Excitation and electron capture in collisions of Li+ with Li

Abstract

Coupled-state calculations have been performed for the collision processes Li++Li(2s)→Li++Li(nl),→Li(nl)+Li+, in which 1, 3, 4, or 6 of the states 2s, 2p0, 2p1, 3s, 3p0, 3p1 were included on each nuclear center as traveling-wave atomic orbitals. The rectilinear path impact parameter method was used, and cross sections for populating the various channels were determined over the energy range 0.4–400 keV. The atomic wavefunctions were obtained by solving the time-independent Schrödinger equation for a single valence electron in the field of the nucleus plus an exponential inner charge cloud. This pseudopotential was carried over into the dynamical calculation as the potential holding the single electron to the effective Li+ center. Therefore, our atomic wavefunctions are essentially exact solutions of the asymptotic Hamiltonian at large nuclear separations. The same numerical methods used previously for (H+, H) and (He++, H) collisions were used here. The inclusion of the 2p states had a strong effect on the calculated cross section for total electron capture, but higher states did not. When the 2p states are included, we find that the large oscillations in the curve of total capture cross sections vs energy are almost completely damped out. This is in disagreement with experiment, where oscillations are observed. However, the oscillations derive mostly from small impact parameters, where our method is least reliable. The over-all magnitude and energy dependence are in good agreement with experiment. Our calculations also predict a very large cross section [∼ 60 Å2 at 100 keV] for direct excitation of Li by Li+ at high collision energies. No data are available for comparison at present.