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Coulomb fragmentation processes in collisions of Kr8+ ions with nitrogen molecules were studied at projectile energies between 19 and 171 eV/u. At low impact energies (less than or similar to50 eV/u) and small impact parameters, the electron distribution of the fragmenting molecular N-2(q+) ion becomes strongly polarized. This is due to the presence of the slow highly charged projectile during the fragmentation process. Microscopic calculations simultaneously incorporating electron transfer and molecular dissociation give insight into the dynamics of this newly found polarization effect.

We have Studied the ionization and fragmentation Of C-60 in collisions with protons and deuterons at impact energies of 50-300 keV. The time-of-flight spectra of all fragment ions were measured in coincidence with one of the emitted electrons. The cross sections of multiple ionization as well as relative yields of even-mass fullerene-type ions of various charges are deduced from experimental spectra. A theoretical model based on a statistical approach is developed for the explanation of the experimental data. The model describes the p +C-60 collision as proceeding in three steps: (i) energy deposition by the projectile, (ii) emission of prompt electrons, and (iii) C-2 evaporation and delayed electron emission. The observed distribution of fragment fullerenes indicates that the delayed electron emission is an essential part of the process and that it occurs before the equilibration is achieved.

We have studied energy transfer, fragmentation and ionization in collisions between protons and C-60 molecules. The collision energy is varied between 1 and 300 keV and we thus cover a velocity range from 0.2 to 3.5 atomic units. The distributions of intact and fragmented C-60 ions have been measured by means of time-of-flight mass spectrometry using pulsed beams or emitted electrons to clock the collision events. The processes leading to multiple ionization and evaporation of neutral dimers are analyzed as functions of the projectile velocity. In contrast to recent findings for He+-C-60 collisions by Schlatholter et al., our results show that the cross sections for ionization and evaporation first increase with velocity, have maxims at around 1-2 a.u., and decrease again for higher velocities. This velocity dependence closely resembles the one for the electronic stopping power indicating that electronic excitations are the main means of energy transfer to C-60 and that this energy is rapidly distributed among the vibrational degrees of freedom. This conclusion is supported by Monte Carlo calculations of nuclear and electronic energy loss for protons passing through and closely outside the C-60 cage.

The dissociation of molecular ions (N-2)(q+) produced by 4.7 MeV/amu Bi-57+,Bi-25+ and 5.9 MeV/amu Xe-43+,Xe-18+ ion impact on N-2 has been studied using the coincidence technique with a position- and time-sensitive multiparticle detector. The kinetic energy release distributions for all observed ion pairs Nq1+ +Nq2+ are measured. Analysis of the distributions is made with emphasis on the highly charged-molecular ions, q>3. In these cases the most probable total kinetic energy of the two fragments is quite well described by the Coulomb explosion model for point charges, while the measured width of the distributions is much larger than that predicted by this simplified model. We therefore suggest a more elaborate model of Coulomb fragmentation based on a statistical description of the individual potential energy curves of the molecular ion This enables us to describe both the position and the width of the kinetic energy release distributions. We suggest that the main cause of the large width of the distributions for the high q values is the spread of the potential energy curves at the equilibrium internuclear distance; this is mainly due to the dispersion interaction arising from the mutual polarization of the atomic ions.