Experimental studies of fusion and fragmentation of fullerenes

Abstract

Energy dependences of the fusion cross section for the collisions C + 60 +C 60 , C + 60 +C 70 and C + 70 + C 70 have been measured. Fusion occurs for energies above a sharp energetic barrier V B. The fusion barrier lies in the region between 60 and 80 eV and increases with increasing number of atoms participating in the collision. For energies beyond the barrier the fusion reaction cross section increases with collision energy to a maximum value and then decreases very rapidly. The highly excited fusion compound decays on the experimental timescale, and the resulting fragmentation pattern has been studied as a function of collision energy. For energies up to 200 eV the fragmentation behaviour can be modelled in terms of successive evaporation of C 2 units from the hot fusion product. At higher energies this model breaks down and another fragmentation mechanism has to be invoked. The overall results are in very good agreement with quantum molecular dynamics simulations and the predictions of simple phenomenological fusion models.

Fullerene-fullerene collisions

“…The values for the fusion thresholds extracted with both models are given in table I along with predictions from molecular dynamics simulations. Excellent agreement is found if the calculations assume an initial temperature of 2000 K which is a realistic value under our experimental conditions [8]. The average fusion probability is also given in table I and is seen to increase more rapidly than the geometrical crosssection.…”

“…The experimental set-up has been described in detail before [7,8] and will be only briefly described here. A positively charged fullerene ion beam is produced by evaporating fullerene powder from an oven at a temperature of about 500 o C and ionising by electron impact.…”

“…2(a) as a function of centre-of-mass collision energy. The details of how we extract the absolute cross-sections from the time-of-flight data are given in [8]. In order to extract these cross-sections we considered both the non-fragmented C + 120 which we detected in the mass spectrometer as well as all fragments from this species that could be observed with the detector at 0 0:6 .…”

“…A better fit to the experimental data in the barrier region is found by using an extension to the absorbing sphere model which takes into account the steric effects [8][9][10][11][12][13]. This model can reproduce the curvature of the experimental data points near the threshold if the fusion probability is assumed to be approximately proportional to the collision energy [8].…”

“…1. We can model this behaviour by assuming a simple Arrhenius expression for the evaporation rate constant and using a Monte Carlo procedure to simulate the effect of successive C 2 loss on the experimental timescale [8]. In this way we can reproduce the experimental results in the linear regime very satisfactorily by using a dissociation energy for C 2 loss of 6.4 eV, as shown by the full lines in the figure.…”