Line positions and intensities of the ν₄ 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, CH₃I, in the C[sbnd]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 ν₁ band centered at 2971 cm^-1, and the asymmetric stretch ν₄ band centered at 3060 cm^-1. The spectra of the ν₄ band and the nearby ν₃+ν₄-ν₃ 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 ν₄ band with a standard deviation (observed – calculated) of 0.00034 cm^-1, and 831 transitions to the ν₃+ν₄-ν₃ hot band with a standard deviation of 0.00084 cm^-1. For comparison, in the earlier work using standard FT-IR with 162 MHz resolution [Anttila, et al., J. Mol. Spectrosc. 1986; 119:190–200] 1830 transition were assigned to the ν₄ band, and 380 transitions to the ν₃+ν₄-ν₃ hot band, with standard deviations of 0.00083 cm^-1 and 0.0013 cm^-1, respectively. The hyperfine splittings due to the ¹²⁷I nuclear quadrupole moment are observed for transitions with J ≤ 2 × K. Finally, intensities of 157 isolated transitions in the ν₄ band are reported for the first time using the Voigt line shape as a model in multispectral fitting.