Implanting very low energy atomic ions into surface adsorbed cage molecules: the formation/emission of Cs/C60+

Full Text: We demonstrate the formation of an endo-complex via a collision of energetic ions with molecular overlayers on a surface. An incoming atomic ion is encapsulated inside a very large molecule or cluster by implanting the primary ion into the target species, which then recovers its original structure or rearrange itself around the implanted ion in some stable configuration. Here we describe an experiment resulting in the formation and ejection of an endo-complex, within a single collision. We study the formation and emission of endohedral fullerenes, Cs/C60+ and Cs/C70+, following a single collision of Cs+ ion with a sub-monolayer of C60 (steady state coverage) on gold and silicon surfaces and with a sub-monolayer of C70 on gold. A continuous low energy (E0=35-220 eV) Cs+ ion beam hit the Cs+ covered surface and the collisional formation and ejection of the endohedral Cs/Cs60+ complex, within a single Cs+/C60 collision was observed and characterized. Several experimental observations clearly demonstrate the single collision nature of the combined atom penetration endo-complex ejection event. The fullerene molecule is actually being picked up off the surface by the penetrating Cs+ ion. The evidence for the trapping of the Cs+ ion inside the fullerene cage is given both by the appearance of the Cs/Cs(602-2n)+ (n=1-5) sequence and its termination at Cs/Cs50+. Kinetic Energy Distributions (KEDs) of the outgoing Cs/Cs60+ were measured for two different Cs+ impact energies under field-free conditions. The most striking observation is the near independence of the KEDs on the impact energy. Both KEDs peak around 1.2 eV with similar line shapes. A simple model for the formation/ejection/fragmentation dynamics of the endohedral complex is proposed and is found to be in good agreement with the experimental results