Excitation of positive ions by low‐energy electrons: Relevance to the Io torus

Abstract

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 Laboratory's 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.