High‐resolution Photoelectron Spectroscopy

The evolution of photoelectron spectroscopy of neutral molecules in the gas phase, from its first applications in the early 1960s to the present day, is reviewed with emphasis on the description of methods that have contributed to the improvement of resolution. In the early days, photoelectron spectroscopy was very successfully used to study the electronic structure of molecules. As the resolution improved, the emphasis gradually shifted to studies of the structure and dynamics of molecular cations. Current methods enable one to fully resolve the rotational structure in the photoelectron spectra of polyatomic molecules and even to measure the fine and hyperfine structures of positively charged atoms and small molecules. Several of the methods providing high resolution make use of the long lifetimes and electric‐field‐ionization properties of high Rydberg states and give access to detailed information on molecular ionization channels and molecular photoionization.