X-ray photoelectron spectra of solids

Both UPS and XPS of solids are useful techniques. So far as studies of adsorption by surfaces are concerned we would expect UPS, involving only valence orbitals, to be more sensitive. For example, if we wish to determine whether nitrogen molecules are adsorbed onto an iron surface with the axis of the molecule perpendicular or parallel to the surface it would seem that the valence orbitals would be most affected. This is generally the case but, because ultraviolet photoelectron spectra of solids are considerably broadened, it is the X-ray photoelectron spectra that are usually the most informative. 

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