Isomer-specific excitation of formic acid in collisions with helium atoms

An accurate estimation of the molecular abundances of isomers in the interstellar medium (ISM) is necessary to unravel the underlying chemistry and physics. After the recent detection of both isomers of formic acid (cis- and trans-HCOOH) in dense dark cold clouds, their accurate molecular line modeling became of interest. The conditions of these environments do not necessarily follow the local thermodynamic equilibrium, thus taking into account the competition between the radiative and collisional processes is required. This involves knowledge of the rotational excitation data for collisions with the most abundant interstellar species - He and H₂. In this paper, the first potential energy surface (PES) for the interaction of formic acid with He atoms is computed using the explicitly correlated coupled-cluster theory [CCSD(T)-F12]. The obtained interaction potentials for the two rotamers demonstrate qualitative similarities and high anisotropy. The global minima are found with V = -53.0 cm^-1 and V = -46.0 cm^-1 for cis-HCOOH and trans-HCOOH, respectively. Collisional excitation cross sections calculated for total energies up to 100 cm^-1 demonstrate similar propensity rules for both isomers. Quantitative differences between the cross sections associated with the two rotamers are also discussed.