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Photon Polarization in Radiative Recombination of Bare High – Z Ions

The radiative recombination (RR) and radiative electron capture are two well–established processes from the study of electron–ion and atom–ion collisions. In these processes, a free or quasi–free electron is captured into a bound state of the ion, accompanied by the simultaneous emission of a photon. For bare high–Z ions, for example, the radiative recombination has been explored in detail by Stöhlker and co–workers [1] at the GSI storage ring during recent years. So far, however, most experiments were focused on measuring the total and angle–differential recombination cross sections which are typically found in good agreement with theoretical predictions, based on Dirac’s theory [2]. Much less attention, in contrast, has been paid previously to the polarization of the emitted radiation, mainly because of the lack of efficient polarization detectors for hard x–rays. Only in the light of recent improvements in detector techniques, the observation of the photon polarization has become more likely within the next few years. To predict the polarization of the photons which are emitted in the recombination of free electrons into a bound state of the bare ions, theoretical studies have been carried out in the framework of the density matrix approach [3]. In the past, this approach has been utilized successfully in a large number of case studies on the polarization as well as correlation phenomena in many areas of the nuclear and atomic physics. In fact, this formalism provides us with a tool of great elegance in the theoretical analysis of collision and decay processes. In the framework of the density matrix theory, the photon polarization is directly related to three Stokes parameters (Pi, i=1..3). While the first two parameters are usually used for the characterization of the linear polarization PL = √ P 2 1 + P 2 2 , the third Stokes parameter describes the circular polarization of the photons PC = P3 [4]. As for the computation of cross sections, these parameters are closely connected to the matrix element