What is photoelectron spectroscopy (PES) primarily used for?

Photoelectron spectroscopy (PES) is a technique primarily employed to investigate the electronic structure of atoms and molecules. It operates on the principle of the photoelectric effect, wherein ultraviolet or X-ray photons are used to irradiate a sample, causing the ejection of electrons from its surface. The primary output of this process is the measurement of the kinetic energies of the emitted electrons.

By measuring these kinetic energies, PES allows researchers to gather information on the energy levels of electrons in the sample prior to ionization. This data is instrumental in deducing the binding energies of electrons, providing insights into the electronic environment of the sample, including details about the chemical state and the distribution of electrons across various energy levels.

Thus, the emphasis on measuring the kinetic energies of electrons makes option B the correct choice, as this is the fundamental action and purpose of photoelectron spectroscopy. The other options do not align with the primary application of PES, as they relate to different types of measurements and analyses.