NEW ANALYSIS OF THE CORIOLIS-INTERACTING $\nu_{2}$ AND $\nu_{5}$ BANDS OF $CH_{3} {^{79}}Br$ AND $CH_{3} {^{81}}Br$
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Date
2004
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Publisher
Ohio State University
Abstract
Methyl bromide $(CH_{3}Br)$ 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 $CH_{3}Br$ 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 $year^{a}$. However, until present, no attempts have been made to determine atmospheric concentrations of $CH_{3}Br$ using infrared spectroscopy. Although the line positions in this region have been studied previously at medium spectral $resolution^{b}$, 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 $CH_{3}Br$. The $\nu_{2} (A_{1})$ and $\nu_{5} (E)$ fundamental bands of $CH_{3} {^{79}}Br$ and $CH_{3} {^{81}}Br$ 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 $J \leq 68$ and $K \leq 11$. 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 $7\times 10^{-4} cm^{-1}$. The ground state axial rotational constants $A_{0}$ were redetermined from allowed and perturbation-allowed infrared transitions observed in the $\nu_{2}$ and $\nu_{5}$ bands around the local crossing. The $A_{0}$ values obtained for both isotopomers are more accurate but fully compatible with those obtained previously.
Description
$^{a}$World Meteorological Organization (WMO), ``Scientific Assessment of Ozone Depletion: 1998'', Report 44, WMO Global Ozone Research And Monitoring Project, Chapter 2, Geneva, Switzerland, 1999. $^{b}$R. Anttila, C. Betrencourt-Stirnemann, and J. Dupre, J. Mol. Spectrosc., 100, 54, (1983).
Author Institution: CNRS, Laboratoire de Photophysique Mol\'{e}culaire; Laboratoire de Dynamique, Interactions et R\'{e}activit\'{e}/Spectrochimie Mol\'{e}culaire, Universit\'{e} Pierre et Marie Curie; Facult\'{e} des Sciences, Universit\'{e} de Ngaound\'{e}r\'{e}
Author Institution: CNRS, Laboratoire de Photophysique Mol\'{e}culaire; Laboratoire de Dynamique, Interactions et R\'{e}activit\'{e}/Spectrochimie Mol\'{e}culaire, Universit\'{e} Pierre et Marie Curie; Facult\'{e} des Sciences, Universit\'{e} de Ngaound\'{e}r\'{e}