A semi-empirical concept for the calculation of electron-impact ionization cross-sections of neutral and ionized fullerenes

We have developed a semi-empirical approach to the calculation of cross-section functions (absolute value and energy dependence) for the electron-impact ionization of several neutral and ionized fullerenes C-60(q+) (q = 0-3), for which reliable experimental data have been reported. In particular, we propose a modification of the simplistic assumption that the ionization cross-section of a cluster/fullerene is given as the product of the monomer ionization cross-section and a factor m(a), where "m" is the number of monomers in the ensemble and "a" is a constant. A comparison between our calculations and the available experimental data reveals good agreement for q = 0, 1, and 3. In the case of the ionization of C-60(2+) (q = 2), our calculation lies significantly below the measured cross-section which we interpret as an indication that additional indirect ionization processes are present for this charge state. (C) 2002 Elsevier Science B.V. All rights reserved.