Calculations of X-Ray Spectrum in the Recombination Processes of Electrons into Bare Uranium Ions

Using the fully relativistic Dirac wave functions combining with the channel properties, the radiative recombination (RR) processes of electron into bare uranium (U92+) are investigated in the relative energy approaching to zero, and the X-ray spectrum emitted from the direct RR and sequential cascade processes is simulated. It is found that the direct RR spectrum for capture to the ground and low excited states is in a good agreement with the experimental measurements, but there exist significant discrepancies in the cascade spectrum produced in captures to the high Rydberg states. We also find that the shorter the measurement duration is, the smaller the discrepancies. These results show that the enhancement of RR rate coefficients in the ultra-low energy (< 1 meV) found in numerous experiments mainly comes from the processes for capture to the high Rydberg states. Qualitatively, this conclusion is in harmony with the field-induced recombination (FIR) model proposed recently by Horndl et al. [Phys. Rev. Lett. 95 (2005) 243201], but the theoretical intensity ratios between Ly-alpha(1), and Ly-alpha(2) and between Ly-beta(1) and Ly-beta(2), which are calculated with the FIR effects considered, are in conflict with the measured data. Further experimental and theoretical studies are expected.