Electron Spectroscopy and Molecular Structure

Electron spectroscopy can now be applied to solids, liquids and gases. Some fields of research require ultrahigh vacuum conditions, in particular those directly concerned with surface phenomena on the monolayer level. Liquids have recently been subject to studies and several improvements and extensions of this technique can be done. Much advance has lately been achieved in the case of gases, where the pressure range presently is 10' — 1 torr. Signal-to-background ratios for core lines can be — 1000: 1 and the resolution has been increased to the extent that vibrational fine structures of is levels in some small molecules have been observed. These improvements are based on the monochromatization of the exciting AlK radiation. Under such conditions the background is so much reduced that shake-up structures are more generally accessible for closer studies. ESCA shifts are also much easier to resolve and to measure with higher precision around 0.02 eV. It is interesting to compare the results from electron spectroscopy with those recently obtained from X-ray emission spectra of free molecules. By means of grating at grazing incidence a resolution has been obtained at a sufficient intensity to record not only transitions from resolved molecular orbitals to the is levels but also in some cases vibrational fine-structures inside such separate lines. The photoionization dynamics including atomic and molecular relaxations have been investigated, both experimentally and theoretically. In the valence electron region improvements in energy resolution and in the application of the intensity model based on the MO-LCAO approximation greatly facilitate the assignments of the valence orbitals. Accumulation of empirical evidences gathered from series of similar chemical species and also better methods of calculation, both ab initio and semi-empirical, have gradually resulted in a much better understanding of the molecular orbital description. The experience of the latest ESCA instrument with monochromatization has motivated an attempt to design an optimized apparatus according to the general principles of this prototype. A considerable gain in intensity can be made at an improved resolution set by the inherent diffraction pattern of the focussing spherical quartz crystals. INTRODUCTION Electron spectroscopy has been reviewed several times during the last years and more recently by the author in the conference volume' of the Namur Conference on Electron Spectroscopy (1974). The reader is referred to this book for a more complete account of the present situation in the field and to a comprehensive list of references. In the following I will briefly summarize first some features of this spectroscopy and then illustrate with some recent applications, some of which have not yet been published. Electron spectroscopy was originally started in my laboratory for precise measurements of electron binding energies in atomic and solid state physics. After some ten years development work it gradually became clear that this spectroscopy should have interesting applications in chemistry. With this conclusion in mind the acronym ESCA, Electron Spectroscopy for Chemical Analysis, was coined around 1965. The word Analysis was then taken in a more modern and broader sense than the classical one. For example, one very important application of ESCA is surface analysis. One can analyze both solids, gases and, more recently, liquids. All elements, including the important light elements carbon, nitrogen and oxygen, can be analysed on the surface with high accuracy. A particular feature of ESCA is that in addition to the elemental composition of a compound frequently other useful information can be obtained about the chemical bonds and charge distribution. This information can be deduced from the so-called ESCA shifts of the core level electron lines. When atoms are brought close together to form a molecule the electron orbitals of each atom are perturbed. Inner orbitals, that is, those with higher binding energies, may still be regarded as atomic and belonging to specified atoms within the molecule, whereas the outer orbitals combine to form the valence level system of the molecule. These orbitals play a more or less active part in the chemical bonds which are formed between the atoms in the molecule and which define the chemical properties. The chemical bonds affect the charge distribution so that the original neutral atoms can be regarded as charged to various degrees while the neutral molecule retains a net charge of zero. One may describe the situation by regarding the individual atoms in the molecule as spheres with different charges set up by the transfer of certain small charges from one atomic sphere to the neighbouring atoms taking part in the chemical bond. Inside each charged sphere the atomic potential is constant in accordance with classical electrostatic theory. The result of this atomic potential is to shift the whole system of inner levels in a specified atom by a small amount, the same for each level. Levels belonging to different atoms in the molecule are generally shifted differently, however, and by measuring these ESCA "chemical shifts" for individual atoms in the molecule a mapping can be made of the distribution of charge or potential in the molecule. This is then a reflection of the chemical bondings between the atoms which in turn can be described by the orbitals in the valence level system. It should be remarked that these chemical shifts are different from those obtained in NMR and may even go in the reverse direction. The chemical shift effect has its theoretical importance in the study of molecular electronic structure. In other contexts its usefulness lies precisely in its ability to specify the chemical composition, in particular at sur-