Fritzsche, S.

Gumberidze, A.

Stöhlker, T.

Surzhykov, A.

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Magnetic interactions and retardation in the electron emission fromhighly-charged ionsS. Fritzsche1,2,3, A. Surzhykov∗1,2,4, A. Gumberidze1,5,6, and T. Sto¨hlker1,2,31GSI, Darmstadt, Germany; 2Helmholtz Institute Jena, Germany; 3University of Jena, Germany; 4University ofHeidelberg, Germany; 5FIAS, Frankfurt, Germany; 6EMMI, Darmstadt, GermanyX-ray studies from multiple and highly charged ionshave been found a unique tool for exploring the electron-electron (e-e) and electron-photon interactions in the pres-ence of strong fields [1]. The x-ray spectroscopy of suchsystems have demonstrated for a long time that accurateenergies and cross sections are obtained only if, apartfrom the static Coulomb repulsion among the electrons,the magnetic interactions and retardation as well as leadingquantum-electrodynamical effects are taken into account.In contrast to the spectrocopy of hard x-rays, however,much less is known how relativistic interactions among theelectrons affect their emission and, hence, the dynamics ofelectrons in strong fields.To obtain further insight into the strong-field dynamicsof electrons, we re-analyzed the excitation and autoion-ization of highly charged ions with the goal to separatethe magnetic and retardation contributions to the e-e inter-action from the static Coulomb repulsion. A remarkablechange in the electron angular distribution due to the rela-tivistic terms in the e-e interaction was found especially forthe autoionization of (initially) beryllium-like projectiles,following a 1s → 2p3/2 Coulomb excitation in collisionwith some target nuclei. In this process, the angular distri-bution of the emitted electron is given byW (θ) ∝ 1+∑k=2,4,...Ak(αrJr) fk(αrJr, αfJf )Pk(cos θ),where Ak(αrJr) characterizes the alignment of the inter-mediate state after the excitation, θ is the polar angle withregard to the beam direction and where the fk(αrJr, αfJf )are characteristic functions that describe the dynamics ofthe autoionization. The function fk in this distributionmerely depends on the (reduced) matrix elements of the(frequency-dependent) e-e interactionV = V Coulomb + V Breitthat comprises both, the instantaneous Coloumb repulsionand the (so-called) Breit interaction, i.e. the magnetic andretardation contributions.For the excitation-autoionization process via the1s2s22p3/23P2 resonance, a diminished (electron) emis-sion in forward direction as well as oscillations in the elec-tron angular distribution due to the magnetic and retardedinteractions are predicted especially for the electron emis-sion into the 1s22s 2S1/2 ground and 1s22p 2P1/2 excited∗Work is supported by Helmholtz Association under the project VH–NG–421.0 60 120 180Laboratory angle (deg)4.0e-086.0e-088.0e-081.0e-07Differential cross section (barn/sr)0 60 120 180Laboratory angle (deg)5.0e-101.0e-091.5e-09Figure 1: Angular distribution of electrons emitted in the1s2s22p3/23P2 − 1s22s 2S1/2 (left panel) and 1s2s22p3/23P2−1s22p 2P1/2 (right panel) autoionization of U88+ pro-jectiles with energy Tp = 5 MeV/u.levels of the finally lithium-like ions. This emission pat-tern is in strong contrast to a pure Coulomb repulsion be-tween the bound and the outgoing electrons. For exam-ple, Figure 1 displays the angular distribution of electronsemitted in the 1s2s22p3/2 3P2 − 1s22s 2S1/2 (left panel)and 1s2s22p3/2 3P2−1s22p 2P1/2 (right panel) autoioniza-tion of U88+ projectiles with energy Tp = 5 MeV/u. Re-sults are shown in the laboratory frame and by incorporat-ing only the Coulomb repulsion into the Auger amplitude(blue dashed lines) as well as for a full account of the e-einteraction (black solid lines). The lowering of the electronemission in forward direction (θ ≈ 0o) is significant andenhanced in the laboratory frame due to the Lorentz trans-formation of the energetic electronsIn conclusion, the proposed excitation-autoionizationprocess can be observed at existing storage rings and willprovide novel insight into the dynamics of electrons instrong fields. The most simple signatures of the relativisticcontributions to the e-e interaction in high-Z ions is the re-duced electron emission in forward direction (θ < 5o) aswell as the double-peak structure in the expected angulardistribution; these signatures arise especially at low projec-tile energies < 10 MeV/u and for beryllium-like ions withnuclear charge Z > 70. The electron angular distribu-tion from such projectiles can be analyzed with present-dayelectron spectrometers and provide complementary infor-mation about the electron dynamics in strong fields that isnot accessible from x-ray spectra alone.References[1] S. Fritzsche et al., J. Phys. B 38, S707 (2005).[2] S. Fritzsche et al., New J. Phys. 14, 083018 (2012).PNI-IONS-THEORY-06 GSI SCIENTIFIC REPORT 2012352

Magnetic interactions and retardation in the electron emission from highly-charged ions

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Magnetic interactions and retardation in the electron emission from highly-charged ions

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