L-shell photoionisation cross sections in the S+, S2+, S3+ isonuclear sequence

Abstract

We present absolute L-shell photoionisation cross sections for the S+, S2+, S3+ ions. The cross sections were obtained using the monochromatised photon beam delivered by the SOLEIL synchrotron source coupled with an ion beam extracted from an electron cyclotron resonance source (ECRIS) in the merged dual-beam configuration. The cross sections for single, double and triple ionisation were measured and combined to generate total photoionisation cross sections. For each of the S+, S2+ and S3+ ions, the photon energy regions corresponding to the excitation and ionisation of a 2p or a 2s electron (similar to 175-230 eV) were investigated. The experimental results are interpreted with the help of multiconfigurational Dirac-Fock (MCDF) and Breit-Pauli R-Matrix (BPRM) or Dirac R-Matrix (DARC) theoretical calculations. The former generates photoabsorption cross sections from eigenenergies and eigenfunctions obtained by solving variationally the multiconfiguration Dirac Hamiltonian while the latter calculate cross sections for photon scattering by atoms. The cross sectional spectra feature rich resonance structures with narrow natural widths (typically <= 100 meV) due to 2p -> nd excitations below and up to the 2p thresholds. This behaviour is consistent with the large number of inner-shell states based on correlation and spin-orbit mixed configurations having three open subshells. Strong and wide (typically similar to 1 eV) Rydberg series of resonances due to 2s -> np excitations dominate above the 2p threshold.