Richard J. Mawhorter

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

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 <or= theta <or=17 degrees at 50 eV. These provide the first recorded angular distributions for an optically forbidden transition. It is found that while the cross section for excitation of the 4s 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.

Excitation of positive ions by low-energy electrons : relevance to the Io torus

The importance of measuring electron-ion excitation cross sections in singly and multiply charged positive ions is outlined, and recent results for Mg II and O II ions are given using the Jet Propulsion Laboratorys electron energy-loss merged-beams apparatus. For Mg II, collision strengths are presented for the 3s 2S → 3p 2P(h, k) resonance transition at electron energies from threshold to approximately 9 × threshold (40 eV). Theoretical comparisons are also given with two five-state close-coupling calculations. The energy variation of the collision strength is fitted with a semiempirical analytic function which includes approximations to polarization, resonance, and exchange contributions. In O II, first spectra anywhere of electron excitation of the optically allowed 2s22p3 4S0 → 2s2p4 4P (14.88 eV) and → 2p23s 4P (22.98 eV) transitions are presented. In addition, excitations of the two low lying, optically forbidden transitions 4S° → 2p3 2D0 (3.33 eV) and → 2p3 2P0 (5.02 eV) are detected for the first time.