Differential electron scattering cross sections for the first optically forbidden and resonance transitions in Mg II, Zn II and Cd II

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Abstract
Differential electron scattering cross sections have been measured for dipole-forbidden and resonance transitions in Mg II, Zn II and Cd II in the angular range 4 degrees 2S to 3d94s2 2D transition in Zn II is small, those for the 3s 2S to 3d 2D, 4s 2S (unresolved lines) in Mg II, and the 5s 2S to 4d95s2 2D in Cd II are comparable in magnitude with the cross sections for resonance excitation. In addition, for Cd II one finds that the allowed and forbidden transitions have very similar angular distributions, and it is proposed that excitation to the 2D state may be dominated by a virtual 'double-dipole' transition via the 2P state. Also, the total excitation cross section of the resonance 2P state in Cd II is a factor of four higher than that predicted by the Gaunt factor approximation, suggesting that the accepted value for the oscillator strength may be too low.