Fakultät für Chemie - Gas Electron Diffraction (GED)
Uni von A-Z
Bielefeld University > Department of Chemistry > Research Groups > ac3-mitzel > Gas Electron Diffraction (GED)

What is GED?


Similarly to X-ray diffraction (XRD), the basic principle of gas-phase electron diffraction (GED), is wave interference. According to the dualistic nature of electrons, in high energy electron diffraction, partly particle properties and partly wave properties of the electrons are revealed. If an electron beam hits a molecule, the electrons get deflected from their original trajectory (as well as the molecules). This is mainly due to the coulomb attraction they feel from the strongly localized and positively charged atomic nuclei. The interaction with the diffuse electron cloud of the molecule is almost negligible, which is in complete conrast to XRD where that interaction is the only source of scattering. If like in GED, the electron beams energy is much higher, than the ionisation energy of the molecule, the total energy of the scattered electrons is conserved and only the direction of the electrons trajectory changes (elastic scattering).  


According to the wave nature of the scattering electron, there is no trajectory of the electron passing the molecule somewhere, but rather a wave-interference-like intensity distribution emerging from the molecule as the scattering center. The directional distribution of the scattered electrons contains information on the distances between pairs of atoms in the molecule, which are acting as scattering centers:  

Each atom pair in the molecule contributes a damped sine curve to the scattering total intensity as a function of the scattering angle. The frequency of these sine curves is proportional to the interatomic distance of the respective atom pair in the molecule. Hence a Fourier transformation of the scattered intensities gives a radial distribution curve showing the frequency of occurrence of a certain interatomic distance in the molecules under investigation.  


Finally the problem of the determination of the underlying 3 dimensional structure leading to the 1 dimensional radial distribution curve has to be solved. This is done usually by setting up a so called "model" containing an parameterized version of the atomic coordinates in the form of FORTRAN program. On the basis of this model a theoretical estimation for the scattering data can be calculated. In a least-squares-fitting procedure the best parameter values are determined which are leading to the the smallest disagreement between the theoretical scattering data and the experimental ones. One of the major problems hampering this procedure is the ro-vibrational dynamics of the molecule leading to a brodening of the peaks in the radial distribution curve and hence to a smearing out of the contained information about interatomic distances. Another problem is the occurrence of many similar interatomic distances which is one of the striking characteristics of molecules, but may however be more pronounced for certain molecules (organic compounds with many chemically similar functional groups) while less for others (inorganic heteroatom rich compounds). Finally these problems lead to a restriction of the size and complexity of systems which can be treated by GED. On the other hand-side the GED information can be complemented by structural information yielded by different methods in order to overcome these problems. As formerly mentioned in electron diffraction also the particle nature of the electron shows up. One consequence of that is, that the probability to get a high scattering angle when the electrons feel the nucleis coulomb potential is only low. It depends to a great extend on the probability to of the electrons the cross the nuclei in a close-by trajectory. This is very similar to the scattering of alpha particles by a metal foil. This results in a very high total intensity at small angles scattering and a very low one at lower angels. When looking at the underlying physica it becomes obvious that the largest part of this intensity does not contain the oscillating contributions containing the information of the interatomic distances.  


Its like trying to find the bumps in the skiing slope while inspecting the height lines in the map of the mountain. In order to overcome that problem the technique of the rotating sector has been successfully introduced in the late '50s of the last century.


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