Shattering of Clusters Upon Surface Impact: An Experimental and Theoretical Study

The shattering transition expected upon ultrafast heating has been observed in size selected $({\mathrm{NH}}_3{)}_{n-1\mathrm{}}{\mathrm{NH}}_4^{+}$ clusters, $n=4\mathrm{}$ to 40, upon impact at supersonic velocities on a graphite surface. As a function of the impact velocity, the transition is between the recoil of the intact parent cluster and the appearance of small, $n=1,2,\dots$ charged fragments. As the cluster size increases, the transition becomes a sharper function of the impact velocity. The experimental fragmentation pattern is well accounted for by a distribution of maximum entropy subject to conservation of energy, atoms, and charge.