Green’s Function Calculations of Ionization Spectra of Molecules in the Outer and Inner Valence Region

Abstract

The study of ionic states has become a very important field of research containing an enormous amount of chemical and physical information which can be extracted by different experiments and by calculations. It is a field of research where experiment and theory work hand in hand in order to obtain this information. Among the most important pieces of information are the ionization energies, i.e., the transition energies to the different ionic states. Their calculation will preoccupy us in the sections below. It is generally assumed that the most prominent bands in a photoelectron spectrum (PES) arise from transitions to electronic states that are obtained by ejection of a single electron out of a molecular orbital in the ground state. In addition to these “simple” one-electron transitions two-electron transitions can sometimes be observed, i.e., processes which are ionization of one plus simultaneous excitation of another electron. They appear as satellite lines in a PES and borrow their intensity from the simple transitions. In the outer valence region and in the core region their intensity is small compared to the main line; i.e., a one-electron picture of the ionization process is valid. In the inner valence region, however, this need not be the case as we are going to see.