Absolute line intensities in methyl bromide: The 7-μm region

This work deals, for the first time, with the modeling of absolute line intensities in the fundamental ν2 and ν5 bands of CH379Br and CH381Br at 7 μm. For that, four unapodized absorption spectra of CH3Br (natural abundance, 99% purity, P × L = 0.082 − 0.165 atm × cm, room temperature) were measured in the range 1260-1560 cm−1, at a resolution of 0.002 cm−1 using a Fourier transform spectrometer Bruker IFS 120 HR. For both isotopomers, 313 line intensities were analyzed within the dyad system required to account properly for the strong Coriolis coupling between ν2 and ν5. The intensity fit of experimental data led to the determination of the dipole moment derivatives d2 = ∂μ/∂q2 and d5 = ∂μ/∂q5 relative to the ν2 and ν5 bands, as well as the first-order Herman-Wallis correction in K to d5. The observed line intensities are fitted to 3.0% (3.3%) for ν2 at 1309.9 cm−1 and 2.6% (3.0%) for ν5 at 1442.9 cm−1, respectively for CH379Br and CH381Br. The values derived for the vibrational band strengths of ν2 and ν5 are 55.7(0.6) and 39.2(0.3) cm−2 atm−1 at 296 K, respectively. The corresponding assignments and line positions of the dyad from previous work [F. Kwabia Tchana, I. Kleiner, J. Orphal, N. Lacome, O. Bouba, J. Mol. Spectrosc. 228 (2004) 441] are combined with the present intensity study to provide an improved CH3Br database for atmospheric applications.