Lawrence Berkeley National Laboratory

Experimental and theoretical results are reported for single-photon single ionization of the tungsten ion W4+. 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 W4+. Calculations were carried out for the 4f14 5s2 5p6 5d23FJ, J = 2, ground level and the associated fine-structure levels with J = 3 and 4 for the W4+ ions. In addition, cross sections have been calculated for the metastable levels 4f14 5s2 5p6 5d23P0,1,2, 1D2, 1G4, 1S0. A very satisfying agreement of theory and experiment is found for the photoionization cross section of W4+ which is remarkable given the complexity of the electronic structure of tungsten ions in low charge states.