ion beam can be separated in a magnetic field according to charge, mass, and energy. Since fullerenes with charge states between —2 and +5 can be prepared, a wide variety of fullerene ions can be used as projectiles. As a starting material, C60 powder heated to 300-600°C can be used as a supply for individual Qq molecules in the vapor phase.

One conclusion from these collision-induced fragmentation studies [6.45, 46] is that collision-induced fragmentation occurs by the loss of an even number of carbon atoms, as shown in Fig. 6.13, where the emission from the collision of 200-keV ions on a hydrogen target is plotted in terms of the yield for the various emission species which are selected by their mass-to-charge ratio. In this figure, even mass emission products ranging from CJ, to C5f8 are shown. To explain the various experimental observations described below, it is assumed that in the center-of-mass frame, a gas atom colliding with a C60 ion will experience a number of collisions with individual carbon atoms within the C60 molecule. On a statistical basis, a rift will be opened in some of these charged fullerenes, because of the large local stresses and strains, thus resulting in the sequential and independent emission of m carbon atom pairs. Two characteristic times are introduced to describe the fragmentation process: tl, the time to emit a carbon pair, and t2, the time to close the rift. Figure 6.14 shows a semilog plot of the yield of fullerene fragments vs. m, the number of carbon pairs emitted. The results of Fig. 6.14 imply a pm dependence for the yield, where p is defined below [6.46]. Further analysis shows that the probability

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