  Multiphoton ionization of helium is investigated in the superintense field
regime, with particular emphasis on the role of the electron-electron
interaction in the ionization and stabilization dynamics. To accomplish this,
we solve ab initio the time-dependent Schr\"odinger equation with the full
electron-electron interaction included. By comparing the ionization yields
obtained from the full calculations with corresponding results of an
independent-electron model, we come to the somewhat counterintuitive conclusion
that the single-particle picture breaks down at superstrong field strengths. We
explain this finding from the perspective of the so-called Kramers-Henneberger
frame, the reference frame of a free (classical) electron moving in the field.
The breakdown is tied to the fact that shake-up and shake-off processes cannot
be properly accounted for in commonly used independent-electron models. In
addition, we see evidence of a change from the multiphoton to the shake-off
ionization regime in the energy distributions of the electrons. From the
angular distribution it is apparent that correlation is an important factor
even in this regime.
