Basics: Auger Photoelectron Coincidence Spectroscopy

Auger-photoelectron coincidence spectroscopy (APECS) is a novel experimental technique that enables one to examine the photoelectron distribution from a solid with unprecedented discrimination. As the name suggests, in APECS a core photoelectron and its associated Auger electron are measured in time coincidence. As a result, in true coincidence events both electrons originate from the same photoexcitation event. Therefore, APECS is a highly incisive probe of local electronic structure and photoexcitation/ processes in complex solids.

       Recent Highlights: APECS

Isolate overlapping Auger spectra:  The figure at the left shows a conventional (singles) spectrum [blue curve] in the kinetic energy region of the Cu M23VV Auger spectrum.  An APECS spectrum of the same energy region obtained in coincidence with Cu 3p3/2 photoelectrons is shown as the red data points.  Since the APECS data only includes electrons generated by decay of Cu 3p3/2 core holes, the the lineshape of the isolated M3VV Auger transition is measured.  The green curve is a fit to the coincidence lineshape based on the multiplets of the d8 configuration of the 2-hole Auger final state (contributions shown by the vertical bars).   

Enhanced sensitivity to surface defects:  Singles (red dashed curve) and coincidence (data points with error bars) spectra of the Ti 3p core level region from the TiO2(110) surface.  The singles spectrum shows a symmetric lineshape assoicated with Ti ions in the 4+ oxidation states.  The coincidence spectrum show extra emission in the energy range associated with Ti3+ and Ti2+ ions, indicating that the coincidence spectrum is about and order of magnitude more sensitive to the presence of surface defects (primarily oxygen vacancies) on this surface.  This added sensitivity arises because of the higher electron density on the Ti-site for Ti ions in reduced chemical states.

Auger-Auger coincidence spectroscopy:  Cascade Auger decays lead to a chain of decay processes leaving multiple holes in different atomic levels.  By performing a coincidence measurement where the later transition of a cascade Auger decay is measured in coincidence with Auger electrons generated by earlier steps in the process can enable one to study electron-electron interactions and the effect of different hole configurations.  The spectrum at the left shows a singles photoemission spectrum  from the MnO(100) surface excited with 350 eV photons.  Several members of a cascade Auger chain which leave holes in the  M3 level of Mn.   A singles spectrum of the Mn MVV Auger region is shown in the inset.  Contributions from the last stage of the cascade Auger decay also contribute to the spectrum in the inset.   Spectra of the same Mn MVV region obtained under different coincidence conditions are shown in the figure at the left.  Fig. (a) is the MVV APECS spectrum taken in coincidence with the Mn 3p core photoelectrons, and therefore the cascade contribution is eliminated.  Spectrum (b) shows an AACS spectrum obtained in coincidence with electrons emitted in the LMV Auger decay and spectrum (c) is obtained in coincidence with electrons from the LMM decay.  Note that the AACS spectra [(b) and (c)] are significantly wider than the APECS spectrum (a), in particular, the AACS emission extends to ~ 5 eV higher energy.  This additional width indicates that the multiple valence band holes created in the cascade process participate in the final step of the cascade decay.  

Read more:
Electronic and magnetic properties of thin films probed by Auger photoelectron coincidence spectroscopy (APECS), R. Gotter, F. Offi, F. DaPieve, A. Ruocco, G. Stefani, S. Ugenti, M.I. Trioni, and  R.A. Bartynski,J. Electron Spec. and Rel. Phenom., 161, 128 (2007)

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