Dissociation Energy forC2Loss from Fullerene Cations in a Storage Ring

We have stored positively charged fullerene ions ${\mathrm{C}}_n^{+}$ (n even, from 48 to 70 and 76), ${\mathrm{C}}_{60}^{2+}$ and ${\mathrm{C}}_{70}^{2+}$ in an electrostatic storage ring and have measured the rate of emission of neutral fragments as a function of time. In the time range of the measurements, $50\mu \mathrm{s}$ to a few milliseconds, the rate decreases strongly due to radiative cooling of the molecules. Using the cooling rate predicted from a dielectric model, we have extracted the dissociation energies for ${\mathrm{C}}_2$ loss from the measurements. As expected, the energies are largest for the “magic” fullerenes, ${\mathrm{C}}_{50}$, ${\mathrm{C}}_{60}$, and ${\mathrm{C}}_{70}$, and the value of $9.8\pm 0.1\mathrm{eV}$ for ${\mathrm{C}}_2$ loss from ${\mathrm{C}}_{60}^{+}$ is in reasonable agreement with theory and with other recent experiments.