Electron Spectroscopy for Chemical Analysis

Abstract

When atoms are brought close together to form molecules the orbitals of individual atoms are perturbed and replaced by molecular orbitals. Inner orbitals, i.e. with higher binding energies, may still be regarded as atomic and belonging to specified atoms within the molecule, whereas the external orbitals combine to form the valence level system of the entire molecule. These orbitals take a more or less active part in the chemical bonds which are formed between the atoms in the molecule and which specify the chemical properties. The chemical bonds affect the charge distribution so that the original neutral atoms can be regarded as charged to various degrees and with different signs with a net charge of zero for a neutral molecule. We may describe the situation by regarding the individual atoms in the molecule as spheres with different potentials. Inside each charged sphere the atomic potential, set up by the removal of a certain small charge from its surface to the neighbouring atoms taking part in the chemical bond, is constant according to classical electrostatics. The result of this atomic potential is to shift the whole inner level system of any atom by a small amount, each level being shifted an equal amount. Levels belonging to different atoms in the molecule are generally shifted differently, however, and by measuring these “chemical shifts” for individual atoms in the molecule a mapping can in principle be made of the charge or the potential distribution in the molecule.