Cluster impact chemistry

This paper addresses the interaction of molecular cluster ions with a solid surface in the kinetic energy range of 1-100 eV/molecule. We report experimental results on the energy acquisition by the cluster following its impact on the target, the size distribution and the time scale of cluster fragmentation, and first examples of chemical reactions induced by cluster impact. In particular we show that for a p-type diamond film and moderate collision energies the elasticity of the cluster-surface impact is surprisingly high: The intact cluster recoils with typically 75% of its collision energy. Once, however, the clusters have acquired sufficient internal energy they will shatter, mostly to monomers. In the case of protonated ammonia cluster ions this shattering of clusters upon surface impact is shown to be faster than 80 ps. It provides evidence that the technique of cluster impact allows an ultrafast energy redistribution within superheated cluster ions prior to their fragmentation. The feasibility of this fascinating new approach to femtosecond chemistry is demonstrated with impact-induced chemical reactions of iodomethane clusters to molecular iodine and of trifluoromethane clusters to molecular fluorine. The detected reaction yields are surprisingly high, even for the small cluster sizes investigated so far (n < 16).