J . Phys. Chem. 1994, 98, 1895-1903
1895
Auger Emission Angular Distributions from a Silver Monolayer in the Presence and Absence of an Iodine Overlayer: Evidence for the Predominance of Inhomogeneous Inelastic Scattering of Auger Electrons by Atoms Douglas C. Frank,. Oliver M. R. Chyan,? Teresa Golden,$ and Arthur T. Hubbard’ Surface Center, Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 -01 72 Received: October 7, 1993”
Complete Auger electron angular distributions have been measured from a monolayer of Ag atoms (355 eV) in the presence and absence of a n ordered monoatomic overlayer of iodine atoms. In the presence of the iodine overlayer, the Auger emission from the Ag monolayer is anisotropically attenuated to form a distinct hexagonal intensity distribution, and the Auger intensity does not increase relative to that observed for the bare Ag monolayer along any direction of emission. The extent of attenuation is in good agreement with gas-phase cross section measurements for Xe, which also indicate that inelastic scattering predominates over elastic scattering in this energy range. These results are consistent with previous studies in which Auger emission from an ordered monolayer of adsorbed iodine atoms was found to be attenuated along the interatomic directions. Analysis of the Ag/I bilayer data by use of a model in which atoms behave as isotropic point emitters and spherical scatterers of Auger electron leads to a structure which agrees with the results of LEED, STM, AES, and electrochemical measurements. Data for the bare Ag monolayer reveal that the silver layer is nearly hexagonal and is rotated 27.2’ with respect to the hexagonal Pt( 111) substrate surface.
Introduction Recent measurements in several laboratories of the complete low-energy (300 eV) are due primarily to elastic scattering within rows of atoms (a wave phenomenon) or channeling between them (a particle phenomenon). The iodine monolayer resultsIOJ1 were investigated not only for their structural simplicity but also because Auger electron emission from iodine (-510 eV) occurs well within the kinetic energy range where elastic forward scattering effects have been postulated to predominate.2G22 Thus, it is significant that the distributions of iodine Auger electrons emitted from iodine monolayers contain minima along the internuclear directions, contrary to the prediction^^^ of models which assume that inelastic scattering is homogeneous (the “jellium“ approximation). It is also important to note that the total and inelastic electron scattering cross sections of various noble gases have been accurately measured. In particular, data are available for Xe (atomic number, Z = 54), which should closely resemble iodine (Z= 53). From the Xe data it can be seen that inelastic scattering accounts for about 72%of the total scattering cross section at a KE of 350 eV.24
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* To whom correspondence. should be addressed.
t Present address: Department of Chemistry, University of North Texas, Denton, TX 76203-5068. t Present address: Graduate Center for Materials Research, University of Missouri-Rolla, Rolla, MO 65401. e Abstract published in Advance ACS Abstracts, January 15, 1994.
Faced with these new experimental findings, some authors responded by reasserting various previously published models, suggesting that the new data were incorrect or anomalous.2sJ6 More recently, various authors have attempted to account for the data by postulating that the emission of Auger electrons is a n i ~ o t r o p i c . I ~Others - ~ ~ have suggested that the data be interpreted in terms of a “holographic” model based upon classical wave scattering formali~ms.27-~~ However, all of the above theoretical treatments assume that atomic lattices behave homogeneously in inelastic scattering of Auger electrons.30 That is, the crystal, thin film, or monolayer is assumed to behave as “jellium” with respect to inelastic scattering of Auger electrons. In the jellium case, the degree of attenuation of the Auger intensity depends only on the total distance traveled through the sample, not on the structure of the lattice or the particular path from the emitting atom to the detector. At the same time, those models assume that the lattice behaves inhomogeneously as an elastic scatterer of Auger electrons. Viewed from a quantum mechanical perspective, the dependence upon a jellium model of inelastic scattering is somewhat surprising and may have been motivated more by convenience than by first principles. The commonly accepted underpinning of rigorous discussions of electron scattering is the “overlap” of the wave function of the propagating electron with the wave functions of the ground and excited states of the scattering centers (atoms or ions). See, for example, eq 16.76 and the accompanying discussion in the treatise by Mott and Massey.31 Also, from a practical standpoint, virtual delocalization of all inelastic scattering processes of all scattering centers in all substances with respect to all Auger electron kinetic energies is unlikely. In the present study, complete Auger electron angular distributions were measured from a monolayer of Ag atoms (355 eV) with and without an overlayer of iodine atoms. The results indicate that the iodine overlayer substantially and anisotropically attenuates the Auger intensity from the Ag monolayer, producing a distinct attenuation pattern. In addition, the intensity is found to be smaller along all directions of detection when the iodine overkyer is present, and the degree of attenuation of the Ag signal by the iodine overlayer is in close agreement with the inelastic cross section measured for gaseous Xe.24 In contrast, if the ”forward focusing” model as currently defined23 had been
0022-3654/94/2098-1895%04.50/0 0 1994 American Chemical Society
Frank et al.
1896 The Journal of Physical Chemistry, Vol. 98, No. 7, 1994
predominant, the iodine overlayer would merely have redistributed the Ag intensity so as to produce higher intensities along some directions at the expense of others. Thus, since signal enhancement due to the iodine layer was not observed along any direction, the “forward focusing” model as currently formulated is contradicted by experiment. The distinct attenuation pattern observed in the Ag angular distribution due to the iodine overlayer can be employed to investigate the geometric structure of the Ag/I bilayer. First, the fact that Auger emission from the Ag monolayer decreases in the presence of the iodine monolayer reveals the relatiue positions of the atomic layers, with iodine forming the outermost atomic layer. This straightforward conclusion does not require angle-resolved Auger emission data32333 but is nonetheless important experimental information. Consideration of angleresolved data permits investigation of the relative positions of atoms within and between the layers. From the iodine monolayer studies,lOJl it is clear that emission from monolayer atoms (510 eV) is essentially isotropic and that attenuation of Auger intensity leads to minima in the angular distributions along the interatomic directions. Studies of various monolayers and single crystals1-” indicate that the same is true a t lower KE (
