Terahertz and Mid-infrared Spectra of Cold Formic Acid Aerosol Particles

Fourier transform infrared spectra
of formic acid aerosol particles in situ generated
in a collisional cooling cell at temperatures
ranging between 90 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 modeling
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.