Impact-parameter-dependent stopping of swift ions

Abstract

Quantitative theory of the stopping of channeled ions needs impact-parameter-dependent electronic energy loss as input. In the previous paper, binary stopping theory has been expanded to provide such data primarily for the classical regime, i.e., for beam energies below 100Z12 keV/u. The theory incorporates the Barkas-Andersen effect, charge-dependent projectile screening and pertinent atomic properties. A stringent test on the scheme is channeling measurements under frozen-charge conditions, where minimum energy losses have been measured. We have applied the scheme to the stopping of B, C, N, O and F ions in the (111) planar channel of Au, and of F, Mg, Si, S and Cl ions in the [110] axial channel of Si. The Si target has been characterized in terms of atomic charge distributions for the three principal shells. The Au target has been characterized by atomic charge distributions for all subshells except the 6s electrons which are described in terms of a homogeneous electron gas. Better than 10% agreement has been achieved without the use of adjustable parameters with experimental data for 2 MeV/u ions in Au and 3 MeV/u in Si for all measured charge states. We find that a description in terms of the impact-parameter dependence is essential in the analysis, in particular with regard to the influence of the charge state. The discussion emphasizes the fact that unlike in random motion, the stopping force follows a q12 relation quite closely.