Absolute cross section for Si2+(3s3p 3Po→3s3p 1Po) electron-impact excitation -: art. no. 052702

We have measured the absolute energy-averaged cross section for electron-impact excitation of Si2+(3s3p P-3(o)-->3s3p P-1(o)) from energies below threshold to the turn-on of the 3s3p P-3(o)-->3p(2) P-3 transition. A beams modulation technique with inclined electron and ion beams was used. Radiation at 120.65 nm from the decay of the excited ions to the 3s(2) S-1 ground state was detected using an absolutely calibrated optical system. The fractional population of metastable Si2+(3s3p P-3(o)) in the incident ion beam was determined to be 0.256+/-0.035(1.65sigma). The experimental energy spread ranged from 0.85 eV (full width at half maximum) at the lowest energies to 0.56 eV at the highest. Resonance features consistent with 12-state close-coupling R-matrix calculations are seen.