Optical frequency comb Fourier transform spectroscopy of the CH279Br81Br, CH279Br2, and CH281Br2 isotopologues in the 1180-1210 cm-1 region

Abstract

Quantitative spectroscopic detection of dibromomethane, CH₂Br₂, for environmental monitoring, workplace safety, and exoplanetary studies is limited by the lack of accurate absorption cross-section data and rigorous spectroscopic models. We report the first high-resolution (6.3 MHz point spacing) absorption cross-section of CH₂Br₂ in the 1180 -1210 cm^-1 region, measured using optical frequency comb Fourier transform spectroscopy. This spectral region is dominated by the strong CH₂ wagging (ν₈) fundamental vibration, which is about 50 times stronger than the fundamental C-H stretch around 3077 cm^-1 . The measurements resolve isotopologue-specific rovibrational features of CH₂⁷⁹Br⁸¹Br, CH₂⁷⁹Br₂, and CH₂⁸¹Br₂, and we assign rovibrational transitions of the ν₈ fundamental and the overlapping ν₄+ν₈-ν₄ hot bands using two methods. First, an empirical non-linear least square fit implemented in PGOPHER provides high-precision line assignment and spectroscopic constants, including accurate band origins, rotational constants, and quartic centrifugal distortion parameters, for the three isotopologues, covering rotational levels up to K_a = 25 and J = 144, with an average RMS residual of 0.00037 cm^-1 (11.1 MHz). Compared with previously reported band parameters retrieved from a fit to narrowband (1.78 cm^-1) supersonically cooled spectra (B. E. Brumfield et al., J. Mol. Spectrosc., 2011, 266, 57-62), our fit provides much improved global agreement between measured and simulated spectra. In parallel, an ab initio-based effective Hamiltonian approach was used to model the complete rovibrational polyads, including weak hot-band transitions and polyad interactions inaccessible to purely empirical fits, and provided the first ab initio-based line intensities of CH₂Br₂ in the 8 μm spectral region.