- Gas electron diffraction
Gas electron diffraction (GED) is one of the applications of
electron diffractiontechniques. The target of this method is the determination of the structure of gaseous moleculesi.e. the geometrical arrangement of the atoms from which a molecule is built up.
The geometrical structure of molecules can be worked out directly from the observed electron diffraction pattern by using this method. The principle is the same as that of other electron diffraction methods such as
LEEDand RHEED, but the obtainable diffraction pattern is considerably weaker than those of LEED and RHEED because the density of the target is about one thousand times smaller and the orientation of the target relative to the electron beams is random, which corresponds to incoherent scattering.
In the case of determining the structure of complex molecules, the information obtained from rotational
spectrais combined. The total scattering intensity is given as a function of the momentumtransfer, which is defined as the difference between the wave vectorof the incident electronbeam and that of the scattered electron beam and has the reciprocal dimensionof length. The total scattering intensity is composed of two parts: the atomic scattering intensityand the molecular scattering intensity. The former decreases monotonicallyand contains no information about the molecular structure. The latter has sinusoidalmodulations as a result of the interferenceof the scattering spherical wavesgenerated by the scattering from the atoms included in the target molecule. The interferences reflect the distributions of the atoms composing the molecules, so the molecular structure is determined from this part.
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