  The single particle spectral-weight function (SWF) of the ionic Hubbard model
at half filling is calculated in the cluster perturbation theory approximation.
An abrupt change of regime in the low-energy region, near the chemical
potential, is found at a critical value, $U_{c}$, of the coupling constant
(Hubbard $U$). The SWF at the Fermi points $k_{F}$=$\pm{\pi}/2$ jumps, as $U$
increases, from a two-peak structure, the gap edges, to a four-peak structure
accompanied by a (non-vanishing) minimum of the charge-gap. The two inner peaks
of this structure show very small dispersion (flat bands) away from the Fermi
points, whereas the outer peaks mark the edges of the Hubbard bands. No other
signatures of abrupt change are detected in the SWF. The two regimes are
physically realized in the angle-resolved photoelectron spectra of
$(TaSe_{4})_{2}I$, and the blue-bronze $K_{0.3}MoO_{3}$, respectively.
