15min:

F. KWABIA TCHANA, J. ORPHAL, I. KLEINER, CNRS, Laboratoire de Photophysique Moléculaire, 91405 Orsay Cedex, France; N. LACOME, Laboratoire de Dynamique, Interactions et Réactivité/Spectrochimie Moléculaire, Université Pierre et Marie Curie, UMR CNRS 7075, Case Courrier 49, 4 Place Jussieu, 75252 Paris Cedex 05, France; AND O. BOUBA, Faculté des Sciences, Université de Ngaoundéré, B.P. 454 Ngaoundéré, Cameroun.

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 year. 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 resolution, little is known about the line intensities. The purpose of the present work is to complete and extend the previous studies of the 1200-1600 cm^-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 ₂ (A₁) and ₅ (E) fundamental bands of CH₃⁷⁹Br and CH₃⁸¹Br have been recorded at LPPM with a high-resolution Fourier-transform infrared spectrometer (unapodized resolution of 0.004 cm^-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× 10^-4~cm^-1. The ground state axial rotational constants A₀ were redetermined from allowed and perturbation-allowed infrared transitions observed in the ₂ and ₅ bands around the local crossing. The A₀ values obtained for both isotopomers are more accurate but fully compatible with those obtained previously.