  We investigate numerically the hydrodynamic instability of an ionization
front (IF) accelerating into a molecular cloud, with imposed initial
perturbations of different amplitudes. When the initial amplitude is small, the
imposed perturbation is completely stabilized and does not grow. When the
initial perturbation amplitude is large enough, roughly the ratio of the
initial amplitude to wavelength is greater than 0.02, portions of the IF
temporarily separate from the molecular cloud surface, locally decreasing the
ablation pressure. This causes the appearance of a large, warm HI region and
triggers nonlinear dynamics of the IF. The local difference of the ablation
pressure and acceleration enhances the appearance and growth of a multimode
perturbation. The stabilization usually seen at the IF in the linear regimes
does not work due to the mismatch of the modes of the perturbations at the
cloud surface and in density in HII region above the cloud surface. Molecular
pillars are observed in the late stages of the large amplitude perturbation
case. The velocity gradient in the pillars is in reasonably good agreement with
that observed in the Eagle Nebula. The initial perturbation is imposed in three
different ways: in density, in incident photon number flux, and in the surface
shape. All cases show both stabilization for a small initial perturbation and
large growth of the second harmonic by increasing amplitude of the initial
perturbation above a critical value.
