Identification of the 1s2s2p 4P5/2→1s22s 2S1/2 magnetic quadrupole inner-shell satellite line in the Ar16+K-shell x-ray spectrum -: art. no. 032504

We have identified the dipole-forbidden 1s2s2p P-4(5/2)-->1s(2)2s S-2(1/2) transition in lithiumlike Ar15+ in high-resolution K-shell x-ray emission spectra recorded at the Livermore EBIT-II electron-beam ion trap and the Princeton National Spherical Tokamak Experiment. Unlike other Ar15+ satellite lines, which can be excited by dielectronic recombination, the line is exclusively excited by electron-impact excitation. Its predicted radiative rate is comparable to that of the well-known 1s2p P-3(1)-->1s(2) S-1(0) magnetic quadrupole transition in helium-like Ar16+. As a result, it can also only be observed in low-density plasma. We present calculations of the electron-impact excitation cross sections of the innershell excited Ar15+ satellite lines, including the magnetic sublevels needed for calculating the linear line polarization. We compare these calculations to the relative magnitudes of the observed 1s2s2p-->1s(2)2s transitions and find good agreement, confirming the identification of the lithiumlike 1s2s2p P-4(5/2)-->1s(2)2s S-2(1/2) magnetic quadrupole line.