Collisional alignment and orientation of atomic outer shells. II. Quasi-molecular excitation, and beyond

We present a comprehensive and critical review on atomic alignment and orientation after collisional excitation including a full reevaluation and discussion of data published to date in the literature. The present Vol. II focuses on the quasi-molecular low-energy regime in ion-atom and atom-atom processes. The discussion centers on experimental and theoretical data for planar scattering geometry in which the alignment and orientation parameters are determined as a function of scattering angle (particle-photon coincidence or scattering from laser-excited atoms, the so-called third-generation of experiments). In addition, alignment studies with cylindrical symmetry (second-generation experiments) and integral cross sections (first-generation experiments) are also discussed wherever this clarifies the understanding of the relevant processes. A unified set of parameters is used with reference to the so-called natural coordinate frame (having its z-axis perpendicular to the collision plane): the angular momentum transfer L⊥, the alignment angle γ, the linear polarization P⁺ and the symmetry-changing probability ρ₀₀. This parametrization allows one an intuitive interpretation of these otherwise somewhat abstract quantities. After reviewing the theoretical framework the individual model systems are described, starting with excitation and charge transfer in the genuine one-electron system H⁺+H and ending with He⁺+Rg (Rg = rare-gas atom) collisions involving many electrons and demonstrating the explicit influence of spin-orbit coupling. In several appendices a compilation of the relevant classical deflection functions and many useful formulae for the interpretation of alignment and orientation parameters are reported.