Gas electron diffraction: Difference between revisions

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Latest revision as of 19:30, 30 July 2025

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== Results ==

[[File:Examples P4 P3As.jpg|thumb|440x440px|Figure 5: Examples of molecular intensity curves (left) and their Fourier transforms, the radial distribution 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 yieldyields 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 diffrerencedifference between the experiment and the model, i.e. the quality of fit.

The very simple example in Figure 5 shows the results for evaporated white [[phosphorus]], P₄., Itwhich is a perfectly tetrahedral molecule and has thus only one P-P distance. This makes the molecular scattering intensity curve a very simple one; a sine curve which is damped due to molecular vibration. The radial distribution curve (RDC) shows a maximum at 2.1994 Å with a least-squares error of 0.0003 Å, represented as 2.1994(3) Å. The width of the peak represents the molecular vibration and is the result of [[Fourier transform]]ation of the damping part. This peak width means that the P-P distance varies by this vibration within a certain range given as a vibrational amplitude ''u'', in this example ''u''_T(P‒P) = 0.0560(5) Å.

The slightly more complicated molecule P₃As has two different distances P-P and P-As. Because their contributions overlap in the RDC, the peak is broader (also seen in a more rapid damping in the molecular scattering). The determination of these two independent parameters is more difficult and results in less precise parameter values than for P₄.