Absolute cross section for Si 2 + ( 3 s 2 1 S → 3 s 3 p 1 P ) electron-impact excitation

We have measured the absolute cross section for electron-impact excitation (EIE) of ${\mathrm{Si}}^{2+}{(3s}^{2}{}^{1}\stackrel{\ensuremath{\rightarrow}}{S}3s3p{}^{1}P)$ 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 ${\mathrm{Si}}^{2+}(3s3p{}^{3}{P}^{o})$ metastable state in the incident ion beam was determined to be $0.210\ifmmode\pm\else\textpm\fi{}0.018$ $(1.65\ensuremath{\sigma}).$ The data have been corrected for contributions to the signal from radiative decay following excitation from the metastable state to $3s3p{}^{1}P$ and ${3p}^{2}{}^{3}P,$ and excitation from the ground state to levels above the $3s3p{}^{1}P$ level. The experimental $0.56\ifmmode\pm\else\textpm\fi{}0.08\ensuremath{-}\mathrm{eV}$ energy spread allowed us to resolve complex resonance structure throughout the studied energy range. At the reported $\ifmmode\pm\else\textpm\fi{}14%$ total experimental uncertainty level $(1.65\ensuremath{\sigma}),$ the measured structure and absolute scale of the cross section are in good agreement with 12-state close-coupling R-matrix calculations.