Critical Sizes against Coulomb Dissociation of Highly Charged Sodium Clusters Obtained by Ion Impact

The multiple ionization of alkali-metal clusters by low energy ions is investigated experientially for the first time. Multi-ionized clusters possibly undergoing Coulomb dissociation are formed and detected. Free sodium clusters of a few hundred atoms have been bombarded by different ion beams ( ${\mathrm{H}}^{+}$, ${\mathrm{O}}^{5+}$, ${\mathrm{Ar}}^{8+}$) of velocity ranging from $0.5\times {10}^8$ to $2.2\times {10}^8\mathrm{cm}/\mathrm{s}$. Critical sizes for stability against charge excess have been deduced for cluster charges up to 6. They are found to depend upon the projectile charge. Observed differences are discussed in terms of temperature effects.