Fragmentation dynamics of clusters induced by heavy ion impacts*

Dimers, consisting of two noble atoms, atom-molecule or two molecules, are weakly bound by Van der Waals bond or hydrogen bond and ideal model systems to understand the damage dynamical processes in bio-environments induced by energetic particles. When a dimer is ionized by ion, photon or electron impact, due to the presence of neighboring molecules, some forbidden relaxation channels in isolated monomer may be open. Interatomic Coulombic decay (ICD) [1] is a typical process which consumes excess energy of the ionized system and has been confirmed by experiments [2]. The fragmentation related to ICD mainly involve the virtual photon or electron exchange between the dimer components. Two new cases will be presented that involve massive ion and charge migration over large distances within a cluster resulting in fragmentations. The first case is hydrogen bonded acetylene dimer (C₂H₂)₂ irradiated by 200 keV alpha particles. The electronic and nuclear relaxation of the doubly charged acetylene dimer is accomplished via a novel mechanism involving intermolecular proton transfer [3] or the ICD. The two mechanisms trigger fast Coulomb explosion of the dimer due to creation of charge-separated states. The second case deals with a Van der Waals cluster (N₂Ar) which is doubly ionized by 1 MeV ions Ne⁸⁺. The dissociation of the dimer ion (N₂Ar)²⁺ proceeds via an exotic process of heavy ion N⁺ transfer and NAr⁺ ion formation [4]. Due to the presence of neighboring Ar atom in the dimer the tunneling of N⁺ ion from molecule ion N₂²⁺ becomes accessible, as a result, the breakup of the covalent N⁺-N⁺ bond, and the tunneling of N⁺ ion and the formation of N⁺-Ar bond occur at the same instant, and consequently, Coulomb explosion of N⁺ and NAr⁺ ion pairs comes after immediately. These newly observed ion transfer mechanisms may be general in biochemical environments and have potential importance in understanding the irradiation damage caused by energetic particles, and furthermore, may directly affect the expression of DNA and protein in vivo.

Collaborators: X L Zhu, S Xu, S C Yan, and S F Zhang, IMPCAS and UCAS; X Q Hu, Y Wu and J G Wang, IAPCM, Beijing, China; L S Cederbaum and N V Kryzhevoi, Universität Heidelberg, Germany