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, , 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 500, in agreement with recent measurements. This negative behaviour for the n1D states is opposite to that associated with excitation of the nsup(1,3)P and n3D states for the same range of scattering angles. The qualitatively different characteristic of 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 . As is also the case for excitation of the nsup(1,3)P states, the electron impact coherence parameters for excitation of the nsup(3,1)D states are predicted to be essentially independent of the principal quantum number. (author)