Electrostatic Potential of Free Molecules Derived from Electron Diffraction Results

Abstract

With the extension of the wave theory to electrons, studies of structures at atomic and molecular dimensions became possible. Not only have electron microscopes proven to be powerful tools in biology and medicine, but individual heavy atoms can be followed in their thermal motion on a single crystal surface,¹ and the arrangement of atoms in crystals can be photographed directly.^2,3 The wave nature of electrons has been established many different times, but Moellenstedt et al. gave the most direct demonstration in their biprism interference experiment.⁴ All these experiments have one thing in common: they utilize high-energy electrons (greater than 30 keV), as at this energy the de Broglie wavelength is smaller than the atomic dimensions to be studied. In spite of the theoretically high resolving power of an electron microscope, the structure of a free molecule cannot be projected on the observation screen. This failure is due to the aberrations with which all optical devices are universally afflicted.⁵ Hence the most precise information on the nature of the charge distribution in molecules is of the indirect type obtainable by means of diffraction experiments.