Surface Characterization Using Metastable Impact Electron Spectroscopy of Adsorbed Xenon

In contrast to ultraviolet photoelectron spectroscopy (UPS), photoemission of adsorbed xenon (PAX) can be used for quantitative analysis of solid surfaces. In this paper, it is shown that PAX yields ambiguous results when the Xe features overlap with the substrate signals. On the other hand, with metastable impact electron spectroscopy (MIES) of adsorbed xenon (MAX), the substrate signals are completely quenched, and only the Xe features appear due to the high surface sensitivity of MIES. MAX, therefore, can provide more accurate information than PAX with respect to binding energies, energy positions, and the widths of the Xe 5p states. Several examples are given to show that MAX can be informative for the quantitative characterizations of uniform and nonuniform surfaces, illustrating that, in contrast to MIES, MAX can be used on metals as well as wide band gap materials. Ultraviolet photoelectron spectroscopy (UPS) is quite useful for the characterizations of clean and adsorbate-covered solid surfaces, however, quantifying UPS data can oftentimes be problematic. However, photoemission of adsorbed Xe (PAX) has been shown to be quantitative particularly regarding the outmost surface layer. 1,2 X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES), two other widely used techniques for quantitative surface analysis, integrate over the surface and the near-surface regions. Metastable impact electron spectroscopy (MIES) is a nondestructive technique and extremely surface sensitive because the metastable helium atoms impinge the surface with thermal kinetic energy. Auger deexcitation (AD) is the dominant mechanism that leads to the MIES signal for wide band gap materials or metals with very low work functions. If AD is the deexcitation mechanism, a plot of the intensity of the ejected electrons versus their kinetic energies yields the surface density of states (SDOS) for the topmost layer of the surface. 3 Due to its extremely high surface sensitivity, MIES has been used to characterize subtle differences in various solid surfaces, e.g., small modifications of the surface structure that could not be detected using UPS. 4 However, for metallic and semiconductor surfaces with relatively high work functions, the yield due to the AD process is low because the dominant quenching mechanisms in these cases are resonance ionization (RI) and Auger neutralization (AN). This preference for RI and AN on these surfaces limits the application of MIES to wide band gap materials or metals with very low work functions. 3 In this study it is shown that PAX does not give completely reliable information when superposition of the Xe peaks and the substrate features occurs. A new method, MIES of adsorbed Xe (MAX), is shown to give more accurate information with respect to binding energies, peak intensities, and full width at half-maxima (fwhm) of the Xe 5p states. This work demonstrates that MAX can be used for the characterization of surfaces for which the application of PAX is difficult due to the superposition of the Xe peaks and substrate signals. In contrast to MIES, the application of MAX is not limited to insulating materials and metals with lower work functions, but can be used on metallic surfaces with higher work function.