Spin anisotropy for excitation in collisions between two one-electron atoms

The singlet and triplet contributions of excitation cross sections are stud ied theoretically fur collisions between various two one-electron atoms. Th e spin anisotropy is shown to have a general behavior in the important impa ct energy range. At low energies triplet cross sections dominate completely over the singlet ones while the opposite is true when the active electron and projectile velocities are comparable. Beyond the matching velocity regi me singlet and triplet contributions become identical. We propose a general dynamical interpretation based on the analysis of the time dependency of t he electron probability density and probability current density.