Relativistic $R$-matrix studies on direct and resonant photoionization of magnesium-like Si$^{2+}$ ions

Abstract The direct and resonant photoionization of magnesium-like Si$^{2+}$ ions from the ground state and three low-lying excited states [Ne]$3s3p~^3P_{0, \, 1, \,2}$ is studied using the relativistic R-matrix theory. Special attention is paid to the direct and resonance ionization limits and the resonance structure. To this aim, we compute the energy levels of Si$^{2+}$ and neighboring Si$^{3+}$ ions as well as the direct and resonant photoionization cross sections, using which the direct and resonance ionization limits are determined, being consistent with the NIST results. Moreover, for the ground-state photoionization 25 relatively isolated resonance peaks of different Rydberg series are resolved at the level of configuration or even fine structure by pairing the energy levels with the peak positions, which are determined to be associated with the resonance channels [Ne]$3lnl'$ ($l=p,d$). In contrast, for the excited-state scenarios just several resonance peaks are resolved due to the complexities of resonance structure, though the cross sections are calculated with a step size of $10^{-6}$ Ryd. This work fills the gap in the photoionization study of Si$^{2+}$ ions and, thus, is expected to contribute to the diagnosis and simulation of relevant astrophysical and laboratory silicon plasmas.