Charge-dependent electronic stopping of swift nonrelativistic heavy ions

The stopping of swift nonrelativistic heavy ions has been analyzed theoretically as a function of the projectile charge $q_1.$ Excitation and ionization of target electrons are described within Bohr’s classical theory and the limitations toward Bethe’s quantal description are outlined. For oxygen ions in carbon the Bethe regime is found to be confined to the velocity range where the ion is essentially stripped in charge equilibrium. The effect of projectile electrons is taken into account mainly via screening of the Coulomb interaction, but excitation of projectile electrons is discussed briefly. The significance of projectile screening hinges on a parameter $s$ that depends on projectile and target parameters and varies substantially over the electron shells of the target. Calculated fixed-charge stopping cross sections agree well with measured values in absolute magnitude and in their variation with charge state. $q_1^2$ scaling frequently has been assumed in the literature. A continuous transition is predicted between such scaling and a stopping power that is almost independent of the projectile charge.