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

Abstract

This work deals, for the first time, with the modeling of absolute line intensities in the fundamental ν 2 and ν 5 bands of CH 3 79Br and CH 3 81Br at 7 μm. For that, four unapodized absorption spectra of CH 3Br (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 ν 2 and ν 5. The intensity fit of experimental data led to the determination of the dipole moment derivatives d 2 = ∂ μ/∂ q 2 and d 5 = ∂ μ/∂ q 5 relative to the ν 2 and ν 5 bands, as well as the first-order Herman–Wallis correction in K to d 5. The observed line intensities are fitted to 3.0% (3.3%) for ν 2 at 1309.9 cm −1 and 2.6% (3.0%) for ν 5 at 1442.9 cm −1, respectively for CH 3 79Br and CH 3 81Br. The values derived for the vibrational band strengths of ν 2 and ν 5 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 CH 3Br database for atmospheric applications.