I'm starting to read about two-electron atoms from physics of atoms and molecules by Bransden.
Analyzing Helium spectrum I noticed that the energy of $He^+(1s)+e^-$ is greater than the energy of He, where the energy of $He^+(1s)+e^-$ is calculated using the $E_n$'s formula of hydrogenic atom with Z=2 (-2.0 a.u.). Obviously in order to obtain $He^+(1s)+e^-$ I have to excite He, so it is natural that the energy of $He^+(1s)+e^-$ is bigger that energy of He. But how is it possible that an atom with Z=2 when it has 1 electron this is less bound and when it has 2 electrons these are more bound? It appears to me that the one-electron should be more bound because the nuclear charge is Z=2 and there aren't repulsive effects.
Sorry for bad English and thanks you all.