Spontaneous Decay of Ionized Atomic Clusters: Statistical and Non-Statistical Channels

Abstract

If the energy of an electron (or photon) beam colliding with gas phase clusters is greater than a critical value termed appearance energy, some of these neutral clusters will be ionized. The abundance and the variety of the cluster ions produced from a specific neutral precursor Pn depends on geometric, electronic and energetic properties of the neutral and ionized clusters. Whereas inelastic interaction of electrons with single atoms results only in changes of the electronic configuration, interaction of electrons with van der Waals (vdW) clusters consisting of atomic constituents involves — besides electronic excitation — (i) changes in the nuclear motion (vibrational and rotational excitation), (ii) multiple collisions of the incoming electron, and (iii) subsequent intermolecular reactions within the cluster. All of this leads to the production of hot ions, i.e. the deposition of excess energy into various degrees of freedom. Energy flow between different degrees of freedom leads to spontaneous decay reactions of these hot ions on time scales ranging from a few vibrational oscillations for prompt dissociations up to several 100 µs for metastable dissociations. Depending on the energy storage mode and concomitant decay mechanisms metastable decay reactions can be divided into statistical and non-statistical decay channels.