Cross sections of electron excitation out of metastable helium levels with a fast metastable target produced via charge exchange

Absolute direct cross sections for electron excitation out of the 2 S-3 level and into the 3 D-3, 4 D-3, and 3 S-3 levels of the helium atom from threshold to 500 eV and into the 3 P-3 level over a more limited energy range have been measured using a fast metastable atomic beam target. We produce the metastable atoms via near-resonant charge exchange between a 1.6-keV He+ ion beam and Cs vapor. Because this reaction is highly nonresonant with the ground state of helium, the charge-transfer process yields a primarily metastable beam. We use a thermal detector which we calibrate with ions to measure absolutely the neutral beam flux. The atomic beam is crossed by an electron beam, and we collect the resulting fluorescence at right angles to both the electron and atomic beams. We obtain the cross sections for excitation out of the 2 S-3 level into the various excited levels by monitoring the emission out of the excited level of interest.