Mahmut Ruzi, Rebecca Auchettl, Courtney Ennis, Dominique R.T. Appadoo, Evan G. Robertson
https://pubs.acs.org/doi/10.1021/acsearthspacechem.8b00085
Fourier transform infrared spectra of formic acid aerosol particles in situ generated in a collisional cooling cell at temperatures ranging between 90 K and 210 K are recorded in the mid infrared and THz/far infrared regions. Infrared spectroscopic features are used to identify the formic acid dimer above 200 K, the crystalline β1 phase in the 110 – 200 K temperature range, and amorphous solid formic acid at ~ 90 K. Density functional calculations of discrete clusters and the full periodic β1 crystalline structure help to assign the low wavenumber intermolecular modes by comparison with experimental far IR and Raman data from the literature. Refractive indices of the formic acid crystal in the mid IR range are retrieved. These data are essential for radiative transfer modelling of solid formic acid condensed in the terrestrial atmosphere. THz/far IR spectra, in addition to the well documented mid infrared spectra, may be useful for the identification of solid formic acid in astrophysical environments such as the young stellar object W33A.
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