Strong Polarization of a $J=1/2$ to $1/2$ Transition Arising from Unexpectedly Large Quantum Interference

We experimentally show that the $1s^{2}2s^{2}2p_{1/2}-1s2s^{2}2p_{1/2}^2$ transition in ${\mathrm{Pb}}^{77+}$ emitted in dielectronic recombination of ${\mathrm{Pb}}^{78+}$ is strongly polarized, although it is an intrinsically unpolarized $J=1/2$ to $1/2$ transition. This unanticipated polarization is shown to be due to quantum interference with radiative recombination. The interference effect has been studied on an asymmetric resonance profile but has never been studied on polarization. In this Letter, we show that the effect on polarization can arise from a different cross term than that responsible for asymmetry, resulting in unexpectedly large polarization even for a nearly symmetric resonance suggesting a small interference.

Figure 1

Two-dimensional plot of x-ray spectra from highly charged Pb ions as a function of the electron beam energy (bottom panel). The two diagonal lines correspond to RR into the $j=1/2$ ($2s_{1/2}$ and $2p_{1/2}$) and $j=3/2$ ($2p_{3/2}$) orbitals of He- to F-like Pb ions. The top panel shows the theoretical DR cross sections calculated with the flexible atomic code [45]. The arrows with “on” and “off” represent the energies where the polarization measurement was performed (see the text for details). The middle panel shows the experimental DR spectrum (black squares) obtained by integrating the two-dimensional data along the RR line for $j=1/2$ and the fitted Gaussian functions (red, blue, and green lines). The experimental electron energy scale was normalized to the theoretical resonance energy for Li-like Pb.

Figure 2

X-ray spectra obtained with (a) a Ge detector and (b) EBIT-CC. The red and black curves are the spectra for the on and off periods, respectively. Notably, the off spectrum is shifted by $-170\mathrm{eV}$ (electron energy difference between the on and off periods). The blue curve in (b) represents the DR x-ray spectrum obtained by subtracting the off spectrum (shifted by $-170\mathrm{eV}$) from the on spectrum. The magenta solid line is the simulated spectrum for monoenergetic 73.7 keV x rays.

Figure 3

(a) Atomic number dependence of the degree of linear polarization for pure RR (magenta), pure DR (black), and total recombination including interference between DR and RR (blue). The red solid curve represents $P_{\mathrm{int}}^{\mathrm{DR}}$ defined in Eq. (8). $P_{\mathrm{int}}^{\mathrm{DR}}$ calculated by assuming $\mathrm{\Delta }\delta ={\delta }_s-{\delta }_d=0$ is plotted by the dashed red line, which overlaps with the black line at $P=0$. The present experimental result is plotted by the red square. (b) Atomic number dependence of $|a_{\mathrm{DR}}/b_{\mathrm{RR}}|$ (red, left axis) and $\mathrm{\Delta }\delta$ (blue, right axis).