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 re cent experiments. Results for a higher laser frequency also indicate a comparable very high double-ionization yield for sufficiently short p ulses. It is shown that the Hartree-Fock approximation fails dramatica lly in describing the two-electron dynamics. Also, in a density functi onal theory approach, we demonstrate the need for an improved exchange correlation potential and for more accurate density functionals for t he ionization probabilities.