Energy loss and charge exchange: Statistics and atomistics

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This is a brief summary of recent work by the authors on (I) statistical aspects of the interplay between charge exchange and electronic energy loss and (II) atomistics of charge-dependent stopping of heavy ions. (I) A matrix formalism has been developed that specifies energy-loss spectra differential in entrance and exit state. Examples refer to a simple two-state system where experimental results exist. The precise definition of energy loss needs attention when particles change identity. (II) The relation between stopping cross section and ion charge has been studied on the basis of the Bohr theory. A dimensionless parameter has been identified which governs the significance of projectile screening, indicating that the stopping cross section for target excitation is as sensitive to the target as to the projectile. The contribution of projectile excitation to electronic stopping is discussed qualitatively. Comparison with measured fixedcharge stopping cross sections shows good to satisfactory agreement.