Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 505–509 Experimental evidence for extreme surface sensitivity in Auger-Photoelectron Coincidence Spectroscopy (APECS) from solids A. Liscio a,d,∗ , R. Gotter b , A. Ruocco a , S. Iacobucci d , A.G. Danese c , R.A. Bartynski c , G. Stefani a a Dipartimento di Fisica e Unita’ INFM, Universita’ di Roma Tre, via della Vasca Navale 84, I-00146 Rome, Italy b Laboratorio Nazionale TASC-INFM, Area Science ParkTrieste, Italy c Department of Physics and Astronomy and Laboratory of Surface Modification, Rutgers University, New Brunswick, NJ, USA d CNR-IMIP, Area della Ricerca di Roma, Monterotondo Scalo, Rome, Italy Abstract Core hole creation and subsequent Auger decay processes are studied with unprecedented discrimination by Auger-Photoelectron Co- incidence Spectroscopy (APECS). Early works in this field have already pointed out the intrinsic surface sensitivity of these experiments. However, it was not until recently that a model calculation was developed to quantitatively evaluate it. Here we present the first attempt to experimentally establish an effective target thickness for such experiments. The angular distribution of 3p 3/2 photoelectron with kinetic energy of 160 eV is measured in coincidence with the M 3 VV Auger electron with kinetic energy of 55 eV on a Cu (1 1 1) surface. Coincidence and non-coincidence photoelectron angular distributions display differences that, to large extent, are explained by confining the source of the coincident signal within the first two layers of Cu target, thus establishing an experimental upper limit for the effective target thickness of the APECS experiment. © 2004 Published by Elsevier B.V. Keywords: Coincidences; Surfaces; Auger; Photoelectron; Cu 1. Introduction Auger-Photoelectron Coincidence Spectroscopy (APECS) allows one to study in detail core hole creation and the sub- sequent Auger decay. This capability has been extensively applied to investigate atoms and molecules in the gas phase. In this case, the extra discrimination obtained by resolving the electron pair probability distribution in angle, rather than in energy, has given access to fine details of the core ionisation dynamics, such as core state polarization, post collisional electron–electron interaction, and quantum inter- ference between continuum final states [1]. Already, in the late seventies this spectroscopy was applied to solids aiming at: (i) discriminating individual Auger decay channels; (ii) physically removing the secondary electron background and (iii) singling out individual term of otherwise unresolved multiples [2]. The high photon energy resolution achievable ∗ Corresponding author. by synchrotron radiation light sources allows one to perform APECS with resolution well beyond the limit imposed by the core hole lifetime [3]. More recently, Angle Resolved APECS (AR-APECS) has been shown to be feasible also on solids, opening up to the possibility of discriminating among components of the manifold of two hole Auger final states that are otherwise unresolved [4]. Enhanced surface sensitiv- ity has been claimed to be a peculiarity of these experiments in several papers (see for instance [5] and references therein quoted) but a quantitative assessment of this experimental character is still lacking. It is to be noted that, in spite of the enhanced surface sensitivity, APECS spectra are not immune to multiple elastic and inelastic scattering events, which partially hamper the aforementioned discrimination capabil- ities. Recently, a theoretical study of the effects of multiple elastic and inelastic scattering on the line shape in APECS has confirmed the enhanced surface sensitivity of this spec- troscopy as compared to the ordinary Auger and photoelec- tron spectroscopies [6]. It also shows that surface and bulk excitations are, to large extent, uncorrelated, thus allowing 0368-2048/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.elspec.2004.02.099