Analysis of the fine structure of Sn$^{11+...14+}$ ions by optical spectroscopy in an electron beam ion trap

TL;DR

This study re-evaluates the fine structure of Sn ions using optical spectroscopy and advanced calculations, confirming theoretical predictions and suggesting revisions to previous EUV line identifications.

Contribution

It combines optical spectroscopy with ab initio and semi-empirical calculations to accurately identify and analyze the fine structure of Sn$^{11+...14+}$ ions.

Findings

Confirmation of ab initio calculations' predictive power

Validation of line identifications with semi-empirical methods

Reassessment of EUV line identifications in previous studies

Abstract

We experimentally re-evaluate the fine structure of Sn$^{11+...14+}$ ions. These ions are essential in bright extreme-ultraviolet (EUV) plasma-light sources for next-generation nanolithography, but their complex electronic structure is an open challenge for both theory and experiment. We combine optical spectroscopy of magnetic dipole $M1$ transitions, in a wavelength range covering 260\,nm to 780\,nm, with charge-state selective ionization in an electron beam ion trap. Our measurements confirm the predictive power of \emph{ab initio} calculations based on Fock space coupled cluster theory. We validate our line identification using semi-empirical Cowan calculations with adjustable wavefunction parameters. Available Ritz combinations further strengthen our analysis. Comparison with previous work suggests that line identifications in the EUV need to be revisited.