Excited States of Molecules and the Scattering of Fast Electrons

Abstract

When fast electrons are scattered by molecules which undergo excitation in the course of the scattering process, the patterns associated with discrete energy losses show the characteristic molecular‐diffraction features associated with interatomic distances. A theory is developed here in agreement with experimental results thus far obtained which describes the possible forms for these diffraction features, and shows that the molecular scattering does not necessarily disappear on averaging over all states of excitation, as has been generally assumed for the ``incoherent'' scattering. In developing the theory, the wave function for the molecule is formed from a linear combination of atomic orbitals assumed to be S functions. By means of group theoretical methods the proper linear combinations of S functions may be composed for the possible types of excited electronic states of the molecule. The theoretical analysis also employs the Franck‐Condon principle and the Born scattering theory. From the theory developed here it is possible to predict and interpret gross features of the molecular scattering from molecules in the process of excitation. The accurate prediction of energy changes and atomic background scattering are interesting problems for future consideration. This new electron‐diffraction technique should be a valuable adjunct to the field of photon spectroscopy.