Excitation of He+ to the 2 S-2 and 2 P-2 states by electron impact

The first absolute measurement of the cross section for excitation of a hyd rogenlike ion is reported. The combined cross section for electron impact e xcitation of ground state He+ to the 2 S-2 and 2 P-2 excited states has bee n measured in the near-threshold region from 40.08 to 41.95 eV using the JI LA/ORNL merged electron-ion beams energy-loss technique. The threshold step in the cross section is measured as (8.46 +/-1.52) x 10(-18) cm(2) at 90% confidence level of total uncertainty, which is approximately 20% larger th an values calculated using close-coupling and variational methods. This con trasts with the longstanding disparity of nearly a factor of two between th eory and normalized experiment fur excitation to the 2 S-2 state.