Multiple-electron capture in close nearly symmetric ion-atom collisions

Cross sections for capture of two to five electrons are found to exceed the cross section for capture of a single electron for close collisions of 47 MeV Ca17+ ions with Ar atoms. Close collisions are selected by coincident detection of a Ca K or Ar K x-ray and a Ca ion that has undergone electron capture. The corresponding electron capture charge-state distribution can be fitted with a binomial distribution, which is consistent with independent electron capture and suggests an absence of electron correlation in the multiple-electron-capture process. Total electron capture cross sections, in which close collisions are not selected, are found to be much larger than the x-ray coincidence cross sections, and to decrease monotonically with increasing number of electrons captured. (author)