Electron Energy Loss Spectroscopy in the Study of Surfaces

Abstract

Inelastic electron scattering provides a powerful technique for the study of the excitations of free atoms, molecules (1, 2), and solids (3). However, its most recent and very significant application has been in surface science, where electron energy loss spectroscopy (EELS) is used extensively to study the excitations of clean surfaces and adsorbates. The first such study was reported by Propst & Piper in 1967 (4), who studied the vibrational spectra of W(lOO) surfaces exposed to various gases with a resolution of 50 meV. The resolution of EELS was significantly improved by Ibach, who in 1970 reported the study of the surface phonons of ZnO with a resolution < 20 meV (5). Due to the efforts of Ibach and others the resolution has bcen improving ever since (currently a 2-3 meV resolution can be achieved), and the scope and applications of EELS have been constantly widening (6). Electron excitation has several features that make it particularly appropriate for surface spectroscopy. First, it covers a very wide spectral range. The Fourier decomposition of the electric field at the surface produced by an electron at a distance d from the surface and moving with velocity v has frequency components up to a cut off frequency we vld. Thus the electron behaves like a source of continuum radiation-a poor man's synchrotron. Therefore, low frequency phonons, high frequency intra-adsorbate vibrational modes, and electronic excitations can be monitored under the same scattering conditions, thus allowing a more complete understanding of the system under study. Second, the strong electron-matter interaction provides both a high surface selectivity and