  The ionization of two-active-electron systems by intense laser fields is
investigated theoretically. In comparison with time-dependent Hartree-Fock and
exact two electron simulation, we show that the ionization rate is
overestimated in SAE approximation. A modified single-active-electron model is
formulated by taking into account of the dynamical core polarization. Applying
the new approach to Ca atoms, it is found that the polarization of the core can
be considered instantaneous and the large polarizability of the cation
suppresses the ionization by 50% while the photoelectron cut-off energy
increases slightly. The existed tunneling ionization formulation can be
corrected analytically by considering core polarization.
