Ultraviolet photoelectron spectroscopy widths

Fig. 3.19 Ultraviolet photoelectron spectroscopy (UPS) spectra of silver and gold layers on a Ru(001) substrate show the evolution of the d-band as a function of the silver dimensions. Note also the changes in work function reflected in the width of the spectra of Ag.

XPS spectra were recorded using unmonochromatized Mg K radiation (1253.6 eV), and an unmonochromatized He-resonance lamp was used for ultraviolet photoelectron spectroscopy (UPS). XPS spectra were taken with an analyzer resolution of 0.2 eV, and the net resolution measured as the full width at half-maximum (FWHM) of Au 4f(7/2) was 0.9 eV. The spectrometer is of our own construction and is, e.g., designed to provide optimum angle-dependent XPS or XPS(0) (12,l4). For high 5-values, the photoelectrons leave the sample surface near the grazing angel, and due to the limited escape depth of the electrons, this is a "surface sensitive" mode. In the "bulk sensitive" mode, for low 0-values, the photoelectrons exit near the surface normal, and hence more information from the "bulk" of the sample is obtained (15). 

X-ray photoelectron spectroscopy (XPS) uses X-rays generated by allowing an intense beam of electrons to strike an appropriate target material. The inherent width of X-ray lines is greater than the line sources of ultraviolet photoelectron spectroscopy (UPS), but diffraction from a spherically bent crystal can reduce the line width to around 0.25 eV. 

Although valence band information could be acquired by conventional X-ray sources, analysis of the valence band region is not as simple as the core region, since all the components in the sample contribute in this narrow region (with E of 30 eV or less). Due to the broad line width of conventional X-ray sources and the low ionization cross section. X-ray-excited valence band spectroscopy is less commonly used for surface analysis. Instead, ultraviolet sources (e.g.. He I and He II) are adopted to acquire the valence band spectra, a surface technique called ultraviolet photoelectron spectroscopy (UPS). He I and He n resonance lines have inherently narrow widths of only a few meVs and high ionization cross sections in the valence band. This technique is widely used in the study of adsorption phenomena and valence band structure of metals, alloys, and semiconductors. Work functions can be derived from the Fermi level and the secondary electron (SE) cutoff of the UPS spectrum. 

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