Gas electron diffraction: Difference between revisions

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'''Gas electron diffraction''' (GED) is one of the applications of [[electron diffraction]] techniques. The target of this method is the determination of the structure of [[gaseous molecules]] i.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. Diffraction occurs because the [[wavelength]] of electrons accelerated by a potential of a few thousand volts is of the same order of magnitude as internuclear distances in molecules. The principle is the same as that of other electron diffraction methods such as [[Low-energy electron diffraction|LEED]] and [[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. Since the orientation of the target molecules relative to the electron beams is random, the internuclear distance information obtained is one-dimensional. Thus only relatively simple molecules can be completely structurally characterized by electron diffraction in the gas phase. It is possible to combine information obtained from other sources, such as [[rotational spectroscopy|spectra]], NMR spectroscopy or high-quality quantum-mechanical calculations with electron diffraction data, if the latter are not sufficient to determine the molecule's structure completely.

The total scattering intensity in GED is given as a [[function (mathematics)|function]] of the [[momentum]] transfer, which is defined as the difference between the [[wave vector]] of the incident [[electron]] beam and that of the scattered electron beam and has the [[reciprocal dimension]] of [[length]]. The total scattering intensity is composed of two parts: the [[atomic scattering intensity]] and [[the molecular scattering intensity]]. The former decreases [[monotonically]] and contains no information about the molecular structure. The latter has [[sinusoidal]] modulations as a result of the [[Interference (wave propagation)|interference]] of the scattering [[spherical waves]] generated 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.