Photoelectron spectroscopy without photoelectrons: Twenty years of ZEKE spectroscopy

Zero Kinetic Energy (ZEKE) spectroscopy, originally developed as a high resolution form of photoelectron spectroscopy, promised a means to the unambiguous determination of ionic (ro)vibrational states. Since its original development, it has spawned numerous methodological offshoots and has become one of the default methods of choice for high resolution spectroscopy of the ion. This tutorial review describes the historical development of the method, provides some insight into how it works and assesses the impact of the technique by reviewing some of the highlights of the past 20 years as well as some of the more recent developments and applications.