Photoionization of tungsten ions: experiment and theory for W$^{2+}$ and W$^{3+}$

Abstract: Experimental and theoretical results are reported for single-photon single ionization of W$^{2+}$ and W$^{3+}$ tungsten ions. Experiments were performed at the photon-ion merged-beam setup of the Advanced Light Source in Berkeley. Absolute cross sections and detailed energy scans were measured over an energy range from about 20~eV to 90~eV at a bandwidth of 100~meV. Broad peak features with widths typically around 5~eV have been observed with almost no narrow resonances present in the investigated energy range. Theoretical results were obtained from a Dirac-Coulomb $R$-matrix approach. The calculations were carried out for the lowest-energy terms of the investigated tungsten ions with levels ${\rm 5s^2 5p^6 5d^4 \; {^5}D}_{J}$ $J=0,1,2,3,4$ for W$^{2+}$ and ${\rm 5s^2 5p^6 5d^3 \; {^4}F}_{J^{\prime}}$ $J^{\prime}=3/2, 5/2, 7/2, 9/2$ for W$^{3+}$. Assuming a statistically weighted distribution of ions in the initial ground-term levels there is good agreement of theory and experiment for W$^{3+}$ ions. However, for W$^{2+}$ ions at higher energies there is a factor of approximately two difference between experimental and theoretical cross sections.