Systematic theoretical calculations on the polarization properties of x-rays emitted in $K$-shell radiative recombination (RR) into bare ions are carried out, with particular emphasis on the role of spin-flip transitions in $\mathrm{Kr}^{36+}, \mathrm{Sn}^{50+}, \mathrm{W}^{74+}$, and $\mathrm{Bi}^{83+}$ ions. The calculated linear polarization of $K$-shell RR x-rays from bare $\mathrm{Kr}^{36+}$ ions with spin-flip contribution shows good agreement with previous measurement performed at the Tokyo electron-beam ion trap [Rev. Sci. Instrum. 92, 063101 (2021)], demonstrating the necessity of accounting for spin-flip contributions to the angle-differential cross sections. This agreement provides clear identification of spin-flip contribution to the polarization of RR x-rays for the lightest bare ions studied so far. In addition, we show that spin-flip contribution to the linear polarization of the $K$-shell RR x-rays increases significantly with both incident electron energy and atomic number $Z$. This trend suggests that further measurements of the linear polarization of $K$-shell RR x-rays by varying the electron energy and atomic number $Z$ may offer enhanced sensitivity to spin-flip transitions.

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• Atomic & molecular collisions