Energy levels, radiative rates and electron impact excitation rates for transitions in C III

Abstract: We report energy levels, radiative rates (A-values) and lifetimes for the astrophysically-important Be-like ion C III. For the calculations, 166 levels belonging to the $n \le$ 5 configurations are considered and the {\sc grasp} (General-purpose Relativistic Atomic Structure Package) is adopted. Einstein A-coefficients are provided for all E1, E2, M1 and M2 transitions, while lifetimes are compared with available measurements as well as theoretical results, and no large discrepancies noted. Our energy levels are assessed to be accurate to better than 1\% for a majority of levels, and A-values to better than 20\% for most transitions. Collision strengths are also calculated, for which the Dirac Atomic R-matrix Code ({\sc darc}) is used. A wide energy range, up to 21 Ryd, is considered and resonances resolved in a fine energy mesh in the thresholds region. The collision strengths are subsequently averaged over a Maxwellian velocity distribution to determine effective collision strengths up to a temperature of 8.0$\times$10$^5$ K, sufficient for most astrophysical applications. Our data are compared with the recent $R$-matrix calculations of Fern{\'a}ndez-Menchero, Del Zanna & Badnell [A&A 566 (2014) A104], and significant differences (up to over an order of magnitude) are noted for several transitions over the complete temperature range of the results.