Line Positions and Intensities of the ν4 Band of Methyl Iodide Using Mid-Infrared Optical Frequency Comb Fourier Transform Spectroscopy

Abstract We use optical frequency comb Fourier transform spectroscopy to measure high-resolution spectra of iodomethane, CH3I, 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, frep, of 125 MHz. A series of spectra with sample point spacing equal to frep are measured at different frep 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 standard deviation (observed – calculated) of 0.00034 cm-1, and 831 transitions to the ν3+ν4-ν3 hot band with 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 deviation 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.