FRAGMENTATION OF DOUBLY-CHARGED ARGON CLUSTERS

Fragmentation of doubly charged argon clusters is reported. Neutral ar gon clusters are excited with monochromatized synchrotron radiation in the energy regime of the argon L3/L2 absorption edges (240-260 eV) le ading predominantly to cluster dication formation. All charged particl es are detected in a photoelectron-photoion-photoion-concidence (PEPIP ICO) experiment. Symmetric and asymmetric charge separation reactions (Coluomb explosion) are identified for clusters below the critical siz e of stable dication formation. The peak shapes of the coincidence sig nals are investigated as a function of neutral cluster size. Character istic changes in peak shape are observed which are used to derive frag mentation mechanisms involving sequential evaporation of neutrals befo re and after charge separation. The spectra indicate in accordance wit h low kinetic energy releases occurring in charge separation of large dissociative cluster dications (Arn2+, with n > 50) that due to large charge separation distances the momenta of both singly charged fragmen ts are not any more directed into opposite direction, as it is typical for Coulomb explosion. The results are compared to collision induced fragmentation of mass selected argon cluster dications as well as phot on stimulated desorption spectra of condensed argon.