Electron transfer and fragmentation in fullerene collisions

Detta är en avhandling från Stockholm : Fysikum

Sammanfattning: In this thesis, we present results from detailed gas phase studies of intrinsic properties of fullerenes (C60) and clusters of fullerenes as probed by slow multiply charged (atomic or cluster) ions in combination with coincidence time-of-flight mass spectrometry. We have investigated the structures, stabilities, and the electron mobilities of multiply charged clusters of fullerenes, (C60)nr+ (r=2-5). We found that the (C60)nr+ cluster ions are weakly bound by van der Waals forces and the electric conductivity is high in strong contrast to the typical characteristics of other van der Waals type clusters and fullerene crystals (fullerite), where the charge is strongly localized. The high charge mobility was rationalized within the framework of a novel classical static over-the-barrier model for two conducting spheres used to describe multiple charge transfer processes between two neighboring fullerenes in the cluster. The model results showed that electron transfer is possible as soon as the C60-C60 system is charged, consistent with earlier experimental results from slow C60q+ + C60 collisions where an electric contact is established (during the very short interaction time of the collision) between the two molecules at distances outside the binding distance in the C60-C60 system. The present electrostatic model was also used to guide the interpretation of the measured kinetic energy releases in the fragmentation of multiply charged dimers, (C60)2r+ ? C60r1+ + C60r2+. In like manner, we have measured kinetic energy releases in the break-ups of multiply charged monomer fullerene ions with the aid of fragment-ion momentum spectroscopy. This yielded an excellent platform for investigations of the projectile and target dependencies on various intrinsic features such as the ultimate Coulomb stability limits for C60r+ and C70r+ ions, competition between different reaction pathways, and multi-fragmentation processes. The experimental results for the stability limits for multiply charged fullerenes are discussed in view of recent high level Density Functional Theory calculations of C60r+ ? C58(r-1)+ + C2+ transition states and our electrostatic model. The Density Functional Theory results are also used to check the validity and limitations of the classical model regarding the description of fragmentation processes, while a comparison with advanced molecular dynamic calculations of Ar8++Na40 collisions were made for investigating its applicability for charge transfer processes. We found that the present model indeed is useful for describing main features in inherent complex molecular processes at sufficiently low collision velocities. In addition, we propose an extension of the present model to consider two dielectric spheres immersed in a dielectric medium, which might be applied also outside the cluster research field.