## Abstract

We use optical frequency comb Fourier transform spectroscopy to measure high-resolution spectra of iodomethane, CH_{3}I, 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 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 a standard deviation (observed – calculated) of 0.00034 cm^{-1}, and 831 transitions to the ν_{3}+ν_{4}-ν_{3} 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 ν_{4} band, and 380 transitions to the ν_{3}+ν_{4}-ν_{3} hot band, with standard deviations of 0.00083 cm^{-1} and 0.0013 cm^{-1}, respectively. The hyperfine splittings due to the ^{127}I nuclear quadrupole moment are observed for transitions with J ≤ 2 × K. 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.

Original language | English |
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Article number | 107263 |

Journal | Journal of Quantitative Spectroscopy and Radiative Transfer |

Volume | 255 |

DOIs | |

Publication status | Published - Nov 2020 |

MoE publication type | A1 Journal article-refereed |

## Keywords

- Fourier transform spectroscopy
- High-resolution spectroscopy
- Methyl iodide
- Optical frequency comb

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