The delocalized state of the electrons is shown by the dominant fragmentation channels in the dissociation of such objects after a multi-ionization, where the charge deficit tends to be evenly distributed on the fragments. Thus, for a diatomic molecule (N₂ or CO for example), the paths leading to the production of atomic fragments of equal charges will always be preferred. For the very same reasons, the fragmentation of large size clusters (C₆₀, Na_n, ...) leads mainly to singly charged fragments.

These questions are legitimate, since it is also shown that the mobility of charges is an important factor conditioning the susceptibility of materials to irradiation. Thus, damages depend heavily on their character of conductor or insulator.

The role of the mobility of charges in simple molecular systems has been investigated by a Franco-Japanese collaboration, including the CIMAP group. The study focused on clusters of rare gas consisting of only a few atoms. These aggregates, weakly linked by van der Waals interactions (i.e. dipole-dipole interaction) are produced in a supersonic beam. The soft ionization without any transfer of vibrational energy is obtained very efficiently by collisions with multiply charged ions of low energy (Ar⁹⁺ at 152 keV), provided by the ARIBE beam line in GANIL. The rapid passage of the multicharged ion, close to the target, induces the ionization by capture of outer electrons (ie valence e^-).

The details of the fragmentation dynamics (by relaxation of the system) and the measure of the charges have been obtained by the spectrometer COLTRIMS (Cold Target Recoil Ion Momentum Spectroscopy) of CIMAP. This device uses imaging techniques, time of flight measurements and multi-coincidence detection (Figure 1). It gives access to the number of electrons extracted from the system during the collision, the identification of each fragment (charge and mass) together with the measurement of their kinetic energy after the "explosion" of the cluster.

In this context, the recent study of the fragmentation of argon dimers (Ar₂) led to a surprising observation: the kinetic energy of the fragments is higher than the expected value for dissociation into two fragments with the same charge (Fig. 2). Instead, the measured values show that the electrons are pulled out from only one of the two atoms. Then the system dissociates by an excimer process (specifically a donor-acceptor complex process). This localization of the charge reflects the low electron mobility in the system and is also observed on the relative statistical weight of the different fragmentation channels (Figure 3). In the case of Ar₂⁴⁺, the asymmetric channel (Ar⁺ + Ar³⁺) is dominant whereas it is only around 10% of the symmetric channel, for covalent systems (N₂⁴⁺ → N²⁺ + N²⁺).