A newly found fast radiative cooling process of C-6(-) is studied by simulation based on statistical sharing of the internal energy by the vibrational modes both in the electronic ground and excited states. For comparison, a contrasting slow cooling of C6H- is simulated by the same procedure. The simulation demonstrates a distinct difference in the time evolution of energy distributions between C-6(-) and C6H-, and well reproduces the observed fast cooling of C-6(-) by recurrent electronic transitions and the slow cooling of C6H- by vibrational de-excitation. (C) 2014 Elsevier B.V. All rights reserved.