Energy distributions of atomic and molecular ions sputtered by C60+ projectiles

In the process of investigating the interaction of fullerene projectiles with adsorbed organic layers, we measured the kinetic energy distributions (KEDs) of fragment and parent ions sputtered from an overlayer of polystyrene (PS) oligomers cast on silver under 15 keV C60+ bombardment. These measurements have been conducted using our TRIFT™ spectrometer, recently equipped with the C60+ source developed by Ionoptika, Ltd. For atomic ions, the intensity corresponding to the high energy tail decreases in the following order: C+(E−0.4) > H+(E−1.5) > Ag+(E−3.5). In particular, the distribution of Ag+ is not broader than those of Ag2+ and Ag3+ clusters, in sharp contrast with 15 keV Ga+ bombardment. On the other hand, molecular ions (fragments and parent-like species) exhibit a significantly wider distribution using C60+ instead of Ga+ as primary ions. For instance, the KED of Ag-cationized PS oligomers resembles that of Ag+ and Agn+ clusters. A specific feature of fullerene projectiles is that they induce the direct desorption of positively charged oligomers, without the need of a cationizing metal atom. The energy spectrum of these PS+ ions is significantly narrower then that of Ag-cationized oligomers. For characteristic fragments of PS, such as C7H7+ and C15H13+ and polycyclic fragments, such as C9H7+ and C14H10+, the high energy decay is steep (E−4 − E−8). In addition, reorganized ions generally show more pronounced high energy tails than characteristic ions, similar to the case of monoatomic ion bombardment. This observation is consistent with the higher excitation energy needed for their formation. Finally, the fraction of hydrocarbon ions formed in the gas phase via unimolecular dissociation of larger species is slightly larger with gallium than with fullerene projectiles.