PHYSICAL REVIEW A 83, 032702 (2011) Effects of polarization on laser-induced electron-ion recombination T. Mohamed, 1,* G. Andler, 2 M. Fogle, 1 E. Justiniano, 3 S. Madzunkov, 1 and R. Schuch 1,† 1 Department of Physics, Atomic Physics, Stockholm University, S-106 91 Stockholm, Sweden 2 Manne Siegbahn Laboratory, Stockholm University, S-10405 Stockholm, Sweden 3 Department of Physics, East Carolina University, Greenville, North Carolina 27858, USA (Received 16 September 2010; published 3 March 2011) The polarization dependence of laser-induced radiative recombination (LIR) to D + ions was investigated in the electron cooler of the CRYRING storage ring. The LIR gain as a function of wavelength into n = 3 principal quantum states of deuterium was measured at laser beam polarization angles of 0 ◦ and 90 ◦ with respect to the direction of the motional electric field in the interaction region. For the case of the polarization vector parallel to the external field, there is a double-peak structure in the gain curve that indicates a polarization effect in the LIR process. The two polarization directions also reveal a different width for the respective gain curves, giving additional evidence for the polarization effect, clearly seen by the behavior of a defined polarization parameter. The obtained polarization effect indicates a high sensitivity in recombination processes to external fields. DOI: 10.1103/PhysRevA.83.032702 PACS number(s): 34.80.Lx, 42.25.Ja I. INTRODUCTION In the universe, most matter exists in the plasma stage, i.e., it consists of free ions and electrons. Recombination of electrons with ions is balanced by electron-impact ionization processes and the emission of electromagnetic radiation is controlled by inelastic electron-ion collisions. When a free electron is captured into a bound state in the ion, with the simultaneous emission of a photon, it is called radiative recombination (RR). This results in a continuum of photon energies determined by the electron energy distribution and the final quantum state. The basic theory of radiative transitions predicts that the recombination rate can be enhanced by stimulated photon emission using an external photon field such as that provided by an intense, monochromatic laser. Such a process is referred to as laser-induced recombination (LIR). Considering the applied aspects and inherent fundamental processes in nature, it is important to study the dynamic behav- ior of recombination processes involving atomic constituents of matter. An in-depth knowledge of the physics governing these processes can also help us to prepare antimatter in the laboratory, e.g., for studying antihydrogen with high precision by recombining antiprotons with positrons [1]. In particular, LIR can be used to enhance weak recombination rates [2] by orders of magnitude compared to spontaneous RR [3] and also to prepare antihydrogen atoms in well-defined quantum states [4–6]. Heavy-ion storage rings equipped with electron coolers are powerful tools for measuring recombination processes at small relative energies. These electron coolers’ primary function is to reduce the stored ion beam’s emittance with a velocity-matched, cold-electron beam. The collinear electron beam also serves as an electron scattering target for near-zero energy electron-ion collisions for studies of recombination. * Permanent address: Physics Department, Faculty of Science, Beni- Suef University, Egypt. † Author to whom correspondence should be addressed: schuch@physto.se A disadvantage of electron coolers is that recombination photons are difficult to observe. This can, in part, be overcome by LIR, as a specific final state can be selected by setting the photon energy close to its binding energy. Stimulated photon emission is a resonant process, because it will only occur if the energy of the photons matches the energy difference E e − E n , where E e and E n are the energy of the electron and the energy of the bound state, respectively. The ratio between the induced recombination rate to the total spontaneous recombination rate over all final states n is referred to as the gain. In the spontaneous recombination rate, close to zero relative electron collision energy, an enhancement over the theoretical predictions, has been observed in measurements at ion cooler storage rings. This has drawn attention both theoretically [7–11] and experimentally [12–16]. Such an enhancement effect was also investigated for the LIR process and was found by Schramm et al.[17] for C 6+ . There, LIR shows a similar enhancement to that of spontaneous RR at low detuning energies of the merged ion and electron beams. LIR, therefore, represents an ideal tool to probe this effect with high resolution and additional parameters, such as polarization properties and energy detuning of a laser beam. In fact, an additional effect that could be connected to the enhancement is the large contribution below threshold observed in LIR for specific laser tunings to defined principal quantum states [18–21]. Three possibilities could cause this below threshold gain: (1) the formation of quasibound Rydberg states when the ions pass through the toroid magnet of the electron cooler, which merges the electron beam with the ion beam. There is a transverse motional electric field in this area of the electron cooler that acts as a pulsed field (∼ns). This field can induce electrons to be captured into high-lying states. The effect of such a field pulse has been used to explain the observed recombination enhancement [11,22]. Electrons in the high-lying states could be driven by the laser to a lower state and contribute thus to the below-threshold gain. A similar process has been observed in pulsed field recombination [23]. (2) The external electric fields in the interaction region of the electron cooler: The main sources of these external electric fields are (a) the field owing to the electron-beam space 032702-1 1050-2947/2011/83(3)/032702(6) ©2011 American Physical Society