Ab initio determination of the rate coefficient for radiative association of

The interaction potential and electric dipole moment function of the collisional system He(1S) + H+ are calculated at the configuration interaction level of theory using canonical SCF orbitals as orbital basis and employing large atomic basis sets of different flexibility. All the rotation-vibration bound states and rotationally quasi-bound resonance states as well as the corresponding free-bound dipole transition moments are evaluated and used to determine in a rigorous quantum mechanical treatment the rate coefficient for the radiative association He(1S) + H+ → HeH+ + hν and its temperature dependence. The results are compared to those obtained in the same way for a highly accurate potential function which is derived from previous directly correlated wavefunction calculations. It turns out that the rate coefficient at low temperatures contains predominantly contributions from a few low-energy resonances which are very sensitive to the shape of the potential and that reliable results thus require extremely accurate interaction potentials. At higher temperatures contributions from the large number of non-resonant continuum states become increasingly dominant and the results for the total rate coefficient are therefore less affected by small changes in the potential function.