Charge-state equilibration length of a highly charged ion inside a carbon solid

The penetration of 576-keV Arq+ ions with initial charge states qᵢ=8,12,16 through a carbon foil of 310 {} thickness has been investigated. The projectile's energy loss and final charge-state distribution have been measured. Within a resolution of a few percent no dependence of these data on the projectile's initial charge state was found. Furthermore the relative yield of Auger electrons emitted by the projectile before and after the foil was detected. From these yields we could derive that the Ar ions quickly reach an equilibrium excitation state inside the foil that does not depend on the initial charge state qᵢ. From the measured energy-loss data we derive an upper limit for the projectile's average charge-state equilibration length xₑ inside the foil of approximately one carbon layer. This result implies extremely large cross sections for electron transfer mechanisms between projectile and solid. An estimation of this cross section by using the classical over-barrier model hardly explains the observed, very large charge-exchange cross sections between ion and solid.