Gas-Phase Electron Scattering from Free Spatially Oriented Molecules

Abstract

Diffraction methods are often employed for structure analysis. While x-ray diffraction is widely used to analyze condensed matter, electron scattering ^1–3 is the preferred method of investigating gaseous compounds. In the vapor phase it is superior to x-ray scattering since electrons interact much more strongly with the target atoms than photons. This was already demonstrated in the early pioneering work of Mark and Wierl ⁴. A comparison with x-ray scattering can serve to illustrate the role of orientation in diffraction experiments. In the powder diffraction (Debye-Scherrer) technique a sample consisting of randomly oriented crystallites is exposed to a monochromatic x-ray beam. This leads to a characteristic pattern of interference rings which is cylindrically symmetric due to the random orientation of the target components. In this respect the result of electron diffraction from a beam of unoriented molecules with a statistical distribution of their molecular axes is quite similar, although the resulting circular scattering pattern is usually more diffuse. Since there is no preferred orientation, the structure information is only one-dimensional in these cases.