Abstract：Using the device for ion velocity imaging, the laser frequency is doubling with the wavelength in the region of 492-523 nm, and the laser after frequency doubling was used as the light source. The ion spectrum of methyl iodide parent molecular (CH3I+) in the range of 76 500-81 120 cm-1 was obtained by the way of two-photon ionization, with a very high-resolution. The mechanisms of the methyl iodide molecule two-photon ionization were also described, the CH3I+ spectrum obtained in the experiment was marked based on Rydberg formula and the quantum defect, the split arising from p series, d series and f series levels was also explained, and the spectral assignment showed that the two-photon ionization of methyl iodide molecule can not only be used to observe the reported characteristics of single photon ionization, but also can find some transitions which is forbidden in the single photon ionization, such as f series transitions.
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