Dichroic effects in Auger photoelectron coincidence spectroscopy of solids

The Sn $M_5{N}_{45}{N}_{45}$ Auger spectrum from the $(\sqrt{3}\times \sqrt{3})R30°\text{−}\mathrm{Sn}∕\mathrm{Ge}\left(111\right)$ surface has been measured in coincidence with the corresponding $3d_{5∕2}$ photoelectron. By detecting this pair at appropriate emission angles, the contribution from spin-symmetric (triplet) and spin-antisymmetric (singlet) final states can be selectively enhanced or suppressed. This dichroic effect in the Auger photoelectron coincidence spectroscopy of solids provides a probe of the local valence electronic structure with element, chemical states, emission depth, and spin selectivity. The consequences and applications of this dichroic effect are discussed.

Figure 1

Schematic representation of the analyzer frames in the ALOISA chamber.

Figure 2

Noncoincidence Sn $M_5{N}_{45}{N}_{45}$ Auger spectrum obtained from the $(\sqrt{3}\times \sqrt{3})R30°\text{−}\mathrm{Sn}∕\mathrm{Ge}\left(111\right)$ surface excited by 700-eV photons. The electron spectrometer energy resolution was 140 meV. The dotted line is a parabolic background, and the solid curve is a fit generated by the sum of Voigt profiles whose energy and relative intensity are given by the vertical bars indicating the multiplets of the $d^8$ final-state configuration (see Ref. 12).

Figure 3

Auger spectra obtained in coincidence with Sn $3d_{5∕2}$ photoelectrons are shown in (a) and (b) top panels. The data points with error bars are the coincidence data (the superimposed solid line is a guide to the eyes), and the dashed line is the simultaneously acquired single spectrum. The Auger channel energy resolution was 1.4 eV while it was 3 eV for the photoelectron channel. The geometries of the measurements were designed to suppresses the contribution from symmetric-spin states ($A{N}_1$ and $A{N}_2$) or antisymmetric-spin states ($NN$ and $AA$). The bottom panels of (a) and (b) show the dichroic effect in angle-resolved Auger photoelectron coincidence spectroscopy (DEAR-APECS) as derived from the difference between the spectra reported in the corresponding top panels.

Figure 4

Predicted contributions to the ${}^{1}G$ and ${}^{3}F$ multiplets as a function of the difference in magnetic quantum numbers, $\mid \Delta m\mid =\mid m_1-m_2\mid$, of the two holes in the Auger final state. Note that the ${}^{3}F$ state has no weight with $\mid \Delta m\mid =0$ but dominates for $\mid \Delta m\mid ⩾2$.