High Resolution Photoexcitation Measurements Exacerbate the Long-Standing Fe XVII Oscillator Strength Problem

For more than 40 years, most astrophysical observations and laboratory studies of two key soft x-ray diagnostic $2p-3d$ transitions, $3C$ and $3D$, in Fe XVII ions found oscillator strength ratios $f(3C)/f(3D)$ disagreeing with theory, but uncertainties had precluded definitive statements on this much studied conundrum. Here, we resonantly excite these lines using synchrotron radiation at PETRA III, and reach, at a millionfold lower photon intensities, a 10 times higher spectral resolution, and 3 times smaller uncertainty than earlier work. Our final result of $f(3C)/f(3D)=3.09(8)(6)$ supports many of the earlier clean astrophysical and laboratory observations, while departing by five sigmas from our own newest large-scale ab initio calculations, and excluding all proposed explanations, including those invoking nonlinear effects and population transfers.

Figure 1

(Top) Experimental setup: an electron beam (orange) aimed at the trap center produces Fe XVII ions, which are then resonantly excited by a monochromatic photon beam (red). Subsequent x-ray fluorescence is registered by a silicon drift detector. (Bottom) Fluorescence yield and photon energy vs time with three different methods: (1) line scans with open photon shutter; (2,3) by closing it at each step (purple areas), we subtract the electron-beam-induced background at (2) each line center and (3) scanning over a one-full-width-at-half-maximum range.

Figure 2

Fluorescence photon yield and energy vs excitation-photon energy for the Fe XVI $C$, Fe XVII $3C$, and $3D$ transitions recorded by a silicon-drift detector. Black dots: total fluorescence within a 50-eV region of interest. Red solid lines: Fits to $3C$ and $3D$. Red dashed line: fit to $C$.

Figure 3

Present experimental $3C/3D$ ratios compared with previous predictions and experiments. Red band: combined results of the three different methods. Blue open circles: values from databases [51, 52, 53]. Blue crosses: predictions [29, 31, 32, 35, 36, 54, 55, 56, 57]. Note that the validity of theory [36] has been disputed [37]. Blue solid circles: present FAC [58], large-scale CI [39], MR-MCDHF [40], and AMBiT [41] calculations. Blue band: observed line ratios in astrophysical sources [3, 4, 7, 59, 60, 61], with color shades coding the distribution of values weighted by their reported accuracies. Purple band: spread of tokamak results [20]. Open green diamonds: previous EBIT results [18, 21, 25]. Note that the spread seen in various astrophysical sources and in tokamak in part arises from insufficient removal of Fe XVI $C$ line contamination of Fe XVII $3D$ line, at varying Fe XVI/Fe XVII abundance ratios [6, 20]; nonlinear dynamical effects [32, 34, 38] (see main text) may have reduced the LCLS ratio [25].