Target effects on the linear polarization of REC radiation from U91+ ions

In the last decades, the photon emission from highly charged ions has been found a versatile tool for exploring light-matter interactions in strong (Coulomb) fields [1]. Apart from a series of energy and angle resolved experiments, especially for the x-ray emission following the radiative electron capture (REC) into high-Z projectile ions [2], recent emphasis was placed on measuring the linear polarization of this radiation [3], by making use of recent advances in detector technology [4]. However, while most REC calculations for high-Z projectiles are based so far on first-order perturbation theory in the electron-photon interaction, a rigorous analysis of this capture process would require, in general, a timedependent, two-center Dirac description. Of course, such an extensive description is well beyond of what is presently feasible, but it appears at least necessary for angle-resolved and polarization measurements to take into account the structure of the target atoms. This will be true especially if impact-parameter dependent measurements are to be performed in forthcoming years. Therefore, in order to improve our understanding of the target effects upon the REC (linear) polarization from high-Z ions, we applied the impulse approximation as appropriate for fast collisions of heavy projectiles with lowand medium-Z targets [5]. Within the impulse approximation, the collision of the projectile with the target atom is still described in perturbation theory but asymmetrically by introducing an explicit momentum distribution for the target electrons. In this approximation, therefore, the initial state of the captured electron is expressed as