Experimental evidence for the effect of resonance structures on electron excitation rates for SI III

Laboratory experimental data are presented which confirm the effect of resonance structures on the electron excitation rates of SI III and, through them, on line ratios used for astrophysical and laboratory diagnostic purposes. In particular, the spectrum of silicon emitted by low-density magnetic mirror plasma was recorded for the 1100-1400 A range, and the relative ratios of SI III lines emitted around 1300 A were compared with two different sets of theoretical predictions. The experimental results are in agreement with R-matrix calculations which include the effects of resonance structures, but disagree with the distorted wave computations which do not include these resonances.