Photo- and Penning Ionization of Molecules in the Gas Phase and in the Liquid Phase

Abstract

Electron spectroscopy has contributed a great deal to the understanding of the properties of matter, let it be atoms, molecules or matter in the condensed phase. The common feature of these spectroscopies consists in analysing the kinetic energy of electrons that have experienced an interaction with the probe to be investigated. One way of performing the experiment is to start with a beam of electrons of known kinetic energy. If the loss of kinetic energy due to interaction with the probe is recorded one talks about electron energy loss spectroscopy (EELS). The energy loss spectrum is characteristic of the target atoms or molecules, but the detailed interpretation of such spectra is not always straightforward since the theoretical description of the electron molecule interaction is not simple. Still, many studies of this kind have been performed on gas phase molecules ^1,2 and first attempts to use this experimental tool for the investigation of molecules in the liquid phase have been reported ^3,4. Another widely used technique, applicable for primary electron beams of several keV energy, is to observe electrons which originate from ions produced by the impact of the primary beam. If the ion is created by removal of an electron out of a core hole then the ion is in a highly excited state and will decay via an Auger process by emission of a second electron into a doubly charged ion. The spectroscopy of these electrons (Auger electron spectroscopy=AES) has developed into a broad field in atomic and molecular physics ⁵ and has become a routine tool in the analysis of solid surfaces ⁶. Liquid surfaces have been investigated by this technique as well ², but its specific contribution to the understanding of liquid surfaces is yet to be understood.