Electron-impact ionization of Ar7+

Cross sections for the ionization of Ar7+ by electron impact have been calculated using the R-matrix method in two independent 19- and 45-state close-coupling approximations. Autoionization states arising from the 2s22p53s3l configurations together with the ground and excited 2s22p63l states are included in the close-coupling expansion in the 19-state calculation, while additional autoionization states arising from the 2s22p53s4l (l=0,1,2) and 2s2p63s3l configurations and the low-lying bound states 2s22p64l are also included in the 45-state calculation. Several Rydberg series of resonances converging to various autoionizing states are formed due to the resonant-excitation-double-autoionization process and these are found to significantly enhance the cross sections in the autoionizing threshold region. The calculated results are compared with the recent crossed-beam measurements