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

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Abstract
First-order many-body theory predicts that the electron-impact-induced orbital angular momentum transfer to the target, (Lperpendicular to ), associated with excitation of the n1D 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 n1D states is opposite to that associated with excitation of the n1,3P and n3D states for the same range of scattering angles. The qualitatively different characteristic of (Lperpendicular to ) for excitation of n1D compared with the n3D states of helium shows that details of the excitation process itself play an important role in determining the angular dependence of (Lperpendicular to ). As is also the case for excitation of the n1,3P states, the electron impact coherence parameters for excitation of the n3,1D states are predicted to be essentially independent of the principal quantum number (for fixed incident electron energy).