Vibrational Spectra by Electron Impact: Theoretical Models for Intensities

Besides its valuable applications in surface science and to problems of chemisorption and catalysis, vibrational electron energy loss spectroscopy (EELS) is becoming a useful tool in giving information on excited states of molecules and on the electronic structure of the respective negative ions and their unimolecular decomposition. Need for a deeper chemical insight has been also recognized for a variety of electron-molecule collisional problems that have been traditionally a domain of physics. Theoretical basis for EELS is less developed than for other spectroscopies used in chemistry such as infrared, UV/VIS, or NMR. It is a feeling of the authors that the achievements of the electronic structure theory of molecules can be more exploited in the electron scattering theory. The aim of this paper is to present our attempt to develop a simple but rigorously ab initio computational method that can be applied to vibrationally inelastic electron scattering by polyatomic molecules. Some general problems of ab initio calculations on vibrational inelastic differential cross sections are discussed.