[en] The current density effects for fcc, bcc and hcp lattice structures like Al, Ta and Zr bombarded by 3-9 keV Cd⁺ ions have been studied. Our observations support the idea that ionization takes place in vacuum outside the target surface. A semi-empirical formula is also proposed which takes into effect both the low and high current density effects and the results of the measurements have been explained in terms of the formula. (orig.)