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

Relaxation of the spin-orbit excited C+(P-2(3/2)) ion by collisions with H-2 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 H-2 to the ion. To capture the full angular dependence of the C+-H-2 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+(P-2(3/2)) level in collisions with H-2 in its j = 0, 1, and 2 rotational levels. With the assumption that the para-H-2 rotational levels are in Local Thermodynamic Equilibrium (LTE), rate coefficients were then calculated for de-excitation by para- and ortho-H-2 for temperature ranging from 5 to 500 K. The rate coefficient for de-excitation by para-H-2 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-H-2 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.