Empirical line intensities of methanol in the 300-500 cm−1 region

Given the expectations associated with modern observational facilities such as Herschel, ALMA and SOFIA, and the ubiquity of methanol in interstellar gas, precise knowledge of the methanol spectrum is critical. Abundant research has characterized the line positions and quantum assignments in the far infrared, but there is a dearth of line intensity information at most wavelengths. In this work, an empirical database of methanol line intensities from 300 to 500 cm−1 has been compiled from far-infrared measurements recorded on the Bruker IFS 125 HR Fourier transform spectrometer located at the Jet Propulsion Laboratory. The sum of the integrated line intensities has been computed (∑Int=1.13×10−18cm−1/(moleculecm−2)) at 296 K for 1317 measured features. These have been combined with known assignments for 2ν12-gs, ν12-gs, 2ν12-ν12 plus a few other bands with a small number of transitions in the spectral range of this study. Line intensities of 916 unblended features have been compared to available theoretical calculations of Mekhtiev et al., J. Mol. Spectrosc., 194, 171-178 (1999). These results support the analysis of astronomical observations taken from orbit by the Herschel HIFI instrument.

► The first experimental far-infrared absolute intensity measurements of methanol. ► A much more accurate partition function of 36,831.557 at 296 K is derived. ► Experimental intensities qualitatively agree with RAM Hamiltonian calculations. ► The inclusion of an ab initio three-fold dipole component is an improvement.