Photoionization measurements for the iron isonuclear sequence Fe${}^{3+}$, Fe${}^{5+}$, and Fe${}^{7+}$

Cross sections for single photoionization of Fe${}^{3+}$, Fe${}^{5+}$, and Fe${}^{7+}$ ions were measured at spectral resolutions of 0.04, 0.15, and 0.13 eV, respectively, by merging mass- and charge-selected ion beams with a beam of monochromatized synchrotron undulator radiation. The measurements span photon energy ranges beginning at the ionization thresholds and extending several tens of electron volts to include the most important resonant contributions due to $3p$–$\mathit{nd}$ transitions to autoionizing states. The photoion yield spectra are characterized by narrow resonances and also broad features in Fe${}^{3+}$ and Fe${}^{5+}$ that are believed to result from unresolved fast super-Coster-Kronig transitions following $3p$–$3d$ excitation. Absolute photoionization cross-section measurements were also performed using ion beams containing undetermined fractions of ions in their ground and metastable states. A Rydberg series attributed to $3p$–$\mathit{nd}$ transitions from the ${}^2{G}_1$ metastable state in Fe${}^{3+}$ was identified. The data are compared with recently published measurements on Fe${}^{3+}$ and Fe${}^{5+}$ using a similar technique at lower spectral resolution.

Figure 1

Cross-section measurements for single photoionization of Fe${}^{3+}$. The upper panel shows the present measurements at ALS in the energy range 38–84 eV at a fixed photon energy resolution of 0.04 eV. Absolute measurements are shown by open circles with error bars, to which the energy scan data are normalized. The vertical dashed line represents the tabulated ionization threshold of the ${}^6{S}_{5/2}$ ground state at 54.80 eV. The lower panel shows the absolute cross section measured at ASTRID in the same energy range at a photon energy resolution ranging from 0.14 to 0.62 eV [20].

Figure 2

Photoionization of Fe${}^{3+}$ in the region below the ground-state threshold on an expanded energy scale showing the Rydberg series attributed to autoionizing transitions from $3d$${}^5$(${}^2{G}_1$) to $3d$${}^4$(${}^1{G}_1$)$\mathit{np}({}^2{F}^0)$, where $n$ is the principal quantum number starting from $n=7$. The spectroscopic energy limit of this Rydberg series is 53.36 eV [29] (indicated by the vertical dashed line).

Figure 3

Rydberg fits for the $3d^4({}^1{G}_1)\mathit{np}({}^2{F}^0)$ series from the ${}^2{G}_1$ metastable state of Fe${}^{3+}$. The solid curve was obtained by including the bound state ($n=4$) transition [29] (indicated by the open circle) in the fit, while the dashed curve was obtained without this bound transition.

Figure 4

Plot of quantum defects ${\delta }_n$ vs principal quantum number $n$ for the Rydberg series [Ar]$3p^4({}^1{G}_1)\mathit{np}({}^2{F}^0)$ from the ${}^2{G}_1$ metastable state of Fe${}^{3+}$. The open circle represents the transition to the $n=4$ bound state [29]. The dotted line just connects the data points.

Figure 5

Comparison of measurements of the resonance corresponding to the transition $3d$${}^5$(${}^2{G}_1$) to $3d^{4}7p({}^2{F}^0)$ in Fe${}^{3+}$ at experimental energy resolutions $\Delta E$ of 40 and 7.5 meV. The curve is a fit of Voigt profiles to the two resonance features.

Figure 6

Single photoionization of Fe${}^{5+}$ in the energy range 87–142 eV. The upper panel shows the present ALS measurements made at a fixed spectral resolution of 0.15 eV. Absolute cross-section measurements are shown by open circles with error bars, to which the energy scan data are normalized. The vertical dashed line represents the tabulated ionization threshold of the ${}^4{F}_{3/2}$ ground state at 99.063 eV. The lower panel shows the photoionization cross section measured at ASTRID at lower spectral resolution in the same energy range [20].

Figure 7

Absolute cross-section measurement for single photoionization of Fe${}^{5+}$ on an expanded energy scale in the range where the ionization thresholds of the eight [Ar]$3d$${}^3$ terms are expected [29] (indicated by dashed vertical lines). The numbers labeling these thresholds are identified in Table , with 1 representing the ${}^4{F}_{3/2}$ ground state.

Figure 8

Absolute cross section for photoionization of Fe${}^{7+}$ in the energy range 150–172.5 eV measured at a spectral resolution of 0.126 eV. Absolute cross-section measurements are shown by open circles with error bars, to which the energy scan data are normalized. The vertical dashed lines represent the ionization thresholds of the ${}^2{D}_{3/2}$ ground state and the ${}^2{D}_{5/2}$ metastable state.

Figure 9

Measurements of single photoionization of Fe${}^{7+}$ on an expanded energy scale in the region of the [Ar]$3d^2{D}_{3/2}$ ground state and ${}^2{D}_{5/2}$ metastable state ionization thresholds. The vertical lines indicate the tabulated values in the NIST atomic spectra database [29]. Solid circles represent the energy-scan measurements and open circles are the same measurements to which five-point Savitsky-Golay smoothing [46] was applied to reduce the statistical fluctuations.

Figure 10

Comparison of absolute cross-section measurements for photoionization of Fe${}^{7+}$ with the TOPbase theoretical data, shifted by$+$4.059 eV to match the ground-state ionization potential with the NIST tabulated value and convoluted with a Gaussian of FWHM$=$0.126 eV to simulate the experimental energy resolution.