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

In previous work, Saraph, Seaton and Shemming have calculated cross sections for electron impact excitation of the O 2+ ground configuration terms, in the approximation of neglecting collisional coupling to configurations other than 2s 2 2p 2 . In the present work it is shown that collisional coupling with 2s2p 3 produces resonances of the type 2s2p 3 3s 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 2s2p 3 ( 5 S)3s 4 S is calculated allowing for configuration interaction with 2s2p 3 ( 3 S)3s 4 S and 2s 2 2p 3 4 S. 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 2s2p 3 ( 3 D)3s 2 D allowing for configuration interaction with 2s2p 3 ( 1 D)3s 2 D and 2s 2 2p 3 2 D, and for 2s2p 3 ( 3 P)3s 2 P allowing for interaction with 2s2p 3 ( 1 P)3s 2 P and 2s 2 2p 3 2 P. It is found that the 3s 2 D state lies just above the threshold for excitation of 2s 2 2p 2 1 D, and that the 3s 2 P state lies just above the threshold for excitation of 2s 2 2p 2 1 S. A generalized reactance matrix is calculated using a variational expression, and the 2s2p 3 3s functions of the present work and the 2s 2 2p 2 k p 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.