  We have studied the stability of C$_{59}$ anions as a function of time, from
their formation on femtosecond timescales to their stabilization on second
timescales and beyond, using a combination of theory and experiments. The
C$_{59}^-$ fragments were produced in collisions between C$_{60}$ fullerene
anions and neutral helium gas at a velocity of 90 km/s (corresponding to a
collision energy of 166 eV in the center-of-mass frame). The fragments were
then stored in a cryogenic ion-beam storage ring at the DESIREE facility where
they were followed for up to one minute. Classical molecular dynamics
simulations were used to determine the reaction cross section and the
excitation energy distributions of the products formed in these collisions. We
found that about 15 percent of the C$_{59}^-$ ions initially stored in the ring
are intact after about 100 ms, and that this population then remains intact
indefinitely. This means that C$_{60}$ fullerenes exposed to energetic atoms
and ions, such as stellar winds and shock waves, will produce stable, highly
reactive products, like C$_{59}$, that are fed into interstellar chemical
reaction networks.
