The 2ν6/ν2 + ν3/ν3 + ν5 band system of CH3Br revisited: Modeling anharmonic and Coriolis interactions in a three-level system near 2000 cm−1

Abstract The 2 ν 6 (A 1  + E)/ ν 2  +  ν 3 (A 1 )/ ν 3  +  ν 5 (E) band system of CH 3 Br, near 2000 cm −1 , has been studied, for both 79 Br and 81 Br isotopologues, using Fourier transform infrared spectroscopy, with a resolution of 0.003 cm −1 . This band system, revealing anharmonic (Δ k  = Δ l  = 0) and Coriolis (Δ k  = Δ l  =  ± 1) interactions, has been analyzed through a least-squares fit of more than 3000 transitions, for each isotopologue. More than 600 transitions belonging to the very weak ν 3  +  ν 5 combination band were assigned for the first time, for both CH 3 79 Br and CH 3 81 Br isotopologues. Assignments of the weak 2 ν 6 0 parallel band, which is Fermi-interacting with ν 2  +  ν 3 , were also considerably extended with respect to a previous high-resolution study (Najib et al., 1985), thanks to a more accurate knowledge of the Fermi coupling parameters and of the relative positions of the interacting levels. The least-squares fits provided quantitative reproduction of all data belonging to the four above mentioned bands. Moreover, the Coriolis coupling parameters obtained for the ν 2  +  ν 3 / ν 3  +  ν 5 interacting bands show a remarkable consistency with those obtained for the ν 2 / ν 5 ‘fundamental’ system (Kwabia Tchana et al., 2004).