Anisotropic $LMN$ dielectronic resonances from ratios of magnetic-dipole lines

Signatures of multi-keV $LMN$ dielectronic resonances in highly charged $3d^n$ ions of tungsten were detected in the intensity ratios of extreme-ultraviolet magnetic-dipole lines within ground configurations. The measurements were performed with an electron beam ion trap at beam energies of about 6 keV. Large-scale collisional-radiative modeling incorporating magnetic sublevels of autoionizing levels showed the significance of anisotropy effects due to the monodirectional propagation of the electron beam. The observation method allows simultaneous resolved registration of dielectronic resonances from several ions.

Figure 1

Measured spectrum (in analog-to-digital units of CCD) at the nominal beam energy of 6057 eV. The strongest lines of tungsten are identified by their isoelectronic sequences. A strong impurity line at 15.02 nm due to emission from oxygen and xenon ions is shown by an arrow.

Figure 2

Calculated resonance strengths (in s${}^{-1}$) for the $LMn$ ($n=3–5$) resonances in Ca-like W. Arrow shows the approximate range of beam energies where both Ca-like and K-like ions are abundant. The labels show the quantum numbers for the $L$ electron and the recombined free electron. For instance, $2s_{+}3d$ corresponds to the $2s2p^{6}3s^{2}3p^{6}3d^3$ configuration.

Figure 3

Top: circles, measured intensity ratio of the 17.080-nm line in W${}^{54+}$ and the 15.962-nm line in W${}^{55+}$; dot-dashed line, simulation with $m$-averaged autoionizing levels (AL) assuming isotropic collisions; dashed line, simulation with $m$-averaged AL and effective anisotropic corrections; solid line, simulation with $m$-resolved AL; dotted line, simulation without AL/DR. Both the energy uncertainty of $\pm 22$ eV for all data points and the line ratio uncertainties correspond to $2\sigma$. Bottom: resonance strengths for 786 autoionizing states $2s^{2}2p^{5}3s^{2}3p^{6}3d^{2}4l$ in W${}^{54+}$.

Figure 4

Measured line ratios for the Ca-like to K-like ions (top), Sc-like to Ca-like ions (middle), and Ti-like to Sc-like ions (bottom). The theoretical ratios calculated with the anisotropic collisional-radiative model are shown by solid (FWHM = 50 eV) and dashed (FWHM $=$ 40 eV) lines in the top panel. The calculated relative strengths for the $L\left(2s_{1/2}\right)MM$ (blue) and $L\left(2p_{3/2}\right)MN$ (black) resonances are shown by vertical lines in the middle and bottom panels. The dotted lines are shown for an eye guide. Both the energy uncertainty of $\pm 22$ eV for all data points and the line ratio uncertainties correspond to $2\sigma$.

Figure 5

Measured x-ray emission (in arbitrary units) between 7.5 keV and 12 keV registered with a Ge solid state detector. A: $2p$-$4l$ transitions, B: $2p$-$3d$ transitions, C: $2p$-$3s$ transitions.