Linear polarization of x-rays emitted in the decay of highly-charged ions via overlapping resonances

Abstract

View the article online for updates and enhancements. Related contentHigh-precision mass measurements for fundamental applications using highlycharged ions with SMILETRAP Sz Nagy, T Fritioff, I Bergström et al. Abstract. The linear polarization of x-rays, emitted from highly-charged ions, has been studied within the framework of the density matrix theory and the multiconfiguration Dirac-Fock method. Emphasis was placed especially on two-photon cascades that proceed via intermediate overlapping resonances. For such two-step cascades, we here explore how the level-splitting of the resonances affects the linear polarization of the x-rays, and whether modifications in the degree of polarization may help determine small level-splittings in multiply-and highly-charged ions, if carefully analyzed along isoelectronic sequences. Detailed calculations are carried out for the 1s2p 2 Ji = 3/2 → 1s2s2p J = 1/2, 3/2 + γ1 → 1s 2 2s J f = 1/2 + γ1 + γ2 radiative cascade of lithium-like W 71+ ions. For this cascade, a quite remarkable increase of the (degree of) linear polarization is found for the second-step γ2 photons, if the level-splitting becomes smaller than ∆ω 0.2 a.u. ≈ 5.4 eV. Accurate polarization measurements of x-rays may therefore be also utilized in the future to ascertain small level-splittings in multiply-and highly-charged ions.

Level sequence and splitting identification of closely spaced energy levels by angle-resolved analysis of fluorescence light

“…For different γ 1 photon energies of the incident light, the population of these levels is expected to differ relative to each other and so also the angular distribution of the fluorescence emission. Moreover, the (coherent) excitation of the two resonances also depends on the level splitting [20,21] and, this should thus become visible in the angular distribution as well. Figure 4 displays the anisotropy parameter β for the angular distribution of the (1s 2 2s2p 6 3s) 1 3p 3/2 J = 1/2, 3/2 → 1s 2 2s 2 2p 6 3s J f = 1/2 fluorescence emission of sodium as functions of the photon energy ω of the incident light.…”

“…2 2s J f = 1/2 + γ 1 + γ 2 of lithium-like tungsten, which proceeds via such overlapping intermediate resonances [20,21]. While, for an initially aligned 1s2p 2 J i = 3/2 level, a remarkably strong dependence was obtained for the second-step fluorescence photons upon the splitting of the two (overlapping) 1s2s2p J = 1/2, 3/2 resonances, no effect was found with regard to the sequence of these resonances due to the mutual cancelation of the sequence-dependent summation terms.…”

“…However, less attention has been paid to the influence of overlapping resonances upon the atomic fluorescence, in contrast to the autoionization of inner-shell excited atoms [17][18][19]. Only rather recently, we explored the angular and polarization properties of the emitted photons in the two-step radiative cascade 1s2p 2 J i = 1/2, 3/2 → 1s2s2p J = 1/2, 3/2 + γ 1 → 1s 2 2s J f = 1/2 + γ 1 + γ 2 of lithium-like tungsten, which proceeds via such overlapping intermediate resonances [20,21]. While, for an initially aligned 1s2p 2 J i = 3/2 level, a remarkably strong dependence was obtained for the second-step fluorescence photons upon the splitting of the two (overlapping) 1s2s2p J = 1/2, 3/2 resonances, no effect was found with regard to the sequence of these resonances due to the mutual cancelation of the sequence-dependent summation terms.…”