The CoESCA station at BESSY: Auger electron–photoelectron coincidences from surfaces demonstrated for Ag MNN

T Leitner,A Born,I Bidermane,R Ovsyannikov,F.O.L Johansson,Y Sassa,A Föhlisch,A Lindblad,F.O Schumann,S Svensson,N Mårtensson

doi:10.1016/j.elspec.2021.147075

Abstract

In this work, we present the CoESCA station for electron–electron coincidence spectroscopy from surfaces, built in a close collaboration between Uppsala University and Helmholtz-Zentrum Berlin at the BESSY II synchrotron facility in Berlin, Germany. We start with a detailed overview of previous work in the field of electron–electron coincidences, before we describe the CoESCA setup and its design parameters. The system is capable of recording shot-to-shot resolved 6D coincidence datasets, i.e. the kinetic energy and the two take off angles for both coincident electrons. The mathematics behind extracting and analysing these multi-dimensional coincidence datasets is introduced, with a focus on coincidence statistics, resulting in fundamental limits of the signal-to-noise ratio and its implications for acquisition times and the size of the raw data stream. The functionality of the CoESCA station is demonstrated for the example of Auger electron–photoelectron coincidences from silver surfaces for photoelectrons from the Ag 3d core levels and their corresponding MNN Auger electrons. The Auger spectra originating from the different core levels, 3d3∕2 and 3d5∕2 could be separated and further, the two-hole state energy distributions were determined for these Auger decay channels.

Highlights

After core electron photoionisation, an Auger process can occur, where the core hole is filled by an electron from a less bound shell, while a second electron is emitted, carrying the excess energy
Arena et al have previously reported Ag 3d-M45NN Auger PhotoElectron Coincidence Spectroscopy (APECS) measurements [16] and shown how coincidence spectroscopy can be used to determine the intrinsic shapes of overlapping spectral lines
They measured the coincidences with two cylindrical mirror electron analysers (CMA) of 1 eV instrumental resolution

Summary

Introduction

An Auger process can occur, where the core hole is filled by an electron from a less bound shell, while a second electron is emitted, carrying the excess energy. Since the initial vacancy state and the final two-hole state have well-defined energies for each element, the Auger electrons will be emitted with characteristic energies. In this way Auger Electron Spectroscopy (AES) can be used to identify which elements are present in a sample. For Auger transitions involving shallow core-levels, the Auger energies typically are in the range of a few hundred eV. For such electron energies the mean free path is relatively short and the Auger spectral information is surface sensitive. For this reason AES became a very powerful and much used tool in surface science

Methods

Results

Conclusion