  A peculiar ion behavior is found in a Si cluster, moving with a speed of
~0.22c (c: speed of light) in a solid Al plasma: the Si ion, moving behind the
forward moving Si ion closely in a several angstrom distance in the cluster,
feels the wake field generated by the forward Si. The interaction potential on
the rear Si may balance the deceleration backward force by itself with the
acceleration forward force by the forward Si in the longitudinal moving
direction. The forward Si would be decelerated normally. However, the
deceleration of the rear Si, moving behind closely, would be reduced
significantly, and the rear Si may catch up and overtake the forward moving Si
in the cluster during the Si cluster interaction with the high-density Al
plasma.
