The products observed by mass spectra in collision-induced fragmentation are also dependent on the charge of the fullerene projectile and the target molecules. For example, in the case of the H2 target, the various features in the yield spectrum for 300-keV Cg02 are similar to those in the 200-keV Cg0 spectrum (see Fig. 6.13), indicating electron absorption by C6'02 to yield CJ,, upon collision of the C^2 ion with the H2 target.
By measuring the relative yields of the fragmentation process in terms of C2 emission, going from G^ to C^ and from G^ to Cj~4, we can obtain the yields for the daughters in the decay chain. Decay lifetime measurements of the and Cj~6 molecular ions have also been carried out, assuming a simple decay formula I(t) = 70exp(-i/r). Results for t of 337 ¿is for GJ8, and 66 /as for Cj6 have been obtained by several groups [6.47,48]. After decay to a neutral species, the lifetime for the neutral ions is much greater (~10 s), thus allowing better opportunities for experiments to be carried out with these neutral molecular beams.
In collisions of C60 positive ions, such as Cgo, charge can be transferred to the target. For a triply charged C^ ion, such charge transfer occurs, for example, when the third ionization potential IIU for C^ exceeds the ionization potential for the target species. Charge transfer from C^ and Cgo ions can occur when these ions collide with a gas target, if the ionization potentials of the target are less than the second and third ionization potentials of carbon, 9.37 eV and 10.75 eV, respectively [6.49],
Interesting results have been reported for the collision of CJ, ions and various surfaces. Especially popular have been graphite surfaces, where the projectile and target are both carbon atoms. Collisions between energetic C6g ions (with energies up to 200 eV) and graphite surfaces result in large energy transfers to the surface, leaving the ions with final energies of only ~15 eV, almost independent of the incident energy [6.36,37,50], In this collision process, the Cg0 ion is momentarily heated to high temperatures, experiences major distortions, but transfers most of its initial kinetic energy into heating the surface. Only the elastic energy involved in the molecular deformation is retained by the C^,, and the retained energy is only weakly dependent on the incident energy. For higher incident energies (above 200 eV), the Cg0 ions fragment with the loss of C2 units [6.26,36], Similar behavior is observed for Cg0 ions colliding with diamond surfaces, regarding the velocity distribution of the scattered Cg0 ions and the conditions for their fragmentation [6.36].
Similar experiments have been done by studying the collision of Cg0 with an Si (100) surface for energies < 250 eV [6.51,52]. The scattering of the
Cg0 ion by the surface first neutralizes the C60, as it picks up an electron from the surface. The energy of the scattered neutrals is determined by reionization of the C60 neutrals to Cg0 using a pulsed ArF excimer laser (~15 ns, 193 nm, 6.4 eV). Just as for the graphite and diamond surfaces, almost all of the energy of the Cg0 ion incident on silicon goes into heating the surface, with ~15 eV going into internal vibrations of the rebounding fullerene, consistent with molecular dynamics calculations [6.53], showing that the C60 projectile on impact with the surface is compressed to two thirds of its diameter (from ~7 A to ~4.5 A). This intense deformation couples strongly to the vibrational modes, with a dissipation of ~10 eV per C60 into vibrational energy and ~5 eV into its rebound energy (rebound velocity ~1200 m/s).
As another example, collisions of 275-eV C6f0 were studied with a crystalline (111) fullerite film grown on mica [6.54]. Many of the effects that are observed for the collision of Cg0 with graphite, diamond, and silicon surfaces, as described above, are also found for collisions of Cg0 ions with a C60 film surface. One important new feature of the impact of Cg0 ions on a C60 film is the tendency to form higher-mass fullerenes, especially near C120, as shown in Fig. 6.15, with some intensity also found near C70. Of particular interest are the many fullerenes observed between Cno and Cj3q [6.54], Apparently, during the collision between the projectile and target fullerenes, much of the kinetic energy of the projectile is consumed in forming the larger fullerene, which is released from the surface. In contrast, Fig. 6.15 shows that the impact of ions on a graphite surface does not give rise to higher-mass fullerenes.
Fig. 6.15. Mass spectra of fullerene ions released from a surface after impact of CJ, ions (a) on a crystalline fullerite film target (£,„ = 275 eV, a = 25°, v = 2000 m/s) and (b) on oriented graphite (HOPG). Note that the intensity scale for the heavy mass fullerenes has been expanded by a factor of 4 [6.54],
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