Delayed ionization of C60: The competition between ionization and fragmentation revisited

High-resolution kinetic energy release distributions and dissociation energies for fullerene ions Cn+, 42 < or = n < or = 90

We have measured the kinetic energy released in the unimolecular dissociation of fullerene ions, Cn+ --> C(n-2)+ + C2, for sizes 42 < or = n < or = 90. A three-sector-field mass spectrometer equipped with two electric sectors has been used in order to ensure that contributions from isotopomers of different masses do not distort the experimental kinetic energy release distributions. We apply the concept of microcanonical temperature to derive from these data the dissociation energies of fullerene cations. They are converted to dissociation energies of neutral fullerenes with help of published adiabatic ionization energies. The results are compared with literature values.

The dynamics of endohedral complex formation in surface pick-up scattering as probed by kinetic energy distributions: Experiment and model calculation for Cs@C60+

Endohedral Cs@C60 molecules were formed by implanting low energy (E0 = 30-220 eV) Cs+ ions into C60 molecules adsorbed on gold. Both growth and etching experiments of the surface deposited C(60) layer provide clear evidence for a submonolayer coverage. The Cs+ penetration and Cs@C60 ejection stages are shown to be a combined, single collision event. Thermal desorption measurements did not reveal any Cs@C60 left on the surface following the Cs+ impact. The Cs@C60 formation/ejection event therefore constitutes a unique example of a pick-up scattering by endocomplex formation. Kinetic energy distributions (KEDs) of the outgoing Cs@C60+ 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 Cs+ 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. The model leads to a strong correlation between the vibrational and kinetic energy of the outgoing Cs@C60. The KEDs are calculated taking into account the competition between the various decay processes: fragmentation and delayed ionization of the neutral Cs@C60 emitted from the surface, fragmentation of the Cs@C60+ ion, and radiative cooling. It is concluded that the measured KEDs are heavily biased by the experimental breakdown function. Good agreement between experimental and calculated KEDs is obtained.