A Fourier Transform Spectroscopic Study of The 2v₃ Overtone of Methyl Iodide

The overtone spectroscopy has the foundamental importance for the study of the sstructures and behavious of the molecules in high-lying vibrational states. the present paper is one of our recent works, which are devoted to a systematic study on the high-lying vibrational states of various kind of molecules.
The spectrum of the 2v₃ overtone band of methyl iodide was recorded on a Bomem DA3.002 FT Spectrophotometer at resolution of 0.04cm^-1 with 18.5cm^-1 absorpertion length and 6 torr sample pressure. The K structures of some P- and R- brandch lines (J up to 50, K up to 7) and Q- branch lines (J from 3 to 19) were assigned. A least sqares fitting was carried out and the corresponding molecular constants were obtained as shown in the following table:
The molecular constants of 2v₃ state (in cm^-1)

The computed spectrum was calculated by assuming a Boltzmann distribution of spectral line intensities and a Gauss line shape with 0.02cm^-1 for the every spectral lines width. The computed spectrum shows good agreement with experimental spectrum.
The geometrical configuretion of mathyl iodide in v₃ vibrational state was also discussed. From v₃=0 to v₃=1, the increases of I_band I_aare 0.5% and 0.04 respectively, from v₃=1 to v₃=2, the corresponding increases are 0.9% and 0.03%.