Electron correlation effects in the double ionization of He

The double-ionization yield of He is calculated with a one-dimensional fully correlated two-electron model for the low laser frequency of recent experiments. Results for a higher laser frequency also indicate a comparable very high double-ionization yield for sufficiently short pulses. It is shown that the Hartree - Fock approximation fails dramatically in describing the two-electron dynamics. Also, in a density functional theory approach, we demonstrate the need for an improved exchange correlation potential and for more accurate density functionals for the ionization probabilities.