Study On Multiphoton Ionization Dissociation Processes Of Ethyl Iodide And Trichloromethane

Mass Spectrometry (MS) is an effective methed of multiphoton ionization and dissociation, through the laser interrraction with organic compouds, the transition rate is greatly increased by absorbing several photons. Combined with time of flight mass spectrometry, fragment ions from the mass spectrum is analysised to study the cleavage channel and thus provide a certain amount of theoretical data for organic research and development. Multiphoton ionization means that molecules or atoms absorb one or several photons to be triggered into a medium quantum state and then further absorb one or several photons to cause ionization. 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 during the ionization and detection process. Compared with the other methods, the technique has much advancement: high sensitivity, elaborate selectivity, high rsolution 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. Meanwhile it has intersected with many field and has got many returns. This paper mainly studies the mutiphoton ionization dissociation processes of ethyl iodide and trichloromethane at the 800 nm femtosecond laser, combined with the time of flight mass spectrometry to study dissociation channels of the fragment ions.This paper is divide into five chapters. The first chapter is mainly about instruction, concludes a summary of multiphoton ionization dissociation, the definition and classification and characteristics of the principle of TOF-MS, Gaussian 03 methodology and theoretical basis. The second chapter is about experiment description on the instruments used in this paper, which introduces the experimental operation and data acquisition equipment and experimental procedures and experimental methods. The third chapter analysis detaily the processes of multiphoton ionization dissociation (MPID) of ethyl iodide was studied at 800 nm. The time of flight mass spectra of ethyl iodide were obtained. We draw the conclusion that it mainly undergoes the 3+3 resonant-enhanced multiphoton ionization (REMPI) process. The figure of laser power index changing with the laser power is given. We measured the percentage of fragment ions to the total ion current and the dependence on laser intensity of the molecule. Based on the experimental results, the multiphoton ionization fragmentation mechanism of ethyl iodide was discussed. Fragement ions are produced by dissociating C2H5I+, this is consistent with the parent-ion dissociation staircase. The main possible channel of the MPI and photodissociation of C2H5I+ are analyzed. At the same time, the two possible channels of parent ion are analyed, but the C-I dissociation is the main channel. In addition, we also use Gaussian 03 calculate the energy of ethyl iodide parent ions and main fragment ions at B3LYP/3-21G basis set, the theortical results further demonstrate the experiment.The four chapter analysis the mutiphoton ionization peocess of trichloromethane and discussed the trichloromethane molecules multiphoton ionization dissociation channels and dissociation fragment ions channels. The results shows that the main trichloromethane dissociation channel is the mother molecular absorbs three photons to reach the electronic excited state, then absorbs five photons become mother ions. We draw the conclusion that it mainly undergoes the 3+5 resonant-enhanced multiphoton ionization (REMPI) process. Then mother ions dissociation and ionization other ions. The figure of laser power index changing with the laser power is given. At the same time, changing the laser intensity, we obtained the percentage of part fragment ions to the total ion current and dependence on laser intensity of the molecule. According to Getanken's theory, we concluded that the primary MPI process of trichloromethane is the fragment-ion dissociation staircase. We also discussed some formation process of the fragmentcations CHCl2+,CHCl+ CH+,Cl+.The five chapter is the summary.