Kinetic energies of Cn+ fragment ions (58⩾n⩾4) produced by electron impact on C60

Abstract

We used mass spectrometric techniques in conjunction with the ion deflection method to determine the kinetic energies of Cn+ fragment ions (58⩾n⩾4) produced by controlled electron impact on C60 under single collision conditions. The recorded ion beam profiles for the various fragment ions were analyzed using two independent methods. One method extracts the average kinetic energy of the fragment ion under study from the measured half-width of the ion beam profile (half-width method), whereas the second method analyzes the entire measured beam profile (profile method). For each Cn+ fragment ion, the kinetic energies obtained were interpreted assuming two possible formation pathways, (i) the fission of the excited parent C60+ ion in a single-step, two-fragment break-up, C60+→Cn++C60−n and (ii) the sequential decay of excited C60+ into Cn+ via the successive removal of Cm units (m=1, 2, or 3). Both the half-width method and the profile method yield very similar results for the kinetic energies of the Cn+ fragment ions for both formation mechanisms, viz., an average kinetic energy of roughly 0.45 eV in the case of the single-step fission and a linearly increasing average kinetic energy from 0.43 eV for the formation of C58+ to roughly 12 eV for the formation of C4+ in case of the sequential decay mechanism (in this case an essentially constant energy of also about 0.45 eV was obtained for the last fragmentation step in each case, which was confirmed by a third method using a fitting procedure). These results will be discussed in the context of the most likely fragmentation mechanism leading to the formation of the various fragment ions.