Tight-binding molecular dynamics simulations of radiation-inducedC60fragmentation

Abstract

The radiation-induced fragmentation of the ${\text{C}}_{60}$ fullerene was investigated by tight-binding molecular dynamics simulations based on the parametrization of Papaconstantopoulos et al. [Tight-Binding Approach to Computational Materials Science, edited by P.E.A. Turchi, A. Gonis, and L. Colombo, M.R.S. Symposia Proceedings No. 491 (Materials Research Society, Pittsburgh, 1998), p. 221] and employing novel models for nonadiabatic excitation and charge redistribution. The resulted fragment size and fragment charge distributions, averaged over large ensembles of trajectories corresponding to total ionization states up to $+24e$ and excitation energies up to 1000 eV, have been used to analyze the fragmentation statistics in terms of several derived quantities. For moderate excitation energies, the fragment size profiles reproduce the experimentally observed U shape and bimodal dependence. Even though for high excitation energies and high total charges, predominantly multifragmentation occurs, a genuine power-law dependence sets in only beyond 1000 eV. A well-defined phase-transition region is found in the total charge-excitation energy plane, which appears to be delimited by a roughly parabolic critical line. The overall average critical excitation energy estimated from the simulations amounts to 55 eV and agrees with the experimental findings.