Study On Multiphoton Ionization Mass Spectra Of 3-Picoline And Pyridine

The interaction of laser pulses with matter remains still to be an important problem and receives more and more attenuation in modern laser physics. The development of ultrashort pulse laser technology promotes in-depth studies on the laser interaction with atoms and molecules, and more and more new physical phenomena are observed. Resonance enhanced multiphoton ionization (REMPI) technique developed and improved following the appearance and development of laser in late 1970s. Cooperating with time of flight mass spectra, it can achieve dual selections (spectroscopy selection and mass spectrum selection) durring the ionization and detection process. Compared with the other methods, the technique has much advancement:high sensitivity, elaborate selectivity, high resolution etc. Ever since then the method has been playing an important role in environment pollutant detection, material composition analysis, molecular energy levels, and so on. Mean while it has intersected with many fields and has got many returns.In this paper, we combined the femtosecond laser with time of flight mass spectrometry to study the processes and mechanism of multiphoton ionization and dissociation of 3-picoline and pyridine on the base of resonance enhanced multiphoton ionization and flight spectroscopy technique, this paper includes the following parts:The first chapter is about instruction; include the development and advantage of laser mass spectra, the classification and characteristics of multiphoton ionization dissociation, the mechanism of multiphoton ionization and dissociation, the meaning of the subject.The second chapter is about the research method; conclude the experimental instruments, experimental methods and Gaussian03 research methodology.The third chapter is about the processes and mechanism of multiphoton ionization and dissociation of 3-picoline. We got the mass spectrum of 3-picoline at different laser energy, and observed the double-logarithm relation between the intensity of the parent ion and the laser intensity ,the dependency relation between the laser intensity and the signal percent of fragment ions out of all ions based on the experimental results. The multiphoton ionization fragmentation mechanism of 3-picoline is onization before dissociation. The molecule produced parent ion from 4+2 resonance enhanced multiphoton ionization at first, and then the parent ion absorbed photons occurred ring open reaction to produced C5H6+. The dissociation process of C5H6+ is complex, the ion can occurred fracture reaction between ring and branch chain produce C4H3+ or CH3+, or it can occurred ring-open produced C3H3+ or C2H3+. We observed the energy of ions by DFT at B3LYP/6-311G++(d, p) level, the result match with the experiment. The excited states of parent ion were studied by TD-DFT; it shows that the resonance between parent ion energy level and laser wavelength is a key factor that effects the fragmentation.The forth chapter is about the processes and mechanism of multiphoton ionization and dissociation of pyridine. We got the mass spectrum of pyridine, and the laser power index of ions, the possible dissociation channels are analyzed. We optimized the geometry and calculate energy by DFT and CIS on the B3LYP/6-31G (d) level of ground state and excited state. We compare the geometry change between them, and observed the infrared and raman spectrum of the ground state molecule. The energy calculate match with the experiment result.