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

## TL;DR

This study combines experimental measurements and theoretical calculations to accurately determine the photoionization cross sections of W$^{4+}$ ions, revealing strong agreement despite the complex electronic structure of tungsten.

## Contribution

It provides the first detailed experimental and theoretical analysis of W$^{4+}$ photoionization, employing advanced photon-ion merged-beams techniques and Dirac-Coulomb R-matrix calculations.

## Key findings

- Excellent agreement between experiment and theory for W$^{4+}$ cross sections.
- Comprehensive data for multiple fine-structure and metastable levels.
- Demonstrates the effectiveness of combined experimental-theoretical approaches for complex ions.

## Abstract

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.