Line positions and intensities of the ν₄ band of methyl iodide using mid-infrared optical frequency comb Fourier transform spectroscopy

I. Sadiek, A. Hjältén, F. S. Vieira, Ch. Lu, M. Stuhr, and A. Foltynowicz

We use optical frequency comb Fourier transform spectroscopy to measure high-resolution spectra of iodomethane, CH₃I, in the CH 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.

J. of Quantitative Spectroscopy and Radiative Transfer Vol. 255, p. 107263 (2020)

https://doi.org/10.1016/j.jqsrt.2020.107263