  The ultrafast relaxation of small clusters immediately after ionization is
studied. For small Hg$_n$ clusters we determine the fragmentation-time
distributions induced by ionization. A dramatic change of the fragmentation
behaviour occurs when the temperature before ionization reaches the melting
point of the neutral clusters. The ultrafast dynamics depends nonlinearly on
initial conditions regarding atomic positions and velocities. The resultant
largest Lyapunov exponent increases strongly upon ionization.
