Strong influence of configuration interactions on the orientation and alignment dichroism in the $3p$ photoelectron spectra of free laser-polarized Fe atoms

The $3p$ photoelectron spectra of oriented and aligned free Fe atoms are presented. The atomic polarization was achieved by optical pumping. For this purpose single-mode ultraviolet continuous wave laser radiation was produced by second harmonic generation in an external ring resonator. The ground state Fe $3d^{6}4s^2$ ${}^{5}D_4$ was oriented by circularly polarized laser radiation and aligned by linearly polarized laser radiation. Switching from right handed to left handed circularly polarized laser radiation or by changing the polarization angle of the linearly polarized laser radiation by 90° allowed for the measurement of the orientation or the alignment dichroism in the Fe $3p$ photoelectron spectra excited by linearly polarized synchrotron radiation. The spectra are compared to the predictions of the single configuration $LS$-coupling model and the results of single and multiconfiguration calculations. Strong configuration interactions in the final core-hole states manifest themselves in marked deviations from the characteristic patterns. The comparison of the spectra of the free Fe atoms with spectra of thin magnetized Fe films reveals similarities but also marked differences between the dichroism curves of free and bound Fe atoms. The common atomic origin of the dichroism of free and bound Fe atoms is discussed; changes in the intra-atomic electron interactions as well as changes in valence electronic configuration when going from free to bound Fe atoms are addressed.

Figure 1

$3p$ photoelectron spectrum of unpolarized Fe atoms. The experimental spectrum (upper panel) is taken from [27]. The assignment of the peaks is based on the single configuration (SCA) Hartree-Fock calculations shown in the middle panel. The lower panel presents the spectrum calculated taking configuration interaction (CI) into account. For details about the calculations see text.

Figure 2

(Color online) $3p$ photoelectron spectra of laser-oriented Fe atoms (upper panel) in the low binding energy region of the high-spin states for opposite atomic orientations corresponding to right (black line) and left (red line) circularly polarized laser radiation (upper panel) and their difference (second panel) giving the dichroism curve (LMDAD). The dichroism curves LAD (0∣90) and LAD $(45\mid -45)$ for aligned atoms for two perpendicular alignments of the atoms corresponding to angles of (0∣90) and $(45\mid -45)$ of the linearly polarized laser radiation with respect to the polarization axis of the ionizing synchrotron radiation are shown in the lower panels without the corresponding photoelectron spectra.

Figure 3

Experimental (dots) and theoretical SCA-HF (solid lines) photoelectron spectra (upper panel) and LMDAD (middle panel) and LAD(0∣90) (lower panel) dichroism curves. Only transitions from the Fe ${}^{5}D_4$ ground state have been taken into account in the calculations.

Figure 4

Experimental (dots) and theoretical CI (solid lines) photoelectron spectra (upper panel) and LMDAD (middle panel) and ${\mathrm{LAD}}_{(0\mid 90)}$ (lower panel) dichroism curves. Only transitions from the Fe ${}^{5}D_4$ ground state were taken into account in the calculations.

Figure 5

(Color online) Experimental LMDAD dichroism curves for photoionization with different photon energies $90.3\mathrm{eV}$ (dashed red line) and $122.6\mathrm{eV}$ (solid black line).

Figure 6

(Color online) Model calculations for the $3p$ dichroism of Fe in comparison to the experimental data of Roth et al. [3]. The dashed lines (red) give the results obtained by van der Laan within a one-electron model [13]. The full lines (blue) and the dotted line (blue) represent the results calculated by Martins [25] within the SCA-HF approximation. For details see text.