Knowledge Bank

Methyl bromide $(CH3Br)$ has been identified as one of the major sources of atmospheric bromine. Atmospheric methyl bromide originates from both natural (algae, phytoplankton) and anthropogenic sources (agricultural fumigant): the tropospheric mixing ratio of $CH3Br$ is 9-11 pptV in the Northern hemisphere and about 8 pptv in the Southern hemisphere, with an increase of about 0.15 pptV per $yeara$. However, until present, no attempts have been made to determine atmospheric concentrations of $CH3Br$ using infrared spectroscopy. Although the line positions in this region have been studied previously at medium spectral $resolutionb$, little is known about the line intensities. The purpose of the present work is to complete and extend the previous studies of the $1200-1600cm-1$ spectral range and to provide a prediction of line positions and intensities accurate enough to determine optimal spectral windows for future atmospheric detection of $CH3Br$. The $\nu 2(A1)$ and $\nu 5(E)$ fundamental bands of $CH3\mathrm{<mrow\; data-mjx-texclass="ORD">79</mrow>}Br$ and $CH3\mathrm{<mrow\; data-mjx-texclass="ORD">81</mrow>}Br$ have been recorded at LPPM with a high-resolution Fourier-transform infrared spectrometer (unapodized resolution of $0.004cm-1$). For both isotopomers, we assigned 3037 lines for the parallel bands, 4530 for the perpendicular bands, and in addition 80 perturbation-allowed transitions, with $J\le 68$ and $K\le 11$. By taking into account the xy-Coriolis interaction between the two bands, it has been possible to generate an accurate prediction of the whole spectrum, with a standard deviation of better than $7\times 10-4cm-1$. The ground state axial rotational constants $A0$ were redetermined from allowed and perturbation-allowed infrared transitions observed in the $\nu 2$ and $\nu 5$ bands around the local crossing. The $A0$ values obtained for both isotopomers are more accurate but fully compatible with those obtained previously.