X-ray transitions in highly charged neonlike ions

Wavelength measurements of n=3 to n=2 transitions in neonlike ${\mathrm{Xe}}^{44+}$, ${\mathrm{La}}^{47+}$, ${\mathrm{Nd}}^{50+}$, and ${\mathrm{Eu}}^{53+}$ have been made using a high-resolution Bragg-crystal spectrometer on the Princeton Large Torus tokamak. The measurements cover the wavelength regions 2.00--3.00 A\r{} and include the electric dipole and the electric and magnetic quadrupole transitions. The measured wavelengths are compared to energy levels obtained from a multiconfigurational Dirac-Fock calculation. Systematic differences between the experimental and theoretical values are found, which vary smoothly with atomic number. The magnitude of the differences depends on the particular type of transition and ranges from -2.8 eV to +2.2 eV. Inclusion of electron correlation corrections due to ground-state correlations and (super-) Coster-Kronig-type fluctuations in the theoretical energies is shown to reduce the differences for some but not all types of transitions.