L. Augustovičová

Radiative association of He+2 revisited

Aims. Rate coefficients for spontaneous and stimulated radiative association of the He + molecular ion are presented as a function of temperature, considering the association with rotational-vibrational states of the ground X electronic state from the continuum states of the lowest three excited A,C ,a ndE electronic states. Methods. The rate coefficients are obtained from the cross sections for radiative association, which are calculated by solving the Schrodinger equation for bound and continuum states supported by the corresponding Born-Oppenheimer potential energy curves. Results. The rate coefficients for the A → X spontaneous process are in excellent agreement with previous calculations. For temperatures up to about 10000 K, the spontaneous C → X and E → X processes are dominant, exceeding the A → X rate coefficient by several orders of magnitude. Stimulation of the radiative association by black-body radiation has significant influence only on the A → X process at temperatures below 1000 K. The black-body radiation temperature would have to be well above 50000 K in order to have any noticeable effect on the C → X and E → X processes.

DEPOPULATION OF METASTABLE HELIUM BY RADIATIVE ASSOCIATION WITH HYDROGEN AND LITHIUM IONS

Depopulation of metastable He(23S) by radiative association with hydrogen and lithium ions is investigated using a fully quantal approach. Rate coefficients for spontaneous and stimulated radiative association of the HeH+, HeD+, and LiHe+ molecular ions on the spin-triplet manifold are presented as functions of temperature considering the association to rotational-vibrational states of the lowest triplet electronic states a 3Σ+ and b 3Σ+ from the continuum states of the b 3Σ+ electronic state. Evaluation of the rate coefficients is based on highly accurate quantum calculations, taking into account all possible state-to-state transitions at thermal energies (for spontaneous association) or at higher background energies (stimulated association). As expected, calculations show that the rate coefficients for radiative association to the a state are several orders of magnitude larger than the one for the b state formation. A noticeable effect by blackbody background radiation on the radiative association is only obtained for the b → b process. Aspects of the formation and abundance of the metastable HeH+(a 3Σ+) in astrophysical environments are briefly discussed.

Non-radiative inelastic processes in lithium-helium ion-atom collisions

Aims. The aims are to estimate efficiencies of non-radiative inelastic processes in lithium-helium ion-atom collisions and to compare them to those for radiative processes. Methods. Non-radiative inelastic cross-sections and rate coefficients for different lithium-helium ion-atom collisions are estimated by means of the recently proposed branching probability current method, which is based on the accurate ab initio adiabatic BornOppenheimer potentials that have been recently calculated for the low-lying 1,3 Σ + and 1,3 Π states of the LiHe + ion. Results. It is shown that at low temperatures the radiative depopulation in Li + + He and Li + He + collisions dominates over the non-radiative processes, while in Li + + He(2s 3 S) collisions the non-radiative processes dominate over the radiative association at temperatures above 3000 K, which can be expected to have some influence on depopulations of metastable He in high temperature astrophysical environments.

Formation of molecular ion LiHe+ by radiative association of metastable helium He(23P) with lithium ions

Radiative association of metastable helium He(23P) with lithium ions is theoretically investigated on the triplet-sigma manifold. Cross-sections and the corresponding rate coefficients for spontaneous and stimulated processes are calculated. Results are compared to other radiative and non-radiative processes in collisions of metastable helium with lithium ions.