Charge transfer between alkali cluster ions and atoms in the 1 to 10 keV collisional energy range

The cross-sections for collisional charge transfer between singly charged free clusters M$_n^+$ (M = Li, Na; n=1...50) and atomic targets A (cesium, potassium) have been measured as a function of collisional relative velocity in laboratory energy range 1-10 keV. For each cluster size, the experimental values of the charge transfer cross-section $\sigma(v)$ are fitted with an universal parametric curve with two independent parameters $\sigma_m$ and vm, the maximum cross-section and the corresponding velocity. For small size clusters ($n \leq 15$), the $\sigma (v)$ characteristic parameters show strong variations with the number of atoms in the cluster. Abrupt dips observed for n=10 and n=22 are attributed to electronic properties. Charge transfer patterns observed for various collisional systems present similarities, which appear more sensitive to cluster quantum size effects than to collision energy defects. In their whole, the $\sigma_m$ and vm parameters show differences in both their size evolution and their absolute values discussed in term of projectile and target electronic structures