Abstract Photo-ionised plasma is commonly found in a variety of settings in astrophysics, across a range of temperature and density scales. The photo-ionisation cross sections most often used to model emission and absorption in these settings are total cross sections calculated by the Opacity Project. While these high quality data are a standard in atomic physics, use of total cross sections in modelling plasma may incorrectly model level populations and ion fractions under certain conditions. Using cross sections resolved by both initial and final level are the best way to avoid such problems. New photo-ionisation calculations for S+1 − 3 have been carried out using the atomic package Autostructure. Level-resolved cross sections from these and other calculations have been made available in one format for all the low charge states of C, N, O, Ne, Si and S. The IDL ionisation equilibrium routines from the latest version of CHIANTI have been modified so that this process can be included in ion fraction calculations.
Dufresne et al. (Fri,) studied this question.