Slow isocharged sequence ions with helium collisions : Projectile core dependence

The collisions of the isocharged sequence ions of q=6 (C{sup 6+}, N{sup 6+}, O{sup 6+}, F{sup 6+}, Ne{sup 6+}, Ar{sup 6+}, and Ca{sup 6+}), q=7 (F{sup 7+}, Ne{sup 7+}, S{sup 7+}, Ar{sup 7+}, and Ca{sup 7+}), q=8 (F{sup 8+}, Ne{sup 8+}, Ar{sup 8+}, and Ca{sup 8+}), q=9 (F{sup 9+}, Ne{sup 9+}, Si{sup 9+}, S{sup 9+}, Ar{sup 9+}, and Ca{sup 9+}) and q=11 (Si{sup 11+}, Ar{sup 11+}, and Ca{sup 11+}) with helium at the same velocities were investigated. The cross-section ratios of the double-electron transfer (DET) to the single-electron capture (SEC) {sigma}{sup DET}/{sigma}{sup SEC} and the true double-electron capture (TDC) to the double-electron transfer {sigma}{sup TDC}/{sigma}{sup DET} were measured. It shows that for different ions in an isocharged sequence, the experimental cross-section ratio {sigma}{sup DET}/{sigma}{sup SEC} varies by a factor of 3. The results confirm that the projectile core is another dominant factor besides the charge state and the collision velocity in slow (0.35-0.49v{sub 0}; v{sub 0} denotes the Bohr velocity) highly charged ions (HCIs) with helium collisions. The experimental cross-section ratio {sigma}{sup DET}/{sigma}{sup SEC} is compared with the extended classical over-barrier model (ECBM) [A. Barany et al., Nucl. Instrum. Methods Phys. Res. B 9, 397 (1985)], the molecular Coulombic barriermore » model (MCBM) [A. Niehaus, J. Phys. B 19, 2925 (1986)], and the semiempirical scaling laws (SSL) [N. Selberg et al., Phys. Rev. A 54, 4127 (1996)]. It also shows that the projectile core properties affect the initial capture probabilities as well as the subsequent relaxation of the projectiles. The experimental cross-section ratio {sigma}{sup TDC}/{sigma}{sup DET} for those lower isocharged sequences is dramatically affected by the projectile core structure, while for those sufficiently highly isocharged sequences, the autoionization always dominates, hence the cross-section ratio {sigma}{sup TDC}/{sigma}{sup DET} is always small.« less