Thermionic Emission and Fragmentation ofC60

We analyze the rate of delayed (thermionic) ionization of photoexcited ${\mathrm{C}}_{60}$ molecules. The rate has a power-law dependence, indicating the presence of a continuum of rate constants. The value of the exponent provides information about the competition between delayed ionization and unimolecular fragmentation; it is equal to the ratio of the ionization energy and the activation energy for fragmentation. This result provides a novel method to measure the controversial bond dissociation energy of neutral ${\mathrm{C}}_{60}$. We obtain a value of $11.9\pm 1.9\mathrm{eV}$.