ASYMPTOTIC DEPENDENCE OF THE ELECTRON-CAPTURE CROSS-SECTION ON THE N-QUANTUM NUMBER IN SLOW HE-2+-H COLLISIONS

In slow He-2+-H collisions the Rydberg levels of the helium atom are populated by a direct excitation mechanism, the so-called S-promotion of electrons. The saddle point mechanism, which can populate the Rydberg levels of both atoms, only becomes important at very low collision energies, typically much lower than 5 keV amu-1. An analytic expression for the electron capture cross section of the reaction He-2+ + H --> He+ (n) + H+ is derived, which is valid when the saddle point mechanism is negligible, i.e. when the principal quantum number n of the isolated helium atom is large enough. As an estimate one may use n > 5.75/v0, where v0 is the internuclear velocity. The asymptotic behaviour of the electron capture cross section is shown to be sigma(n) is-proportional-to n-3. The appendix contains the correct formulae (17) and (18) of our first article for the positions of the T-type hidden crossings.