  The melting of 2D dust clusters caused by one additional particle in the
lower layer has experimentally been observed to undergo a two-step transition,
which divides the phase of the cluster into three stages. The first transition
is a jump of the dust kinetic energy due to the onset of an instability of the
lower-layer particle, shifting the cluster from an ordinary to a hot
crystalline state. The second transition is the actual phase transition into a
liquid state, which occurs at a decisively lower gas pressure. The detailed
dynamical properties of the system during the transition were determined in
terms of the normal mode analysis.
