Shape Resonances in the Photoionization Spectra of Free and Chemisorbed Molecules

Abstract

With the emergence of synchrotron radiation as a powerful spectroscopic tool, it has become possible to study in detail the photoionization cross sections immediately above the various ionization potentials, not only of core levels (which can also be done by x-ray absorption) but also of all the different valence levels (though the use of photoelectron spectroscopy). The most prominent features in these cross sections are in molecular physics known as shape resonances and constitute major modifications of a simple hydrogenic photoabsorption cross section. Essentially the same phenomenon is sometimes in the condensed matter literature referred to as XANES (x-ray absorption near edge structure), to contrast it with EXAFS (extended x-ray absorption fine structure). This very rich field has over the last five years developed into several different directions. The purpose of the present paper is to summarize some fairly recent experiments concerned with shape resonances in free diatomic molecules and to show how this knowledge can be used in surface physics to extract information otherwise difficult to obtain. Many of the properties of free molecules are retained after chemisorption. It is possible to perform much more accurate theoretical calculations for free molecules than for the molecule- surface complex. By cautiously extrapolating these calculations to the chemisorbed phase much new insight has been gained.