Softening and activation of vibrational modes during the nonthermal fragmentation of

The vibrational modes of a C 60 molecule irradiated with ultrafast laser pulses are studied, using a tight binding approach with variable occupations. It is shown that for the same level of electronic excitation, measured through the electronic temperature, each mode suffers a distinct softening. Excited trajectories below and above the damage threshold, are examined. It is shown that the bond breaking processes and the ulterior fragmentation might be described in terms of the activation of softened vibrational modes; and that the changes in frequency can be dramatically larger than those estimated with the isothermal model.