Photoionization of tungsten ions: experiment and theory for ${{\rm{W}}}^{4+}

Experimental and theoretical results are reported for single-photon single ionization of the tungsten ion ${{\rm{W}}}^{4+}$. Absolute cross sections have been measured employing the photon-ion merged-beams setup at the Advanced Light Source in Berkeley. Detailed photon-energy scans were performed at 200 meV bandwidth in the 40–105 eV range. Theoretical results have been obtained from a Dirac-Coulomb R-matrix approach employing basis sets of 730 levels for the photoionization of ${{\rm{W}}}^{4+}$. Calculations were carried out for the $4{f}^{14}5{s}^{2}5{p}^{6}5{d}^{2}{}^{3}{{\rm{F}}}_{J}$, J = 2, ground level and the associated fine-structure levels with J = 3 and 4 for the ${{\rm{W}}}^{4+}$ ions. In addition, cross sections have been calculated for the metastable levels $4{f}^{14}5{s}^{2}5{p}^{6}5{d}^{2}{}^{3}{{\rm{P}}}_{0,1,2},{}_{}{}^{1}{\rm{D}}_{2}^{},{}_{}{}^{1}{\rm{G}}_{4}^{},{}_{}{}^{1}{\rm{S}}_{0}^{}$. In conclusion, a very satisfying agreement of theory and experiment is found for the photoionization cross section of ${{\rm{W}}}^{4+}$ which is remarkable given the complexity of the electronic structure of tungsten ions in low charge states.