Formation of HeH+ by radiative association of He+ + H. An advanced ab initio study

Rate coefficients of all possible radiative association reactions for He+ + H are calculated over a wide temperature range. For this purpose the potential energy and electric dipole moment functions of the X̃1σ+ ground electronic and the first excited ã3σ+ and Ã1σ+ states of HeH+ are determined at the configuration interaction level of theory and the electronic transition dipole moment function between the two 1Σ states is evaluated using the state interaction method of Malmqvist and Roos. From the potential energy functions, all rotation-vibrational bound and rotationally quasi-bound states are obtained for each electronic state. Whereas formation of the ã3Σ+ and Ã1Σ+ excited states in He+ + H collisions have small rates, the most efficient process leads to the formation of the HeH+ ground electronic state. The temperature dependence of the corresponding rate coefficient shows a typical behavior with a maximum of 6 × 10−15 cm3 s−1 around 10 K and a monotonic decrease with increasing temperature. Accurate potential functions derived from previous directly correlated wavefunctions are used to assess the reliability of the calculations.