Line Positions and Intensities of the 4 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 1 band centered at 2971 cm-1, and the asymmetric stretch 4 band centered at 3060 cm-1. The spectra of the 4 band and the nearby 3+4-3 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 4 band with standard deviation (observed - calculated) of 0.00034 cm-1, and 831 transitions to the 3+4-3 hot band with standard deviation of 0.00084 cm-1. The hyperfine splittings due to the 127I nuclear quadrupole moment are observed for transitions with J≤2xK. Finally, intensities of 157 isolated transitions in the 4 band are reported for the first time using the Voigt line shape as a model in multispectral fitting.