Electron-impact excitation collision strengths and theoretical line intensities for transitions in S III

We present Maxwellian-averaged effective collision strengths for the electron-impact excitation of S III over a wide range of electron temperatures of astrophysical importance, log T_e (K) = 3.0-6.0. The calculation incorporates 53 fine-structure levels arising from the six configurations—3s ²3p ², 3s3p ³, 3s ²3p3d, 3s ²3p4s, 3s ²3p4p, and 3s ²3p4d—giving rise to 1378 individual lines and is undertaken using the recently developed RMATRX II plus FINE95 suite of codes. A detailed comparison is made with a previous R-matrix calculation and significant differences are found for some transitions. The atomic data are subsequently incorporated into the modeling code CLOUDY to generate line intensities for a range of plasma parameters, with emphasis on allowed ultraviolet extreme-ultraviolet emission lines detected from the Io plasma torus. Electron density-sensitive line ratios are calculated with the present atomic data and compared with those from CHIANTI v7.1, as well as with Io plasma torus spectra obtained by Far-Ultraviolet Spectroscopic Explorer and Extreme-Ultraviolet Explorer. The present line intensities are found to agree well with the observational results and provide a noticeable improvement on the values predicted by CHIANTI.