Analysis of fragment size distributions in collisions of monocharged ions with the C₆₀molecule

Abstract

Fragmentation of the C60 molecule is investigated using a multicorrelation technique. We first focus on the transition from asymmetrical dissociation (AD) to multifragmentation (MF). These processes are studied in collisions between H+x(x = 1–3) hydrogenic projectiles and C60 fullerene in the gas phase, in the 2–130 keV collisional energy range. A rather sharp transition from pure AD to predominant MF is observed when plotting the AD/(AD + MF) ratio against the average deposited energy Edep; it occurs in the 80–240 eV Edep range; this ratio is also found to be independent of the projectile species (scaling law). The evolution of the size distribution shape is also discussed and compared with other data available in the literature. A pure power law is never reached in the present experimental conditions. Finally, an event-by-event analysis of the fragmentation data is developed for the first time in the study of the C60 molecule fragmentation and discussed in terms of the predictions of the percolation model near a critical behaviour. Moments of order 2, 3 and 5 are determined for each correlation event. Moments of order 3 and 5 follow a linear behaviour when plotted against the moment of order 2, as predicted, and the exponent τ that is extracted takes a value near 2. The Campi scatter plot is also determined and discussed for total and multiplicity-selected events. Both slopes of the two branches in the Campi plots and τ value are near those that are expected in the percolation of a 2D lattice.

Multiple electron capture, excitation, and fragmentation inC6+−C60collisions

“…[78]). The measured excitation energy distributions in the fragmentation of C 4+ 60 are in the energy range 40-100 eV centered at ∼60 eV both for C 2 and C + 2 emission on the experimental time scale [79]. Intact fullerene ions were observed for excitation energies of about 30 eV while about 80 eV was required for observation of C 2+ 58 and C 3+ 58 on the [79], which is consistent with our simulations.…”

Radiation-induced fragmentation of fullerenes

“…U-shaped size distributions (with absent C monomers) have been evidenced experimentally among others by Rentenier et al 3 and can be identified for the total ionization charge under consideration only in the case of the larger C 96 (up to about 130 eV), since C 96 , in particular, carries the smallest specific ionization charge (q 2 tot /N atom ∼ 1e 2 /atom) and is thus subject to moderate electrostatic interactions. The smaller fullerenes no longer show U-shaped distributions due to the stronger Coulomb repulsion per atom, which leads to the successive disintegration of large fragments into smaller ones.…”

“…For moderate excitation energies, the fragment size profiles have been found to reproduce the experimentally observed U shape and bimodal dependences. 3 One of the main findings of our simulations was the existence of a phase transition region in the total chargeexcitation energy plane, featuring a critical excitation energy around 90 eV for neutral C 60 cages and decreasing roughly parabolically with increasing total ionization charge. The charge-averaged critical excitation energy (55 eV) agrees fairly well with the experimental findings of Rentenier et al, 3 but could not possibly have been reproduced if charged fragments were neglected.…”

“…Overall, the size distribution patterns appear to be similar to those from a wide variety of fragmentation experiments with atomic projectiles. 3,[16][17][18][19] The average fragmentation probability, providing the most synthetic description of the overall features of the fragmentation process, is defined for each trajectory ensemble as the ratio of the number of dissociative trajectories to the total number of trajectories. The excitation energies corresponding to the lowest panels of Figs.…”

“…1,3,18,19 Nevertheless, irrespective of the excitation energy, in highly ionized states the U-shaped size distributions disappear, since the strong electrostatic repulsion between and within clusters produces further disintegration into smaller fragments. A simple square root dependence on the fullerene size of the limiting ionization charge still producing U-shaped distributions has been found to hold for all studied fullerene species.…”