Theoretical Simulations For Electron-impact N₂Vibrational Excitations And Terahertz Spectroscopy Of Methedrine

Ab initio calculation is an important simulation approach in physical research, which is able to analyze and predict physical characteristics of materials, and it is becoming more and more important in specific fields.Nitrogen gas is one of the most common medium in fundamental researches. Therefore, more deeply understanding on physical properties of N2plasmas are of particular importance in atmospheric physics, chemical physics and gas discharge fields, such as the investigation on potential energy curves, spectral constants and electron-impact cross sections. In the first part of the thesis, ab initio calculations of vibrational excitation cross sections for electron collisions with nitrogen molecules in low-lying states using similarity function approach, such as a-a’, a-w, B-B’ and B-W transition systems, are investigated for the first time. One of the great challenges in understanding nitrogen plasma elementary processes is the lack of an available database of the cross-sections of electron-impact excitations and radiations. In order to obtain the cross sections, accurate spectroscopic constants and transition dipole moments have been investigated. Potential energy curves and other electronic transition dipole moments for the low-lying states of N2have been evaluated using complete active space self-consistent field (CASSCF) approach with aug-cc-pVqZ basis set. Comparing with other available results, the new spectral constants such as the dissociation energy De, equilibrium bond distance Re and vibrational frequencies ωe have higher accuracy and are getting a better consistent with experimental data. The calculated cross-sections could provide a database for studying the elementary processes and the properties in N2plasma.In the second part of the thesis, the terahertz time-domain spectra (THz-TDS) of crystalline methedrine, which is one of the illegal drugs, was performed using molecular dynamics simulation by the Fourier transform of time derivative auto-correlation functions of the dipole moment. In order to accurately detecting the drugs from samples, it is necessary to build a complete database for terahertz spectra under the different external conditions by theoretical calculation, which are hardly obtained from the experiments directly. Our results show remarkable consistency with the available experimental data indicating that the presented method has significant capability to predict terahertz spectra at various conditions. We investigated the effects of temperature and pressure on THz-TDS by simulating the system in temperature range between78.4K and400K at pressures up to100atm. Results show the spectral features of THz-TDS both in intensity and profile are highly sensitive to the variation of temperature and with a lower magnitude to the variation of pressure. The vanishing, rebuilding and shifting of spectral peaks are due to the complex mechanisms such as the anharmonicity, shifting in the vibration energy levels, formation and destruction of hydrogen-binding and the deformation of the potential energy surface during the environment changing. This improves our understanding for complicated THz-TDS of crystalline methedrine and would be useful for assignment of the practical measurements.