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

The first absolute measurement of the cross section for excitation of a hydrogenlike ion is reported. The combined cross section for electron impact excitation of ground state He(+) to the 2 (2)S and 2 (2)P excited states has been measured in the near-threshold region from 40.08 to 41.95 eV using the JILA/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 than values calculated using close-coupling and variational methods. This contrasts with the longstanding disparity of nearly a factor of two between theory and normalized experiment fur excitation to the 2 (2)S state.