Fragment ion distribution in charge-changing collisions of 2-MeV Si ions with C₆₀

We have measured positive fragment ions produced in collisions of 2 MeV Si^q+ (q=0, 1, 2, 4) projectiles with a C₆₀ molecular target. The measurement was performed with a time-of-flight coincidence method between fragment ions and charge-selected outgoing projectiles. For all the charge-changing collisions investigated here, the mass distribution of small fragment ions C_n⁺ (n=1–12) can be approximated fairly well by a power-law form of n^-λ as a function of the cluster size n. The power λ derived from each mass distribution is found to change strongly according to different charge-changing collisions. As a remarkable experimental finding, the values of λ(loss) in electron loss collisions are almost the same for the same final charge states k irrespective of the initial charge q, exhibiting a nearly perfect linear relationship with k. We also performed calculations of the projectile ionization on the basis of the semiclassical approximation and obtained inelastic energy deposition for individual collision processes. The estimated energy deposition is found to have a simple correlation with the experimentally determined values of λ(loss).