Gas electron diffraction: Difference between revisions

Revision as of 19:02, 8 April 2022

Revision as of 08:19, 6 July 2022

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The scattering pattern consists of diffuse concentric rings (see Figure 2). The steep decent of intensity can be compensated for by passing the electrons through a fast rotation sector (Figure 3). This is cut in a way, that electrons with small scattering angles are more shadowed than those at wider scattering angles. The detector can be a [[photographic plate]], an electron imaging plate (usual technique today) or other position sensitive devices such as [[hybrid pixel detector]]s (future technique).

The intensities generated from reading out the plates or processing intensity data from other detectors are then corrected for the sector effect. They are initially a function of distance between primary beam position and intensity, and then converted into a function of scattering angle. The so -called atomic intensity and the experimental background are subtracted to give the final experimental molecular scattering intensities as a function of ''s'' (the change of [[momentum]]).

These data are then processed by suitable fitting software like [http://unexprog.org/ UNEX] for refining a suitable model for the compound and to yield precise structural information in terms of bond lengths, angles and torsional angles.

== Theory ==

[[File:GED scattering.jpg|left|thumb|440x440px|Scheme 2: Schematic scattering procesprocess of an electron passing a positively charged atomic nucleus]]

[[File:Electron waves.jpg|thumb|440x440px|Firgure 4. Electron wave scattered at a pair of atomic nuclei at different distances]]

GED can be described by scattering theory. The outcome if applied to gases with randomly oriented molecules is provided here in short:<ref>{{Cite book |title=Stereochemical Applications of Gas‐Phase Electron Diffraction, Part A: The Electron Diffraction Technique |last=Hargittai |first=I. |publisher=VCH Verlagsgesellschaft |year=1988 |___location=Weinheim |isbn=0-89573-337-4}}</ref><ref name=":1" />

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with <math>\lambda</math> being the electron [[wavelength]] defined above, and <math>\theta</math> being the scattering angle.

The above -mentioned contributions of scattering add up to the total scattering

: <math>I_\text{tot}(s) = I_\text{a}(s) + I_\text{m}(s) + I_\text{t}(s) + I_\text{b}(s),</math>

where <math>I_\text{b}(s)</math> is the experimental background intensity, which is needed to describe the experiment completely.

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== Results ==

[[File:Examples P4 P3As.jpg|thumb|440x440px|Figure 5: Examples of molecular intensity curves (lefteleft) and their Fourier tranformedtransforms, the radial districutiondistribution curves of P4 and P3As.]]

Figure 5 shows two typical examples of results. The molecular scattering intensity curves are used to refine a structural model by means of a [[Least-squares function approximation|least squares]] fitting [http://unexprog.org/ program]. This yield precise structural information. The [[Fourier transformation]] of the molecular scattering intensity curves gives the radial distribution curves (RDC). These represent the probability to find a certain distance between two nuclei of a molecule. The curves below the RDC represent the diffrerence between the experiment and the model, i.e. the quality of fit.