Shell effects observed in exit charge state distribution of 1-30 keV atomic projectiles transiting ultrathin carbon foils

We have investigated the exit charge state distribution of 1-30 keV H, He, C, N, O, Ne and Ar ions that transit ultrathin carbon foils (average thickness of approximately 44 Å). The empirical charge state distribution are used to examine the ratio of the electron loss cross section σ_l for neutrals to the electron capture cross section σ_c for positive ions. The ratio, σ/σ_c, is shown to have a quadratic dependence on the projectile velocity and inverse dependence on Z ^{2 3} where Z is the projectile atomic number. The data is fit to the equation A(E_F-E_T)/mZ ^{2 3} where E_F is the exit projectile energy, m is its mass, and A and E_T are constants. A pronounced shell effect is observed: the constant A is dependent on the principal quantum number of the projectile, and E_T exhibits variations with the projectile outer shell electronic structure.