Atomic data for opacity calculations: XVIII. Photoionization and oscillator strengths of Si-like ions Si^0, S^2+, Ar^4+, Ca^6+

Photoionization cross sections, oscillator strengths and energy levels of the silicon-like ions, Si^0, S^2+, Ar4 and Ca^6+ are calculated in the close coupling approximation using the R-matrix method. A large number of bound states with n ≤ 10 have been considered and oscillator strengths for transitions among all these states and photoionization cross sections of all the bound states are obtained. Partial photoionization cross sections of the ground state into various excited states of the residual ion are also obtained for each ion. Detailed comparisons have been made for the calculated energies, oscillator strengths and photoionization cross sections with available theoretical and experimental values. Whereas we find good agreement for the oscillator strengths with recent experimental data for Si^0 and S^2+, there are very few previous data available for Ar^4+ and Ca^6+.The ground-state photoionization cross sections for Si^0 agree well with the previous close coupling calculations, however the present R-matrix results show considerable differences from the background cross sections for Si^0, S^2+ and Ar^4+ in the central field approximations.This work was partially supported by the National Science Foundation (AST-8996215 and PHY-9115057). SNN also acknowledges a research Fellowship by the College of Mathematical and Physical Sciences of the Ohio State University. The computations were carried out on the Cray Y-MP at the Ohio Supercomputer Center in Columbus, Ohio