One-electron and two-electron-one-photon electric dipole transitions in the Mg-like argon spectrum

In charge exchange collisions of Ar 7+ with He at 1.75 keV amu -1 , electron capture selectively populates the n=4 shell. Ar 6+ (n=4) then decays radiatively. A full analysis of the spectrum observed in the VUV range 100-1000 AA allows identification of a series of unknown transitions including two-electron-one-photon transition electric dipole lines, with upper levels 3s4p and 3s4d. Their branching ratios are compared with calculated values. It is thus possible to deduce that they are mostly due to cascade paths from the initial level of direct capture rather than to a combination of excitation and capture.