFull Text:PDF
GTID:1118360242492013Subject:Control Science and Engineering
Abstract/Summary:PDF Full Text Request
Terahertz (THz, 1THz = 1012Hz) radiation with a frequency range of 0.1-10.0 THz, sandwiched between microwave and infrared, belongs to far infrared region. In the last decade, rapid progress in ultra-fast laser technology provides a steady and reliable optical source for the THz pulses generation, which greatly promotes the research in the THz waveband that is difficult to access before. Terahertz time domain spectroscopy (THz-TDS) is a new technique that applied to spectroscopic measurement based on ultrafast femtosecond laser. The temporal waveform of THz pulse is recorded by electro-optical or photoconductive sampling straight in time domain. By fast Fourier transform operation, the information of amplitude and phase of the THz wave are obtained in frequency domain, and then the refractive index and absorption of the material in THz region are extracted. THz-TDS can analyses the components and the tiny change in the structure of the materials based on the distinct absorbance of THz radiation. So, it is widely used to the study the optical characteristics of chemical and biological molecules in the terahertz region in recent years. To date, although THz-TDS has been developed for more than ten years, commercially available THz spectrometer can only make some qualitatively rather than quantitatively analysis.THz-TDS has been used to measure the spectral properties of several typical amino acids found in nature, vitamin and pesticides at room temperature. The refractive indices and absorption spectra are obtained. The results show that the samples have the spectral response in terahertz spectral range and the fact demonstrates the terahertz spectra could be used to study the molecular structure and vibration of sample. The assignments of vibrational modes in the spectral region were proposed using quantum chemical calculation. The theoretical results agree well with the experimental data. The far-infrared absorption features of bimolecular are highly sensitive to the structure and spatial arrangement of molecules, and then THz-TDS can be used for discrimination of compounds which have minor differences. The major contributions in this dissertation are as follows:(1) The theories concerned about terahertz wave technology, including the electromagnetism, quantum chemistry, quantum mechanics and semiconductor physics were discussed in detail. Based on these theories, the principles of generation and detection of terahertz radiation were proposed. The transmission THz-TDS, including the data processing, making the samples, alignment of ray and implement and measurement methods were introduced clearly.(2) THz time domain spectroscopy has been used to study the far-infrared spectroscopic characteristics of three typical amino acid biomolecules (glutamic acid, tyrosine and aspartic acid) and their enantiomers and racemic compounds in the nitrogen surroundings at room temperature. Their frequency domain spectra and well-resolved absorption spectra were obtained between 0.22.2 THz using the physical model of extracting the THz optical parameters of materials developed by Dorney etc. The results show that the spectra between the enantiomers and their racemic compounds have distinct differences. The reasons are that they have the different crystalline structure, and which lead to the difference of crystal lattice vibrational modes and the effect of intermolecular hydrogen bonding in the crystal. THz-TDS is highly sensitive to molecular structure and special arrangement, so it can be used to distinguish the amino acid bimolecular and their enantiomers and racemic compounds. This research provides a new, efficient and nondestructive measurement method for inspection of the food safety and chiral medicine identification.(3) By the terahertz spectroscopic investigation of B vitamin B1, B2 and B6, we found that the three molecules have distinct absorption peaks in the 0.2-2.2 THz range. And the absorption peaks have the large differencies, which come from the differencies of the molecular structures. It is noted that the refractive index varies with the location of the absorption peak. This study provides the fingerprint spectra database of the vitamin molecular and the important experimental results for the deep analysis of conformation and configuration of biomolecules.(4) The THz spectra of seven familiar pesticides molecules have been made, and the frequency domain spectra and absorption spectra were obtained using the THz-TDS in the frequency range 0.2-2.2 THz. The results indicate that the absorption spectra have distinct peaks which are different greatly. The absorption peaks come from not only the collective vibrations in the inter-molecular but also the weak interaction between the intra-molecular. In addition, the effective range of frequency domain spectra can be enlarged by mixing the polyethylene powder with sample in different ratio during the process of making the samples. This research provide the fingerprint spectra database of pesticides in the THz range, and which form a good base for applying the THz-TDS in food security, and the measurement of residual pesticide.(5) In order to better understand the origin of the absorption features observed in our experimental spectra, quantum chemical calculations were carried out with the Gaussian03 program for glutamic acid molecular and acephate molecular. For the glutamic acid molecular, density functional theory (DFT) with software package Gaussian 03 using B3LYP theory and the 6-311 basis set was employed for optimization and vibrational analysis, and the results of theoretical calculation agree well with the experimental ones. The vibrational modes of glutamic acid in 0.2-2.2 THz were assigned in detail by the visual software (GaussView 3.0). As for the acephate molecular, three kinds of quantum chemical calculations, Hartree-Fock, AM1 and DFT were adopted respectively. Since the density functional theory (DFT) takes into account the energy of electron exchange and electron correlation effectively, and adopts the large basis set (6-311+G(d,p)), it has higher calculation precision and efficiency than Ab initio HF method and semi-empirical AM1 method. The basis set (6-311+G (d, p)) can add diffuse functions to all the atoms in the system, add d polarized functions to heavy atoms, and add p polarized functions to H atoms.Some suggestions were put forward for the further study of bimoleculars and pesticides molecules THz spectroscopy in the last part of the paper.
Keywords/Search Tags:terahertz time domain spectroscopy, amino acid, vitamin, pesticide, collective vibration, quantum chemical calculation
PDF Full Text Request
Related items