Barriers for asymmetric fission of multiply charged C_{60} fullerenes

We have measured kinetic energy releases in asymmetric fission, C-60(r+)-->C-58((r-1)+)+C-2(+) (r=6-9) and evaporation C-60(r+)-->C-58(r+)+C-2 (r=2,3), following multiple-electron removal from C-60 in He2+ and Xe17+ collisions at 3q keV (q=2,17). We used the recoil-ion momentum technique and limited the initial momentum distribution of the target molecules by collimation of the effusive C-60 jet. This yielded a resolution of 3 meV for the final kinetic energies of the charged C-58 fragments, mapped out as two-dimensional position distributions at the end of a linear time-of-flight mass spectrometer. The present results for asymmetric fission are in agreement with earlier ones deduced from time-of-flight C-2(+) peak-shape and sector-field C-58((r-1)+)-energy analysis. Model calculations treating C-58((r-1)+) and C-2(+) as conducting spheres indicate that the autocharge-transfer process, which has been proposed to link asymmetric fission to neutral C-2 emission, most likely is inactive for all r. Using a charge-independent activation energy for C-2 emission from C-60(r+) of E-a=10 eV, we deduce fission barriers indicating lower (semiempirical) and upper (model) C-60(r+)-stability limits of r=11 and r=18, respectively.