Radiative-lifetime and absolute-oscillator-strength studies for some resonance transitions of Si I, II, and III

Radiative-lifetime studies have been carried out for resonance transitions of silicon I, II, and III in the ultraviolet using an electron-beam phase-shift apparatus. Lifetimes ranging from 0.03 to 220 nsec were determined. Significant discrepancies with earlier results were found for oscillator strengths of certain transitions, e.g., the Si II 1190-1194-A multiplet, the SiII 1808-A multiplet, and the SiIII 1206-A multiplet, which are important transitions in the establishment of the interstellar abundance of silicon. In making these measurements, some very short lifetimes were encountered, requiring that we make strenuous efforts to avoid errors in the reference phase. This led to redeterminations of the radiative lifetimes for the cascade-free transitions in NeII at 462 A and in NII at 1085 A, with lifetimes of 0.15 ± 0.03 and 3.0 ± 0.1 nsec, respectively.