Introduction

Photoemission studies may be said to have begun in 1887 with the observation by Heinrich Hertz that a spark between two electrodes was obtained more easily if the electrodes were illuminated (HeTrtz, 1887), although this occurred before the discovery of the electron. Improved experiments during the next several decades were important to the development of quantum mechanics. The modern era of photoemission spectroscopy arguably may be said to have started in 1964 with the papers of Berglund and Spicer (1964a,b). Although there had been prior experimental work and calculations in which Bloch electrons were assumed and dipole matrix elements calculated, it was these two papers, and papers by Gobeli et al. (1964) and Kane (1964), which stimulated a large amount of work, coming as they did nearly simultaneously with the widespread calculation of accurate electronic structures of many materials and the commercial availability of ultrahigh vacuum components. The first paper of Burglund and Spicer worked out what is called the three-step model for photoemission, and the second applied it to new data on Cu and Ag.

In the following we outline several approaches to the description of photoelectron spectra, starting with the most simple conceptually, then progressing to a more sophisticated picture. As we do this, we point out some assumptions and approximations often made, sometimes tacitly, and whether or not they may be important for photoelectron spectroscopic studies of high-temperature superconductors. Experimental considerations are presented in Chapter 4.