Abstract
Extreme ultraviolet (EUV) optical transitions of lanthanide highly charged ions have been studied. The $[\mathrm{Ni}]4{s}^{2}4p--[\mathrm{Ni}]4{s}^{2}4d$ and $[\mathrm{Ni}]4{s}^{2}4p--[\mathrm{Ni}]4s4{p}^{2}$ 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 $4{p}_{1/2}$ and $4{p}_{3/2}$ 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 $[\mathrm{Ni}]4{s}^{2}4d$ and $[\mathrm{Ni}]4s4{p}^{2}$ configurations leads to an avoided crossing in the apparent wavelength curves. The configuration mixing makes both the $[\mathrm{Ni}]4{s}^{2}4p--[\mathrm{Ni}]4{s}^{2}4d$ and $[\mathrm{Ni}]4{s}^{2}4p--[\mathrm{Ni}]4s4{p}^{2}$ lines well visible in the EUV spectra near the level crossing point. The levels are found to cross between $Z=62$ and 63.
Highlights
The spectral data and the knowledge of dynamics in extreme ultraviolet (EUV) optical emissions from highly charged heavy ions are of interest in a variety of fields such as the development of short-wavelength light sources for semiconductor lithography [1], the development of microscopes using shorter-wavelength light sources [2], or the diagnostics of nuclear fusion plasma [3,4] in magnetic confinement, as well as inertial confinement reactors
Among the highly charged atomic ions of heavier elements, lanthanide atomic ions are considered as potential EUV light source elements for a shorter-wavelength range
The concentration of the impurity ions is less than 0.1%, assuming that the impurities are uniformly expanded in the plasma
Summary
The spectral data and the knowledge of dynamics in extreme ultraviolet (EUV) optical emissions from highly charged heavy ions are of interest in a variety of fields such as the development of short-wavelength light sources for semiconductor lithography [1], the development of microscopes using shorter-wavelength light sources [2], or the diagnostics of nuclear fusion plasma [3,4] in magnetic confinement, as well as inertial confinement reactors. For higher charged ions such as 4s or 4p open valence subshell ionic states of lanthanide elements, isolated lines are mainly observed [3,4] In those ions, the relativistic effects in atomic orbitals come to play an important role. We consider several EUV emission lines which are realized by transitions of lower excited states of Ga-like lanthanide atomic ions to their ground states. The configuration mixing of the excited-state configurations makes the optically forbidden [[Ni]4s24p−] 1 –[[Ni]4s4p2+] 3 transition visible due to the contribution of the optically allowed [[Ni]4s24p−] 1 –[[Ni4s24d−] 3 transition, because the actual excited atomic state is a linear combination of [[Ni]4s4p2+] 3 and [[Ni]4s24d−] 3 configurations This feature of the spectral lines produces complexities and difficulties in the analysis of the experimental spectra.
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