Absolute cross section for Si2+(3s2 1S → 3s3p 1P) electron-impact excitation

We have measured the absolute cross section for electron-impact excitation (EIE) of Si2+(3s(2 1)S --> 3s3p P-1) from energies below threshold to 11 eV above. A beams modulation technique with inclined electron and ion beams was used. Radiation at 120.7 nm from the excited ions was detected using an absolutely calibrated optical system. The fractional population of the Si2+(3s3p P-3 degrees) metastable state in the incident ion beam was determined to be 0.210 +/- 0.018 (1.65 sigma). The data have been corrected for contributions to the signal from radiative decay following excitation from the metastable state to 3s3p P-1 and 3p(2) P-3, and excitation from the ground stare to levels above the 3s3p Ip level. The experimental 0.56 +/- 0.08-eV energy spread allowed us to resolve complex resonance structure throughout the studied energy range. At the reported +/- 14% total experimental uncertainty level (1.65 sigma), the measured structure acid absolute scale of the cross section are in good agreement with 12-state close-coupling R-matrix calculations. [S1050-2947 (99)06008-4].