Collisional excitation of C+(2P) spin-orbit levels by molecular hydrogen revisited

ABSTRACT Relaxation of the spin-orbit excited C+(2P3/2) ion by collisions with H2 is an important process in the interstellar medium. Previous calculations of rate coefficients for this process employed potential energies computed for only collinear and perpendicular approach of H2 to the ion. To capture the full angular dependence of the C+–H2 interaction, the angular variation of the potential has been obtained by quantum chemical calculations in this work. These data were used to compute rate coefficients for the de-excitation of the C+(2P3/2) level in collisions with H2 in its j = 0, 1, and 2 rotational levels. With the assumption that the para-H2 rotational levels are in Local Thermodynamic Equilibrium (LTE), rate coefficients were then calculated for de-excitation by para- and ortho-H2 for temperature ranging from 5 to 500 K. The rate coefficient for de-excitation by para-H2 is ca. 10 per cent higher at temperatures near 100 K but 10 per cent lower at temperatures greater than 300 K than the previous best calculation. By contrast, the de-excitation rate coefficient for ortho-H2 is 15 per cent higher at low temperatures but approximately equal as compared with the previous best calculation. The impact of these new rate coefficients is briefly tested in radiative transfer calculations.