The Terahertz Experimental And Theoretical Study On Organic Molecules With Similar Structure

Terahertz(THz) radiation, named by its central frequency of 1012 Hz, is located in the region between microwave and far-infrared. Generally, THz waves are those with frequencies from 0.1 to 10 THz, sometimes, it also refers to the region of 0.3-3 THz. At the beginning of 90 th, THz technology has found its own way due to the rapid development of untra-short laser. Nowadays, THz time-domain spectroscopy(THz-TDS) has been used in the quality and quantity analyse of materials because of THz waves unique properties of low energy, high spectral resolution, and so on.As far as we know, recent years, the terahertz spectral research of materials are more studied experiementally, or some with very shallow simulation, which deeply hinder the futher application of terahertz spectroscopy and also the research on materials in molecular level. It has been known that the measured THz spectra present the real-time condition of materials, but the molecular strucuture and the origin of THz spectra can only be predicted based on quantum mechanics theory, this is very important, especially in medical. The calculation will provide information about the origin of the new spectrum so as to predict disease when the measurement shows an abnormal feature. In general, THz spectral information of materials not only has the significant scientific research value but also is of great significance in the application of real life, such as industry, agriculture, food, healthy and security. Thus, THz spectra of a series of important biological molecules in daily life, urgent need of discrimination of industrial isomers and great hidden danger to the food safety materials have been studied along with simulation. The thesis is divided into seven parts:The first part(Chapter 1&2) The characters and applications of THz waves are introduced, and current situation and development of THz time-domain Spectroscopy are presented. Later, the common methods of THz radiation generation and detection are introduced, the calculated functions of absorption coefficient and refractive index are summarized, and the different simulation methods are also discussed.The second part(Chapter 3) THz spectra of alanine, phenylalanine and tyrosine have been detected and analysed using THz-TDS. According to their molecular structures, these three amino acids have the common functional groups of amino and carboxyl groups, but their THz spectra are very different. Based on the simulation, the original reasons of these difference spectra have been summarized.The third part(Chapter 4) THz absorption spectra of fructose, glucose and monohydrated glucose have been measured and analysed using THz-TDS. Fructose and glucose are isomers, and monohydrated glucose contains water and glucose molecules together in solid state. It is found THz-TDS can distinguish isomers and hydrates successfully. Also, the origins of the spectra of glucose and monohydrated glucose have been summarized.The fouth part(Chapter 5) Due to the urgent need on isomers discrimination of 1-2-benzenediol, 1-3-benzenediol and 1-4-benzenediol in industry, THz-TDS was performed to study these three materials. It is found that these three isomers can be easily identified by THz-TDS. Solid-state theory has been performed to further interpret the origin of the THz spectra, and also the mid-infrared and THz spectra of 1-2-benzenediol have been compared. Furthermore, the THz spectra of 2-pyridine carboxylic acid, 3-pyridine carboxylic acid and 4-pyridine carboxylic acid also have been presented.The fifth part(Chapter 6) The solid and liquid state THz spectra of benzoic acid and sodium benzoate have been investigated due to their critical influence in food safety. The experimental features imply that these two materials show totally different THz spectra even they have similar molecular structure. And also, the simulation results imply that the hydrogen bond is the critical reason for the big spectral difference.The sixth part(Chapter 7) The dissipation properties of ionic liquids of ethylammonium formate(EAF) have been investigated using pumb-probe infrared spectroscopy. Due to the molecular structure inhomegenous of ionic liquids, the work starts from two parts, ionic(ND and NH stretching) and hydrophobic domains(CH stretching). Time constant of thermal energy relaxation have been obtained after the data fits to kinetic model, then it is found that molecular dynamics of ionic liquids in different parts is different and also local, which depend on their natural formation.The seventh part(Chapter 8) Content of the thesis has been summarized, and the prospect and application of THz-TDS and pump-probe technology in future have been described.