Resonant and Near-Threshold Photoionization Cross Sections of ${\mathrm{Fe}}^{14+}$

Photoionization (PI) of ${\mathrm{Fe}}^{14+}$ in the range from 450 to 1100 eV was measured at the BESSY II storage ring using an electron beam ion trap achieving high target-ion area densities of ${10}^{10}{\mathrm{cm}}^{-2}$. Photoabsorption by this ion is observed in astrophysical spectra and plasmas, but until now cross sections and resonance energies could only be provided by calculations. We reach a resolving power $E/\Delta E$ of at least 6500, outstanding in the present energy range, which enables benchmarking and improving the most advanced theories for PI of ions in high charge states.

Figure 1

Setup for PI measurements using synchrotron radiation and an EBIT equipped with ion extraction. HCIs produced and trapped by a compressed electron beam form a cloud with an area density of $\approx {10}^{10}{\mathrm{m}}^{-2}$ (e.g., ${\mathrm{Fe}}^{14+}$) for axial overlap with a synchrotron beam. Both target ions and photoions are extracted, charge selected, and counted with a position sensitive detector (PSD).

Figure 2

Yield of ${\mathrm{Fe}}^{15+}$ ions versus interaction time with (green triangles) and without (red squares) photon beam. The difference of both curves gives the PI yield (blue dots). The red area above represents the EII yield from metastable $({\mathrm{Fe}}^{14+}{)}^{*}$ ions. (273 and 112 measuring cycles were added with and without photon beam, respectively.)

Figure 3

Photoionization spectra for ${\mathrm{Fe}}^{14+}$ (detected by the ${\mathrm{Fe}}^{15+}$ yield). Peaks are due to autoionizing states resonantly excited by the photon beam. Experiment (${\mathrm{Fe}}^{15+}$ yield, cross section), green step curve or circles, fits: gray line, theories (resonance strength): RMBPT Gu et al. [9]: blue circles; HULLAC and MCDF both this work: brown triangles and orange diamonds, respectively. (c) overview; (a), (b), and (d) details; (e) ionization edge and second-order photon peaks.