Barriers for asymmetric fission of multiply charged C 60 fullerenes

We have measured kinetic energy releases in asymmetric fission, C{sub 60}{sup r+}{yields}C{sub 58}{sup (r-1)+}+C{sub 2}{sup +} (r=6-9) and evaporation C{sub 60}{sup r+}{yields}C{sub 58}{sup r+}+C{sub 2} (r=2,3), following multiple-electron removal from C{sub 60} in He{sup 2+} and Xe{sup 17+} 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{sub 60} jet. This yielded a resolution of 3 meV for the final kinetic energies of the charged C{sub 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{sub 2}{sup +} peak-shape and sector-field C{sub 58}{sup (r-1)+}-energy analysis. Model calculations treating C{sub 58}{sup (r-1)+} and C{sub 2}{sup +} as conducting spheres indicate that the autocharge-transfer process, which has been proposed to link asymmetric fission to neutral C{sub 2} emission, most likely is inactive for all r. Using a charge-independent activation energy for C{sub 2} emission from C{sub 60}{sup r+} of E{sub a}=10 eV, we deduce fission barriers indicating lower (semiempirical) and upper (model) C{sub 60}{sup r+}-stability limits of r=11 and r=18, respectively.

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