  The ionization of a one-dimensional model Helium atom in short laser pulses
using time-dependent density functional theory is investigated. We calculate
ionization probabilities as a function of laser intensity by approximating the
correlation function of the system adiabatically with an explicit dependence on
the fractional number of bound electrons. For the correlation potential we take
the derivative discontinuity at integer numbers of bound electrons explicitly
into account. This approach reproduces ionization probabilities from the
solution of the time-dependent Schroedinger equation, in particular the
so-called "knee" due to nonsequential ionization.
