Experimental and theoretical results are reported for single-photon single ionization of the tungsten ion 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 W$^{ 4+}$. Calculations were carried out for the $4f^{ 14}5s^2 5p^6 5d^2 \; { ^3}{ \rm F}_{ J}$, $J$=2, ground level and the associated fine-structure levels with $J$=3 and 4 for the W$^{ 4+}$ ions. In addition, cross sections have been calculated for the metastable levels $4f^{ 14}5s^2 5p^6 5d^2 \; { ^3}{ \rm P}_{ 0,1,2},{ ^1}{ \rm D}_{ 2},{ ^1}{ \rm G}_{ 4},{ ^1}{ \rm S}_{ 0}$. Very satisfying agreement of theory and experiment is found for the photoionization cross section of W$^{ 4+}$ which is remarkable given the complexity of the electronic structure of tungsten ions in low charge states.
Categories: physics.atom-ph