Absolute measurements of electron excitation cross sections in C³⁺

Spectral emissions from C³⁺ are of importance in solar, planetary and astro-physical plasmas. In these plasmas the population of excited states is often dominated by electron impact excitation. Accurate cross sections for this process are required to determine parameters such as electron and ion densities. The 2s ²S - 2p ²P transition in C³⁺ is of particular interest as it allows direct comparisons among the results of several different experimental and theoretical groups. Employing the electron energy-loss technique we have measured cross sections to several eV above threshold, a region where there is still significant disagreement among previous results.