Absolute cross section for [Formula Presented] electron-impact excitation

We have measured the absolute energy-averaged cross section for electron-impact excitation of [Formula Presented] from energies below threshold to the turn-on of the [Formula Presented] 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 [Formula Presented] ground state was detected using an absolutely calibrated optical system. The fractional population of metastable [Formula Presented] in the incident ion beam was determined to be [Formula Presented] 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.