  Ionization is one of the most fundamental processes in intense laser-matter
interaction. It is extremely efficient for clusters in laser fields and often
leads to surprisingly high charge states at moderate laser intensities. Here we
reveal a novel ionization mechanism in laser-cluster interaction through
particle-in-cell simulations. As the laser field ionizes a cluster, Brunel
electrons pushed back into the clustered plasma form an attosecond bunches,
impulsively exciting plasma oscillation. The resulting localized wake field
further ionizes the cluster, causing a highly ionized rod-like core along the
polarization axis. This anisotropic ionization is prominent using few-cycle
pulses and may be washed out using longer pulses due to collisional ionization.
This newly identified ionization channel can potentially provide complicated
site-specific control of ionization in nanometer-scale targets.
