Intra-atomic Double Scattering of Binary Encounter Electrons in Collisions of Fast Heavy Ions with Atoms and Molecules

Observation of double differential electron emission spectra after ion-atom collisions has proven to be a major tool in the understanding of complex as well as fundamental ionization processes. Aside from the possibility to improve theories of ion-atom interactions in comparison of predictions and experimental data, interest for electron emission cross sections is especially widespread in applied particle physics such as radiobiology and condensed matter physics. About 2y3 of the energy loss of the ion is transferred into kinetic energy of the d electrons [1] (electronic stopping). The maximum possible momentum transfer is received by the so-called binary encounter electrons (BEe), which are produced in hard, knock-on collisions between the ion and the target. They are therefore responsible for radiation effects in biological or other material in regions far from the primary ionization events [2] leading to induction of latent tracks. In this context, our observation of high energy electrons emerging under large angles from single-ion atom or molecule collisions described in this Letter are not only of significant importance for basic scattering theory. Moreover, the treatment planning for the hadron cancer therapy at GSI and elsewhere relies on the calculation of relative biological efficiencies [3] on the basis of track structure models. In the classical impulse approximation (IA) [4], the BE electrons are described as being elastically scattered by the projectile potential in the center-of-mass (CM) system, thus being a simple two-body interaction [5]. The target nucleus determines only the properties of the electron initial state, i.e., their binding energy and momentum distribution (Compton profile). Theoretical treatment in this framework has shown excellent agreement between theory and experiment for specific collision systems. To a good approximation, the CM system can be considered to be the projectile rest frame, due to the large mass difference. In the rest frame, the target electrons travel at the speed of yP before and after the collision and are scattered into an angle ue between 0 ‐ and 180 ‐ as a function of the impact parameter. For comparison with experimental cross sections, the results of IA calculations have to be transformed to the laboratory system. For bare ion impact on light targets like H2 and He, only slight deviations have been found between experimental data and predicted cross sections [6]. According to the Rutherford scattering formula, the BE production cross section for fast, light ion impact was found to scale with