Measurement of the background in Auger-photoemission coincidence spectra (APECS) associated with inelastic or multi-electron valence band photoemission processes

Abstract

Auger Photoelectron Coincidence Spectroscopy (APECS), in which the Auger spectra is measured in coincidence with the core level photoelectron, is capable of pulling difficult to observe low energy Auger peaks out of a large background due mostly to inelastically scattered valence band (VB) photoelectrons. However the APECS method alone cannot eliminate the background due to valence band photoemission processes in which the initial photon energy is shared by two or more electrons and one of the electrons is in the energy range of the core level photoemission peak. Here we describe an experimental method to determine the contributions from these background processes and apply this method in the case of Copper M 3 VV Auger spectrum obtained in coincidence with the 3p 3/2 photoemission peak. A beam of 200 eV photons was incident on a Cu(100) sample and a series of coincidence measurements were performed using a spectrometer equipped with two cylindrical mirror analyzers (CMAs). One CMA was set at series of fixed energies that ranged between the energy of the core and the VB peaks. The other CMA was scanned over a range corresponding to electrons leaving the surface between 0eV and 70eV. The set of measured spectra were then fit to a parameterized function which was extrapolated to determine the background in the APECS spectra due to multi-electron and inelastic VB photoemission processes. The extrapolated background was subtracted from the APECS spectrum to obtain the spectrum of electrons emitted solely as the result of the Auger process. A comparison of the coincidence spectrum with the same spectrum with background removed shows that in the case of Cu M 3 VV the background due to the inelastic scattering of VB electrons is negligible in the region of the Auger peak but is more than half the total signal down in the low energy tail of the Auger peak.

Electron correlation effects in the exchange coupling at the Fe/CoO/Ag(001) ferro-/antiferro-magnetic interface

“…The cure achieved by the time-coincident detection of Auger and photoelectrons is evident: in APECS spectra the background is completely eliminated before the Auger onset, because Co M 23 VV and Fe M 2 VV Auger electrons, and other prior secondary electrons cannot be detected in coincidence with the Fe 3p 3/2 photoelectrons; only an intrinsic background remains, which can be easily simulated by a Shirley background [49,60] whose intensity is determined by the effective sampling depth of the experiment that equals the inelastic mean free path of the electron pair [61,62].…”

A multi-stop time-of-flight spectrometer for the measurement of positron annihilation-induced electrons in coincidence with the Doppler-shifted annihilation gamma photon

“…Multiple electron emission following the Auger decay of a core hole forms the foundation for explaining the extra intensity found in the inelastic tail of Auger peaks originating from shallow core levels [20][21][22][23][24]. The intensity and energy distribution of the multiple Auger electrons from different samples can provide an estimate of the strength and role of correlation of core holes with valence holes and valence holes with other valence holes in the photon-, electron-or positron-induced electron emission process.…”

“…The coincidence measurement capabilities, when applied to PAES, are expected to be useful in the study of emergent properties on the surfaces of topological insulators [12] or graphene [13] and in the study of new processes involving positron-surface interactions [14][15][16][17][18]. The application of the multi-electron ToF spectrometer to measure the energies of positron annihilation-induced Auger electrons is expected to resolve the contribution of multi-electron Auger decay pathways of the core or deep valence holes which in turn can give critical information on the strength of electron correlation [19][20][21][22][23][24].…”

Positron Annihilation-Induced Auger Electron Spectroscopy Measurements of a TiO2(110) Surface

“…In PAES, positron beam energies well below the electron work function can be used to initiate the Auger process allowing for the complete elimination of the obscuring secondary electron background found in other similar techniques [2]. Experimental determination of the background-free, spectra of electrons emitted as a result of Auger transitions, down to 0 eV, is important for unravelling the complex, correlated Auger decay pathways of core and deep valence holes [3][4][5][6][7]. PAES studies of TiO 2 (110) can also provide important insights relevant to the understanding of positron-stimulated desorption of O + from TiO 2 (110).…”