Linear dichroism of the 4 f photoemission in the giant resonance of atomic europium J. Schulz,* Ph. Wernet, † M. Martins, and B. Sonntag Institut fu ¨r Experimentalphysik, Luruper Chaussee 149, 22761 Hamburg, Germany R. Mu ¨ ller, K. Godehusen, ‡ and P. Zimmermann Institut fu ¨r atomare Physik und Fachdidaktik, Hardenbergstrasse 36, 10629 Berlin, Germany Received 23 August 2002; published 21 January 2003 The linear dichroism in the 4 f photoelectron spectra of atomic Eu excited at photon energies in the range of the giant 4 d -4 f resonance have been determined. Dramatic changes of the dichroism have been observed when tuning the photon energy through the giant resonance. The dichroism patterns are compared to the predictions of a model based on LS coupling which takes the interaction of the discrete resonances with the ionization continua into account. DOI: 10.1103/PhysRevA.67.012502 PACS numbers: 33.60.Cv, 32.80.Fb I. INTRODUCTION Dichroism in the photoemission of rare earths and rare earth compounds is a powerful tool for probing the magnetic structure of surfaces and thin films 1,2, and references therein. Gadolinum Gd with its half filled 4 f shell plays a key role in these studies. The Gd 4 f -shell is well localized in the solid state explaining the good agreement of the spectra with the predictions of ionic models 3–6. The atomic ap- proach is further supported by the close similarities of the dichroism in the 4 f and 4 d photoelectron spectra of atomic europium and of thin gadolinium films 7,8. The typical dichroism patterns 9 could be verified in all photoelectron spectra when the fine-structure components of the 4 f and 4 d multiplets were resolved. In agreement with the theoretical predictions the dichroism in the nonresonant 4 f photoelec- tron spectra of atomic Eu vanishes when integrated over all fine structure components of the 4 f multiplet. For photon energies close to atomic resonances this sum rule is expected to break down. The dichroism observed in the photoabsorp- tion of solid Gd in the photon-energy range of the giant 4 d -4 f resonance is a clear evidence for this breakdown 10. In order to get a deeper understanding for this effect we studied the linear alignment dichroism LAD and the linear magnetic dichroism LMDAD in the Eu 4 f photoelectron spectra excited by photons in the energy range of the giant resonance. The experimental results are compared to the pre- dictions of a model based on Fanos description of atomic resonances 11 and the description of the angular distribu- tion of photoelectrons within LS coupling 12. Our theoret- ical approach is similar to the model by Starke and cowork- ers 13 describing the magnetic circular dichroism in the photoelectron spectra of solid Gd excited in the giant reso- nance, but the Fano-type energy dependence of the dipole matrix elements has been included explicitly and the theory of the angular distribution of photoelectrons 12 has been used in order to extend the theory to linear dichroism in the angular distribution. A similar model calculating the angular distributions of photoelectrons within a resonance has been published 24. II. EXPERIMENTAL SETUP A thermal atomic europium beam was produced by a re- sistively heated oven. The atoms were ionized using linearly polarized undulator radiation monochromatized by a SX700 monochromator at the BW3 beamline of the DORIS storage ring in Hamburg Germany. The 4 f photoelectrons were detected with a scienta SES200 electron analyzer set at the magic angle with respect to the electric-field vector of the undulator radiation. The individual bandwidths of the spectra are given in the figure captions. All bandwidths have been determined by fitting Gaussian lines to the spectra and by rare-gas calibration measurements. For unpolarized atoms the photoelectron spectra are proportional to the total 4 f photoionization cross section. The Eu atoms were aligned oriented by optical pumping with a linearly circularly po- larized laser beam tuned to the 4 f 7 6 s 28 S 7/2 →4 f 7 6 s 6 p 8 P 5/2 transition at a wavelength of 466.2 nm. The laser beam propagated antiparallel to the ionizing syn- chrotron radiation. For a more detailed description of the experimental setup see Refs. 8,14. III. HIGH-RESOLUTION 4 f PHOTOELECTRON SPECTRUM It is well established that final-state configuration interac- tions can influence the photoelectron spectra of the rare-earth metals significantly 15,16. In the Eu 4 f photoionization the 4 f 6 6 s 27 F 0...6 final states strongly overlap with states of the configurations 4 f 6 5 d 5 s and 4 f 6 5 d 2 . This leads to a complicated Eu 4 f photoelectron spectrum displaying more than the seven lines predicted by the LS-coupling model 17. Figure 1 shows the Eu 4 f photoelectron spectrum taken at a photon energy of 49 eV. This spectrum corroborates the spectrum presented in Ref. 17 but reveals additional lines *Electronic address: Joachim.Schulz@maxlab.lu.se; present ad- dress: MAX-lab, Box 118, 221 00 Lund, Sweden. † Present address: SSRL, 2575 Sand Hill Road, MS 69, Menlo Park CA 94025. ‡ Present address: BESSY GmbH, Albert-Einstein-Strasse 15, 12489 Berlin, Germany. PHYSICAL REVIEW A 67, 012502 2003 1050-2947/2003/671/0125028/$20.00 ©2003 The American Physical Society 67 012502-1