Z-dependent crossing of excited-state energy levels in highly charged galliumlike lanthanide atomic ions

Extreme ultraviolet (EUV) optical transitions of lanthanide highly charged ions have been studied. The [Ni]4s(2)4p-[Ni]4s(2)4d and [Ni]4s(2)4p-[Ni]4s4p(2) transition lines of galliumlike lanthanide elements have been spectroscopically measured in large helical device (LHD) plasma and electron-beam ion trap (EBIT) plasma. The wavelengths for Eu ions agreed within 0.03% between the LHD and EBIT measurements. Due to the enhancement of spin-orbit interactions along with the increase of atomic number Z in 4p atomic orbitals, energy splitting between 4p(1/2) and 4p(3/2) orbitals increases significantly from lanthanum to lutetium, causing the crossing of Z-dependent wavelength curves. The spectral line positions and strengths are theoretically calculated by means of the multiconfiguration Dirac-Fock method. The mixing of the [Ni]4s(2)4d and [Ni]4s4p(2) configurations leads to an avoided crossing in the apparent wavelength curves. The configuration mixing makes both the [Ni]4s(2)4p-[Ni]4s(2)4d and [Ni]4s(2)4p-[Ni]4s4p(2) lines well visible in the EUV spectra near the level crossing point. The levels are found to cross between Z = 62 and 63.