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A Brief Overview of X-ray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy also uses X-rays to excite electrons from molecular orbitals into the continuum. However, conventional XPS does not measure absorption while scanning through the absorption edge as is done for XAS. Rather, XPS is conducted by by using a fixed energy source to excite electrons from the sample and then measuring their kinetic energy. Since their kinetic energy is dependent on their binding energy (and, strictly speaking, a correction factor called the work function), different chemical species can be identified based on their distinct binding energies. These experiments are commonly carried out with Al or Mg anodes in the laboratory; these ****ls release X-rays when a high voltage is applied that is monochromatic and useful for XPS. However, synchrotron-based XPS (S-XPS) offers many benefits. Most importantly, since the excitation energy is variable, the kinetic energy of photons can be varied continuously. This is useful because the distance that an electron can travel though the solid (called the escape depth) varies with kinetic energy. Synchrotron-based XPS permits the kinetic energy for a given orbital to be tuned to a value (typically about 50-100 eV) where the escape depth reaches a minimum (about 1 monolayer or a few Ångstroms). Consequently, S-XPS is highly sensitive for surfaces. The incident energy can then be tuned to determine the thickness of any surface layers. Synchrotron-based XPS has other advantages over other methods; it affords superior resolution to conventional instruments, permitting the differentiation of similar chemical species. S-XPS also is useful for light elements (those with atomic number of Ca and lower), because they are difficult to analyze by XAS since their X-rays are too low in energy to pass though air as well. Therefore, elements such as Al, Si, N, and C usually are analyzed by XPS. Fortunately, complementary XAS information can usually be obtained for these light elements concurrently to XPS in synchrotron-based XPS systems.
It should be mentioned that there are limitations associated with this technique because it detects electrons. First, since electrons do not pass through air, these experiments must be run in vacuum. This also means that samples must be dried (water evaporates in a vacuum); this drying may impact the system or result in many chemical transformations. Also, since this technique is highly surface sensitive, it is less useful to analyze bulk properties; in fact, it may make it impossible to identify anything but interferences on a dirty-surface. Consequently, XPS is often done with clean, freshly-cleaved single crystals.
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