Theoretical studies of atomic cluster - cluster collisions

Abstract

A theoretical study on collisions between fullerenes for the systems C + 60 + C 60 , C + 70 + C 60 and C + 70 + C 70 is presented covering a wide range of collision energies (50 < E < 250 eV in the centre-of-mass frame). Quantum molecular dynamics (QMD) simulations enable a detailed insight into the underlying mechanism of the different reaction channels. The fusion barriers for the investigated systems are determined and compared with experimental data taking into account the finite temperature of the colliding fullerenes. Structural as well as energetic aspects of the reaction mechanism are discussed from a microscopic point of view. As a counterpart to the QMD simulations, a simple phenomenological fusion model is described and compared to the experimental fusion cross section as a function of the collision energy.

Fullerene-fullerene collisions

“…This can be regarded as an average probability for fusion to occur in the reactions after all the energetic and impact parameter constraints have been taken into account by the model. Detailed quantum molecular dynamic (QMD) simulations [13] have shown that this is due to a "bouncing off " mechanism which is a consequence of the strongly directional covalent bonding in the fullerene cage. Trajectories at the same collision energy and impact parameter can lead to very different results depending on the relative orientation of the hexagons and pentagons in the two colliding fullerenes.…”

“…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].…”

Adiabatic and non-adiabatic cluster collisions

“…Our systematic investigation of fullerene-fullerene collisions using QMD simulations includes at present C + 60 + C 60 [5,6], C + 70 + C 60 and C + 70 + C 70 collisions [4]. In this section, the fusion-relevant range of collision energies 50 E cm 250 eV (in the centerof-mass frame) is considered.…”

“…1) due to the rearrangement of atoms in the contact zone, a fusion event will be detected, whereas if the increase of kinetic energy proceeds to a large enough extent (lower left part of fig. 1) the effective repulsion of the (more or less) intact fullerene structure leads to a scattering event [4]. In fusion events, strongly deformed (and highly excited) compound clusters are formed which resemble a "peanut"-like structure (see upper right part of fig.…”

“…Large deformations have to be induced, i.e. the closed fullerene structure has to be broken, before a rearrangement of atoms in the overlap region can lead to stable "intercluster" bonds [4].…”