QUANTUM-MECHANICAL STUDIES OF RADIATIVE ASSOCIATION REACTIONS: FORMATION OF HeH⁺, NeH⁺ AND ArH⁺

Radiative association processes play an important role in the formation of interstellar molecules by gas phase ion-molecule reactions. Due to experimental difficulties, only a few radiative association reactions have been studied in the laboratory. Therefore, most processes of interstellar importance must have their rates obtained by theoretical methods. Especially statistical approaches were applied in the past in the attempt to predict the association rates of larger molecular complexes. A full understanding, however, of the different possible reaction mechanisms and their dynamics can only be provided by detailed quantum-mechanical studies. Such rigorous ab initio calculations of the radiative association formation of noble gas hydride ions XH⁺ (X =He,Ne,Ar) are described here to discuss the requirements and limitations of this approach. Rate coefficients of all possible radiative association reactions resulting from X + H⁺ and from X⁺ + H collisions are calculated over a wide temperature range. For this purpose, the potential energy and electric dipole moment functions of the corresponding ground electronic and the first excited and states are determined at the configuration interaction level of theory and the electronic transition dipole moment functions between the two ¹Σ⁺ states are evaluated using the state interaction method of Malmqvist and Roos. From the potential energy functions, all rotation-vibration bound and rotationally quasi-bound levels are obtained for each electronic state. In the present diatomic systems the only efficient formation occurs in the two-state process in which radiative stabilization is achieved through transitions from the initial quasibound levels of the excited ùΣ⁺ state to the bound rotation-vibration levels of the ground electronic state. The temperature dependence of the corresponding rate coefficients show the typical behavior with a maximum at low temperatures and a rapid fall-off when temperatures rise above 100 K. In case of the HeH⁺ system, potential functions obtained from previous highly accurate directly correlated wavefunction calculations are used to assess the reliability of the present results.