| Methyl iodide has been extensively studied in the past. Ionization and dissociation mechanism of methyl iodide induced by 100fs femtosecond laser (I~1014W/cm2) light at 800nm was studied by means of a time-of-flight mass spectrometer. We observed that the mass spectra of CH3I irradiated by ths fs laser consists of the highly charged ions of iodide and carbon . In our work I7+and I8+are possible but they would be difficult to distinguish from the O+ and H2O+ in the mass spectrum. The excited states 3Q0,3Q1and 1Q1,which are optically accessible from the ground state ,are dissociative ,leading to the production of CH3 and I neutral fragments. These states can be reached through absorption of two 800nm photons, while more photons are needed for molecular ionization.The recorded mass spectra, at first glance, could be attributed to fragmentation which is followed by ionization. On the contrary, El-Sayed and coworkers have shown that the multiphoton ionization dissociation is the dominant mechanism in the CH3I, when this molecule is irradiated by picosecond laser pulses(30ps at 266nm).C.Kosmidis and coworkers investigated the molecule also at 1064nm, 532nm with ps , ns pulse duration and proved the El-Sayed's conclusion right. The results from the P.Graham and Castleman's study on CH3I also supported this conclusion In the mass spectra ,the parent ion peak was clearly recorded .Multiply charged molecular ions have been also observed and all In+ and Cp+ have spectral peaks which were assigned to the Coulomb explosion of multiply charged molecular ions in P.Graham and Castleman's respectivel femtosecond experiments. But the peak profiles also presented in C.Kosmidis' experiments at lower laser intensities with picosecond and nanosecond laser pulses.So we have observed complex ion peak profiles not to be assigned arbitrarily to the the Coulomb explosion of multiply charged parent ions, especial to in our lower intensities experiment than the other correlative fs experiments. In order to have more insight, we have calculated the kinetic energies (Ekin) of the fragments ejected to the backward and forward direction to detector by the difference the time they got to detector. The kinetic energy values of I ions increase as their charge values increase, which implies that the higher charged I ions weren't derived of the lower charged ones by further excitation ejected from the parent ion. Therefore, the Coulomb explosion mechanism within a multiple charged unstable parent ion should be invoked in our experiments. We observed the peak of the CH22+, not observed the peaks of the CH2+ and CH32+. We optimized their structure and analyzed their vibrational frequencies respectively with MP2/6-3111g* basis set by Gaussian98 on the personal computer. And it is found that the CH2+ and CH32+ aren't stable structure but the CH22+ is stable. If the CH2+ and CH32+ were produced in the experiment, their lives presented were too short to be recorded by the detector. We analyzed also the CH3I2+ by the bigger basis set and found it is unstable in the electronic ground state, so its disappearance is inexplicable which need to be validated further.P.Graham and co-workers considered that only the fragments CH3I+, I+,CH3+ were produced but not highly charged ions in their work at lower intensities of about 1014W/cm2 .But we observed the highly charged ions of the iodine and carbon at 800nm with about 1014W/cm2. In our experiment we observed three-peak structures of iodide ions which were different from the structure reported by Castleman.By theoretical calculation and computer simulation it is found that the middle peaks are the zero-kinetic-energies-peaks of iodide ions. We considered that these iodide ions come from the multiphoton ionization of the iodide atoms.We also observed that the intensities of iodide ions have orderliness contribution which was considered that depend on the space contribution of the laser intensity and also is relative to the slit that was placed in the front of the traction electr...
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