Charge exchange dependent ion stopping and scattering - Experiments, Ehrenfest Dynamics, and practical BCA simulations for LEIS

The ion charge state is a determining factor in slow ion stopping and scattering. Its dynamic evolution during scattering poses a challenge for various modeling approaches. We employ a molecular dynamics simulation for a charged Xe$^{12+}$ ion transmitting through graphene at different impact points where the charge density is self-consistently calculated in a time-dependent density functional theory framework (see Fig. 1 top) [1]. This Ehrenfest dynamics description uses a LCAO basis set [2] which enables fast simulations with reasonable accuracy even for ion charges as high as 12. From these experimentally-benchmarked simulations we find that the dynamic evolution of the charge density imposes a dynamic acceleration and deceleration of the ion, influencing nuclear and electronic stopping contributions in a complex way.

In order to deploy a practical simulation approach for charge changing scattering processes occurring in LEIS, we independently use an accelerated BCA simulation based on IMINTDYN which is decomposed into scattered ion sub-spectra for single, dual, and multiple scattering [3]. Fitting a sum of these three sub-spectra to a measured LEIS spectrum allows to extract the charge fraction and stopping contributions at close collisions. Using this approach, LEIS spectra can be fully simulated by exploiting the full spectral information beyond the single scattering peak for better LEIS quantification in the future.