Excitation mechanisms in moderate-energy Li⁺–He collisions

Excitation mechanisms in Li⁺–He collisions were studied at laboratory collision energies of 350 ≤ E_lab ≤ 2000 eV by measuring double differential cross sections (DCSs) σ(Θ)_k over a wide range of center-of-mass scattering angles, 2.5 ≤ Θ ≤ 175°. At E_lab ≥ 500 eV, two-electron (2e) excitations were observed as well as one-electron (1e) excitations. At the higher collision energies, excitation probabilities P(Θ)_k for the 1e and 2e excitations have characteristic angular dependences, i.e., at E_lab = 1500 and 2000 eV, P(Θ)_1e for the 1e excitations has double maxima around Θ = 20 and 120° and P(Θ)_2e for the 2e excitations has a broad maximum around Θ = 60°. As a first analysis of the experimental data, P(Θ)_k, σ(Θ)_k, and the integral cross sections S_k(E_lab) were calculated by assuming excitations from the 1¹Σ state into the 1¹Π and 1¹Δ states through rotational couplings using the model potentials and couplings. As the next step, ab initio potential energies for the ground and excited states were calculated by a multiconfiguration self-consistent field (MCSCF) method, and then the electronic transitions among the seven states through the radial and rotational couplings were calculated using the theoretical potentials and couplings. Autoionizations from the 2e-excited He**(2s² and 2p²) atoms were also simulated at E_lab = 750–1500 eV and small laboratory angles of θ ≤ 25° by using the MCSCF potentials. The excitation mechanisms were reasonably well understood through these analyses. (author)