Negative helium ions exiting a carbon foil at keV energies - art. no. 155416

He- is observed in the exit charge state distributions of He transmitted th rough a thin carbon foil over an energy range of 8-80 keV. The observed exi t fraction of He- must be formed in the bulk foil or within several Angstro ms of the exit surface and reaches a maximum of 2.6 x 10(-4) at 0.7 upsilon (0) where upsilon (0) is the Bohr velocity. The probability of forming the observed He- from its source population of He+ by two sequential electron- capture events decreases exponentially with increasing velocity. This behav ior is similar to that of He transiting a Cs vapor, for which this dependen ce is the result of individual cross sections that collectively drive the c harge-state distribution toward higher positive-charge states with increasi ng projectile velocity. No isotope effect in the exit charge state distribu tions of He-3 and He-4 is observed within experimental error.