Ultramicroscopy 11 (1983) 299-302 299 North-Holland Publishing Company SURFACE PLASMON SCATI'ERING ON FLAT SURFACES AT GRAZING INCIDENCE * P.E. BATSON IBM Thomas J. Watson Research Center, Yorktown Heights, New York 10598, USA Received 11 April 1983 This paper reports a study of surface plasmon excitation on the flat, oxide-covered aluminum surface, using high energy electrons which pass the surface at'a finite, and sometimes large, impact parameter. The scattering probability dependence on impact parameter agrees qualitatively with classical, quasi-static calculations, which assume that the materials are characterized by homogeneous dielectric constants. A broad intensity maximum near 16 eV is identified as a surface plasmon made possible by the frequency dependence of the aluminum oxide dielectric constant. 1. Introduction 2. Theoretical model The decay of the surface plasmon electric potential away from a surface can be indirectly determined by observing the inelastic scattering of fast electrons as they pass a surface with a finite impact parameter [1]. This possibility has been closely examined recently with the STEM which now routinely produces probe sizes of 0.5-1.0 nm with 100 keV electrons. Observations of the surface plasmon decay on surfaces of MgO have been reported by Marks [2] and Cowley [3]. These show qualitative agreement with the simple scattering theory, although interpretation is difficult due to the complicated dielectric properties of MgO. Bat- son has reported anomalous intensity maxima, at the oxide-vacuum interface on oxide-coated A1 spheres [4], which are not reproduced in the theory [5]. Therefore, the present work on a simpler flat geometry with the oxide-coated aluminum was undertaken to further clarify the extent of the agreement between the experiment and the classi- cal theory. * From a contribution to Workshop on Surface Imaging and Analysis with High Spatial Resolution, Wickenburg, Arizona, January 1983. The model geometry is shown in fig. 1. It includes three layers: a semi-infinite bulk with dielectric constant c2, an A1203 layer of thickness a and dielectric constant E~, and the vacuum oc- cupied by the fast electron passing with an impact parameter, b. The applied potential ~e~, generated by this electron may be written as a function of position y, the momentum parallel to the surface qz, and the energy loss ,o [6]: 4~re e -x(y-b) / to ) ~e~t = c(y, qz,~) ~ 8[qx-- ~ ,, (1) where X 2 = q2 + ¢~2/D2 defines the surface excita- tion wavevector through kinematical restrictions. The total potential in the system is then written for Abminum AI203 (~2 E1 ..2/',, Vaotudm i y=b Y y=O y=a Fig. I. Model geometry for the classical calculation. The fast electron travels roughly parallel to the oxide=coated A1 surface. 0304-3991/83/0000-0000/$03.00 © 1983 North-Holland