Negative ions, mass selection, and photoelectrons

Abstract

Mass spectrometry and photoelectron spectroscopy of negatively charged atoms, molecules, and clusters open many technical and experimental possibilities to analytical chemistry, research on gas-phase reactions, molecular structure studies, investigations of vibrational and electronic states, and thus on chemical bonds. The outstanding features of the combination of negative ion sources, mass separation, and photoelectron detection, the subject of this article, are (i) the possibility of mass separation prior to spectroscopy; (ii) access to neutral systems and neutral states, which are not accessible by neutral neutral transitions; and (iii) accurate and unambiguous determination of electron affinities. The goal of this article is to impart insight into the possibilities and state-of-the-art of photodetachment experiments. This field has grown continuously for the last 30 years, and is now supplying impressive results on systems such as carbon clusters and fullerenes, metal and semiconductor clusters, metal carbon complexes, homogeneous and heterogeneous water clusters, the short-lived complex IHI, a textbook example of a chemical transition state, or triplet states of aromatic molecules and other hydrocarbons. Before results on these systems are presented in Section III, some experimental aspects are discussed in Section II. After a characterization of different types of anions from a physico-chemical point of view, different ways to form anions are explained. Different types of spectroscopy involving neutral anion transitions, i.e., photodetachment spectroscopy, photodetachment photoelectron spectroscopy, and anion-ZEKE (zero kinetic energy electron) spectroscopy, are discussed. In addition, the threshold laws for photodetachment, where a charged particle and a neutral core are separated in opposition to ionization, are briefly presented. At the end of Section II, different variations of experimental concepts are discussed and a sample experimental setup described. © 1999 John Wiley & Sons, Inc. Mass Spec Rev 17: 275–305, 1998