The Study of Molecular Electron Distribution by X-Ray Photoelectron Spectroscopy

Abstract

A molecule contains two kinds of electrons: valence electrons and atomic core electrons. The valence electrons occupy molecular orbitals and are more or less delocalized on the various atoms of the molecule, whereas the core electrons are highly localized on individual atoms of the molecule. In gas-phase X-ray photoelectron spectroscopy (XPS), one irradiates a sample chamber containing the molecules under study with an approximately monochromatic beam of X-rays and measures the kinetic energy distribution of the ejected electrons.1–3 The spectrum is a plot of electron counting rate as a function of kinetic energy, EK. Peaks are observed corresponding to electronic levels with binding energies (EB) less than the X-ray photon energy, hv. By use of the Einstein relation, hν = EB + EK, one can readily calculate the binding energy corresponding to each peak. The X-ray has enough energy to eject not only valence electrons, but also some core electrons. The main use of XPS is in the determination of core electron binding energies, and this paper will be concerned with the information that one can glean from the core binding energies of molecules.4