Molecular fusion of ${\left({\mathrm{C}}_{60}\right)}_N$ clusters in the gas phase after femtosecond laser irradiation

We report observation of molecular fusion of $\left({\mathrm{C}}_{60}\right)_N{}^{+}$ clusters to form large fullerene molecules after excitation by 800-nm femtosecond laser pulses. The fused species decay by undergoing ${\mathrm{C}}_2$ fragmentation. Such fragmentation behavior for excited clusters of fullerene molecules can be understood in terms of the energetics of molecular fusion as determined previously from fullerene-fullerene collision experiments. Within a certain excitation energy window, there is sufficient time for a substantial atomic rearrangement to take place within the clusters before they dissociate. Below this energy window there is not sufficient internal energy for molecular fusion to occur while above the energy window multiple ionization followed by breakup of the clusters dominates.

Figure 1

Time-of-flight mass spectra obtained from femtosecond (800 nm, 200 fs) excitation/ionization of (upper) a molecular beam, cooled to below 100 K, containing clusters of fullerene molecules, (lower) a standard fullerene effusive source heated to a temperature of 520 °C. Note that there is a significant contribution of pump oil in the upper spectrum for arrival times below $50\mu \mathrm{s}$; however, the intensity of the small fragments from the fullerenes is still significantly higher than in the lower spectrum.

Figure 2

Laser fluence dependence of the high mass ions corresponding to fragments from coalesced fullerenes from van der Waals bound clusters. The fragment ion distributions corresponding to each parent cluster ion mass do not change significantly with changing laser fluence.

Figure 3

Expanded regions of the mass spectrum around the fragment groups coming from the (a) dimer, (b) trimer, (c) tetramer, and (d) pentamer cluster parent. The spectrum was obtained for a laser fluence close to that used to obtain spectrum II in Fig. 2. The horizontal arrows indicate the maximum fragment ion size that can be expected in the mass spectrum considering the energetics of the fullerene fusion mechanism. The asterisk marks the expected maximum in the fragment ion distributions assuming that the internal energies of the higher clusters scale with the number of fullerenes (see text for details).

Figure 4

Number of ${\mathrm{C}}_2$ units lost as a function of precursor fullerene cluster size. Squares and full line: mean number of lost ${\mathrm{C}}_2$ units obtained from fitting the experimental data with Poisson distributions. The shaded area indicates $\pm 2$ standard deviations obtained from the experimental data. Circles: Calculated maxima in the fragment distributions assuming that the photoabsorption cross sections scale linearly with the number of fullerene monomers in the precursor, normalized for $N=2$. Lower dashed line: Minimum expected number of ${\mathrm{C}}_2$ units lost considering the molecular fusion mechanism. Upper dashed line indicates the transition to where multiple ionization dominates, leading to the disappearance of the singly charged, large fragment ions in the mass spectrum.