  The process of nonsequential two-photon double ionization of helium is
studied by two complementary numerical approaches. First, the time-dependent
Schr{\"o}dinger equation is solved and the final wave function is analyzed in
terms of projection onto eigenstates of the uncorrelated Hamiltonian, i.e.,
with no electron-electron interaction included in the final states. Then, the
double ionization probability is found by means of a recently developed
approach in which the concept of absorbing boundaries has been generalized to
apply to systems consisting of more than one particle. This generalization is
achieved through the Lindblad equation. A model of reduced dimensionality,
which describes the process at a qualitative level, has been used. The
agreement between the methods provides a strong indication that procedures
using projections onto uncorrelated continuum states are adequate when
extracting total cross sections for the direct double ionization process.
