Photoelectron Spectroscopy

Abstract Photoelectron spectroscopy measures the energies for emission of electrons from molecules or materials through application of the photoelectric effect. For neutral molecules this is a direct measure of the ionization energies. The ionizations are explicitly defined in terms of transitions between the ground state of a molecule and accessible states of the molecular positive ion, but in a first order approximation, each ionization of a molecule can be described as removal of an electron from an individual orbital. The ionization energies can then be considered as measures of orbital energies, and shifts can be interpreted in terms of orbital stabilizations or destabilizations due to changes in electron distributions and bonding. The intensities and profiles of the ionization bands in the photoelectron spectra provide additional information on the characters and bonding natures of the orbitals. All orbitals can be probed, from valence orbitals such as lone pairs and bonds to orbitals in the cores of heavy atoms. Photoelectron spectroscopy is a large field of study that has many subfields and new techniques continue to be introduced. These different subfields all have different instrumental and sample constraints, but taken as a whole photoelectron spectroscopic experiments can be performed on almost any system of interest in inorganic chemistry. This article summarizes the basic physical principles of photoelectron emission caused by one or more photons, the relationship of photoelectron spectra to the electronic structure of neutral and anionic molecules, and the additional considerations for photoionization of bulk samples. Auger electron spectroscopy (AES) and Penning ionization electron spectroscopy (PIES), techniques which are closely related to photoelectron spectroscopy, are also discussed. Case studies are given for applications of photoelectron spectroscopy to study of bioinorganic model complexes, including iron‐sulfur and molybdenum‐sulfur bonding and covalency, electronic structure of porphyrins and metalloporphyrins, negative‐ion photoelectron spectroscopy of phthalocyanines, and the use of the technique to measure inner‐sphere reorganization energies.