What are the two types of PES?

The correct identification of the two types of PES is based on the types of radiation used to excite the electrons in the sample. X-ray photoelectron spectroscopy (XPS) utilizes X-ray radiation to eject core electrons from an atom, providing valuable information about the elemental composition and chemical state of the materials being analyzed. This technique is particularly useful for measuring the binding energies of core-level electrons, which can give insights into the electronic environment around specific atoms.

On the other hand, UV photoelectron spectroscopy (UPS) utilizes ultraviolet light to excite valence electrons rather than core electrons. This method is adept at exploring the electronic structure, especially the valence levels of materials, and can help in probing molecular orbitals and electronic states in more detail.

Both techniques, XPS and UPS, are important tools in surface science and materials characterization, as they provide complementary information about the electronic structure of materials. Their distinction lies primarily in the source of radiation used and the types of electrons being analyzed, making the selection of the correct answer pivotal for understanding the principles of photoelectron spectroscopy.