O-bromotoluene And Bromotoluene Multiphoton Ionization Mass Spectrometry

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 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.Meanwhile it has intersected with many fields and has got many returns.For the moment,the study of multiphoton ionization and dissociation process has already become one of the most important research areas of the atomic and molecular physics region.We have studied the multiphoton ionization time of flight mass spectra(MPI-TOFMS) of o-bromotoluene,p-bromotoluene and toluene using laser mass spectroscopy technique.This paper mainly includes the following content:(1) O-bromotoluene multiphoton ionization and dissociation(MPID) is studied at 355 nm and 266 nm laser radiation,o-bromotoluene TOF mass spectra of multiphoton ionization are obtained.After analysis,we get the conclusion that there is no obvious ionization-dissociation with the efect of the laser of 355 nm,while under the efect of the laser of 266 nm,MPI-TOF mass spectra of o-bromotoluene molecular is rich.By 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 o-bromotoluene is multiphoton dissociation of parent molecule and multiphoton ionization of neutral fragment.Parent molecule mainly has two possible dissociation channels,one is the direct dissociation,the other is the first isomerism latter dissociation,we discussed some formation process of the fragrnentcations,i.e.C₇H₇⁺,C₆H⁺,C₅H₅⁺ etc.By changing experimental condition,we disovered that the laser delay time to the molecular beam,the laser intensity and the pressure of carrier gas have a significant impact for the formation of ions,we given the explanation to their influence.(2) By means of Gaussion 03 ab initio calculations,the hydrogen atom transfer reaction of o-bromotoluene in photolysis process was made the theory calculation,the reaction activation energy was obtained.The result showed that the rearrangement barrier is less than the single-photon laser energy,so o-bromotoluene molecular possibly has ftrst isomerism in the photolysis process,and then the bromine atom dissociates. (3) Using 266 nm laser as excitation source,p-bromotoluene multiphoton ionization and dissociation was studied,p-bromotoluene time of flight mass spectra of multiphoton ionization were obtained.We measured the percentage of part fragment ions to the total ion current and dependence on laser intensity of the molecule.Based on our experimental results the multiphoton ionization fragmentation(MPIF) mechanics of p-bromotoluene was analysed,we discussed some formation'process of the fragmentcations,i.e.C₇H₇⁺,C₆H⁺,C₅H₅⁺ etc.The conclusion is gained that most of ion fragments are produced by ion-fragment dissociation staircase.The peak positions of mass spectra shift was explained in our experiment due to laser intensity change.(4) There is no obvious ionization-dissociation of toluene molecular at 355 nm laser radiation,but MPI-TOF mass spectra of toluene is too rich at 266 nm.The multiphoton ionization fragmentation mechanics of toluene was simply analysed.