We use optical frequency comb Fourier transform spectroscopy to measure high-resolution spectra of iodomethane, CH$_3$I 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, 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 ${ \nu}_1$ band centered at 2971 cm$^{ -1}$, and the asymmetric stretch ${ \nu}_4$ band centered at 3060 cm$^{ -1}$. The spectra of the ${ \nu}_4$ band and the nearby ${ \nu}_3$+${ \nu}_4$-${ \nu}_3$ 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 ${ \nu}_4$ band with standard deviation (observed - calculated) of 0.00034 cm$^{ -1}$, and 831 transitions to the ${ \nu}_3$+${ \nu}_4$-${ \nu}_3$ hot band with standard deviation of 0.00084 cm$^{ -1}$. The hyperfine splittings due to the ${ ^{ 127}}$I nuclear quadrupole moment are observed for transitions with J$\leq$2xK. Finally, intensities of 157 isolated transitions in the ${ \nu}_4$ band are reported for the first time using the Voigt line shape as a model in multispectral fitting.
Categories: physics.chem-ph physics.optics