Prominent Higher-Order Contributions to Electronic Recombination

Intershell higher-order (HO) electronic recombination is reported for highly charged Ar, Fe, and Kr ions, where simultaneous excitation of one $K$-shell electron and one or two additional $L$-shell electrons occurs upon resonant capture of a free electron. For the mid-$Z$ region, HO resonance strengths grow unexpectedly strong with decreasing atomic number $Z$ ($\propto Z^{-4}$), such that, for Ar ions the 2nd-order overwhelms the 1st-order resonant recombination considerably. The experimental findings are confirmed by multiconfiguration Dirac-Fock calculations including hitherto neglected excitation pathways.

Figure 1

Diagrams for $K\mathrm{\text{−}}L$ intershell resonant recombination of 1st, 2nd, and 3rd order—dielectronic (DR), trielectronic (TR), and quadruelectronic (QR) recombination [blue (left), red (middle), and green (right)], respectively.

Figure 2

Experimental photorecombination spectrum of Fe ions for Li-like to O-like species compared to our MCDF calculations. Top: resonance energies for the different recombination orders. Bottom: IS color coded for this spectrum. The region of C-like resonances is indicated.

Figure 3

Comparison of the C-like recombination lines for Ar, Fe and Kr ions. DR, TR and partially QR lines are marked by blue, red and green colors, respectively. The energy scale of the spectra is normalized by $Z^2$.

Figure 4

Ratio of TR to DR resonance strengths for C-like ions versus atomic number $Z$, comparison between experiment and theory—left-side ordinate. Additionally, the calculated DR and TR resonance strengths normalized to the maximum of the DR strengths are shown (right-side ordinate).

Figure 5

Ratio of HO strengths to the 1st-order ones for Fe ions. Red color for the TR/DR ratio (theory: red bars; exp.: red triangles) and green color for the QR/DR ratio (theory: green bars; exp.: green diamonds).