Rotational excitation of protonated carbon dioxide (HOCO+) in collisions with molecular hydrogen

The protonated carbon dioxide (HOCO+) ion has been observed in the interstellar medium and can provide an indirect probe of carbon dioxide, which cannot be detected by millimeter spectroscopic observations. Accurate modeling of HOCO+ spectra requires accurate radiative and collisional rates. While radiative rates are available, collisional rate coefficients for rotational transitions involving collisions with the dominant H2 molecule must be calculated. In this work, the potential energy surface (PES) describing the interaction of the HOCO+ with H2 has been computed through the use of the explicitly correlated coupled cluster method including single, double, and (perturbatively) triple excitations [CCSD(T)-f12a] and a correlation-consistent aug-cc-pVTZ basis. The molecular geometries are taken as fixed. The computed points were fit to a functional form appropriate for time-independent quantum scattering calculations of rotationally inelastic integral cross sections. The well depth De of the HOCO+-H2 complex, computed with an aug-cc-pVQZ-f12 basis set, was found to equal 2423 cm-1 at an equilibrium intermolecular separation of 6.21a0. This PES was used in time-independent close coupling quantum scattering calculations to compute state-to-state cross sections and rate coefficients for transitions between the rotational levels of HOCO+.