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Line Positions and Intensities of the ν$_4$ Band of Methyl Iodide Using Mid-Infrared Optical Frequency Comb Fourier Transform Spectroscopy

We use optical frequency comb Fourier transform spectroscopy to measure high-resolution spectra of iodomethane, CH$_3$I in the C-H stretch region from 2800 to 3160 cm$^{-1}$. The fast-scanning Fourier transform spectrometer with auto-balanced detection is based on a difference frequency generation comb with repetition rate, f$_{rep}$, of 125 MHz. A series of spectra with sample point spacing equal to f$_{rep}$ are measured at different f$_{rep}$ settings and interleaved to yield sampling point spacing of 11 MHz. Iodomethane is introduced into a 76 m long multipass absorption cell by its vapor pressure at room temperature. The measured spectrum contains three main ro-vibrational features: the parallel vibrational overtone and combination bands centered around 2850 cm$^{-1}$, the symmetric stretch $ν_1$ band centered at 2971 cm$^{-1}$, and the asymmetric stretch $ν_4$ band centered at 3060 cm$^{-1}$. The spectra of the $ν_4$ band and the nearby $ν_3$+$ν_4$-$ν_3$ hot band are simulated using PGOPHER and a new assignment of these bands is presented. The resolved ro-vibrational structures are used in a least square fit together with the microwave data to provide the upper state parameters. We assign 2603 transitions to the $ν_4$ band with standard deviation (observed - calculated) of 0.00034 cm$^{-1}$, and 831 transitions to the $ν_3$+$ν_4$-$ν_3$ hot band with standard deviation of 0.00084 cm$^{-1}$. The hyperfine splittings due to the ${^{127}}$I nuclear quadrupole moment are observed for transitions with J$\leq$2xK. Finally, intensities of 157 isolated transitions in the $ν_4$ band are reported for the first time using the Voigt line shape as a model in multispectral fitting.