COLLISION DYNAMICS FOR SMALL HELIUM CLUSTER IONS

Classical trajectories have been carried out for the collision of heli um atoms with He-2(+) and He-3(+) using a potential energy which inclu des the effects of dispersion, polarisation and charge exchange. At a collision energy of 0.1 eV the cross-section for atom exchange in He He-2(+) is calculated to be 21.5 Angstrom(2), exchange occurring thro ugh both simple collisions and long-lived complexes. However, even for very long-lived complexes exchange never exceeds 50% so that complete permutation of atoms in the complex does not occur. The cross-section for exchange in He + He: at the same collision energy is found to be much smaller at 0.7 Angstrom(2) which is attributed to the much smalle r dissociation energy of He-4(+). Rotational energy transfer for both collisions is similar, for impact parameters less than 3.5 Angstrom on ly 50% of collisions transfer more than 10% of translational energy to rotation.