Resonances in cross sections for excitation of forbidden lines in O2+

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Abstract
In previous work, Saraph, Seaton and Shemming have calculated cross sections for electron impact excitation of the O2+ ground configuration terms, in the approximation of neglecting collisional coupling to configurations other than 2s22p2. In the present work it is shown that collisional coupling with 2s2p3 produces resonances of the type 2s2p33s in the near-threshold cross sections. Calculations are made using semi-empirical methods. Radial functions are calculated using a statistical-model potential, and the binding energy of the 3s electron in the state 2s2p3(5S)3s 4S is calculated allowing for configuration interaction with 2s2p3(3S)3s 4S and 2s22p3 4S. A parameter in the potential is then varied so as to obtain agreement with the observed binding energy for this state. Similar calculations are then made for 2s2p3(3D)3s 2D allowing for configuration interaction with 2s2p3(1D)3s 2D and 2s22p3 2D, and for 2s2p3(3P)3s 2P allowing for interaction with 2s2p3(1P)3s2P and 2s22p3 2P. It is found that the 3s 2D state lies just above the threshold for excitation of 2s22p2 1D, and that the 3s 2P state lies just above the threshold for excitation of 2s22p2 1S. A generalized reactance matrix is calculated using a variational expression, and the 2s2p33s functions of the present work and the 2s22p2kp functions obtained by Saraph, Seaton and Shemming. Resonance structures are calculated using methods of quantum defect theory. It is found that the resonances produce important modifications in the cross sections at near-threshold energies of interest for astrophysical applications.

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