Auger electron angular distributions from surfaces: direct comparison with isoenergetic photoelectrons

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Date: Mar. 8, 1991
From: Science(Vol. 251, Issue 4998)
Publisher: American Association for the Advancement of Science
Document Type: Article
Length: 2,277 words

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DURING THE PAST FEW YEARS AUger electron and photoelectron diffraction have become widely accepted and popular techniques for determining structures at single-crystal interfaces--particularly at surfaces [1]. Interpretation of the angular- and energy-dependent electron emission data collected for these studies has been predicated on quantum self-interference of an ejected electron wave that can scatter from nearby atoms. However, Frank et al. [2] reported unexpected Auger electron angular distributions that, they argue, could not be explained within the conventional electron-scattering formalism. They proposed

a controversial physical model to explain their results. Subsequently, numerous commentators [3-6] argued that the model proposed by Frank et al. cannot explain a large body of existing experimental results extending back to the 1940s, and they offered alternative explanations that depend on existing electron-scattering theory. In this report we present empirical evidence that the original model of Frank et al. cannot be correct and that also conflicts with the alternatives presented by the commentators.

From the single-crystal solids used in their study, Frank et al. observe Auger electron angular distributions with a great deal of structure and--owing to the known short escape depth of electrons in solids--one might expect to extract considerable information about the crystalline surface from these measurements. Indeed, previous work [1] would suggest that the largest contribution to the observed angular variation of Auger electron emission from atoms below the surface would be "forward scattering," the modulation of the electron intensity due to interference between an atomic-like electron wave and portions of that same wave that scatter ("focus") in passing through atoms on the path to the detector. To the extent that forward-scattering predominates, one expects increased intensity whenever atoms lie between the emitter and the detector. By working back from the points of high intensity, one obtains information on the positions of atoms surrounding the emitter. Surprisingly, for samples with known crystal structures, Frank et al. observed dips along interatomic axes.

To explain their unexpected results, Frank et al. devised a new model for electron propagation in a solid. The observed dips were attributed to "shadowing," a classical attenuation of electron intensity caused by putative inhomogeneous inelastic electron scattering. Rejecting this as a sudden denial of quantum mechanics, several commentators offered to explain the observed shadows with electron-scattering models that remain consistent with earlier observations of electron emission from solids. These include complex multiple electron scattering [4], strong energy dependence of Auger electron angular distributions at low emission energies [5], forward-scattering phase shifts near [pi] leading to destructive interference, and even sample misalignment [3]. We show that the dips observed by Frank et al. are real and that they are not caused by any of these purported effects.

Our study compares the [M.sub.2][M.sub.4,5][M.sub.4,5] and [M.sub.3][M.sub.4,5][M.sub.4,5] Auger electron angular distribution patterns (ADPs) to the same patterns for 3p photoelectrons from a clean Cu (001) crystal face. We use the same electron kinetic energies for both the primary photoelectron emission and the secondary Auger emission to identify whether any of the scattering effects cited above or...

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Gale Document Number: GALE|A10561210