Angle-resolved APECS: Basics

Auger-photoelectron coincidence spectroscopy enables one to study the Auger or core level photoelectron spectrum of a solid with unprecedented discrimination.  By performing such measurements in an angle-resolved mode, one can take advantage of the constraints placed on the APECS spectrum by the dipole and Coulomb matrix elements.  These measurements, performed at the ALOISA beamline of the ELETTRA synchrotron in Trieste, Italy, use a versatile experimental chamber with seven hemispherical electron energy analyzers, mounted on two independent axes.  Typically, two analyzers are tuned to the kinetic energy of the core level photoelectrons of interest, while the remaining five analyzers collect Auger electrons.  Thus, ten coincidence pairs are collected simultaneously.  This arrangement compensates in part for the small solid angle collected by each analyser.

       AR-APECS: Recent Highlight

Dichroic Effect in Angle Resolved- (DEAR-) APECS: The polarization vector of the linearly polarized synchrotron radiation establishes a quantization axis for the photoexcitation process.  This constrains the l and m quantum numbers of the core hole that forms the initial state of the Auger transition.  Similar, but more complicated, constraints  arising from the Coulomb matrix element of the Auger transition results in further restriction of the possible values of the quantum numbers of the two holes in the Auger final state.  When the two holes have the same principle quantum number, as in the Sn L23M45M45 Auger transition shown in the figure at the left, one can choose a geometry so that all of the spatial quantum numbers of the two holes are identical. Then the Pauli exclusion principle demands that the holes have opposite spin.  In this way, the singlet final states will dominate the spectrum.  This is shown in the "NN" and "AA" curves in figures (a) and (b) at the left. Similarly, other geometries suppress the singlet states and enhance the triplet states.  This behavior is demonstrated by the AN curves in the Figure.  By calculating the difference spectra (AN-NN) or (AN-AA), a geometrically induced dichroic effect in angle resolved APECS (DEAR-APECS)  is observed.  This effect can be used to explore magnetic behavior in complicated systems in an element specific manner.  

Read more: Dichroic effects in Auger photoelectron coincidence spectroscopy of solids
R. Gotter, F. Da Pieve, A. Ruocco, F. Offi, G. Stefani, and R.A. Bartynski, Phys. Rev. B 72, 235409 (2005)

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