Optical frequency comb Fourier transform spectroscopy of the CHBrBr, CHBr, and CHBr isotopologues in the 1180-1210 cm region
arXiv:2604.00244 · doi:10.1039/d6cp01201a
Abstract
Quantitative spectroscopic detection of dibromomethane, CHBr, 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 CHBr in the 1180-1210 cm region measured using optical frequency comb Fourier transform spectroscopy. This region is dominated by the strong CH wagging () fundamental vibration, which is about 50 times stronger than the fundamental C-H stretch around 3077 cm. The measurements resolve isotopologue-specific rovibrational features of CHBrBr, CHBr, and CHBr, and we assign rovibrational transitions of the fundamental and the overlapping +- 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 = 25 and J = 144, with an average RMS residual of 0.00037 cm (11.1 MHz). Compared with previously reported band parameters retrieved from a fit to narrowband (1.78 cm) 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 CHBr in the 8 m spectral region.