The role of symmetry of the final target state in the angular momentum transfer by electron impact excitation of atoms

First-order many-body theory predicts that the electron-impact-induced orbital angular momentum transfer to the target, (L perpendicular to ), associated with excitation of the n 1 D states of helium in the energy range 45-60 eV, is negative for scattering angles less than or equal to 50', in agreement with recent measurements. This negative behaviour for the n 1 D states is opposite to that associated with excitation of the n 1,3 P and n 3 D states for the same range of scattering angles. The qualitatively different characteristic of (L perpendicular to ) for excitation of n 1 D compared with the n 3 D states of helium shows that details of the excitation process itself play an important role in determining the angular dependence of (L perpendicular to ). As is also the case for excitation of the n 1,3 P states, the electron impact coherence parameters for excitation of the n 3,1 D states are predicted to be essentially independent of the principal quantum number (for fixed incident electron energy).