William G. Scott wrote: > Howdie folks: > > I'm preparing my annual X-ray diffraction lectures, and am at the point > where one replaces the atomic scattering factor with the integrated > electron density. > > It has always bothered me how this is a bit obscure, since it really > lies at the heart of the physical description. > > Is there a straightforward way to explain this?
I guess it depends on what level the course is, but you seem to suggest that the students do not have an extensive physics training. We have generally skipped over that bit quite elegantly, which does not in any way detract from the understanding of the crystallography (albeit perhaps the physics part of it.) The point to emphasize is going from a description based on a point scatterer, to realizing that atoms are not infinitely small; indeed their sizes are approximately identical to the wavelength of the X-rays. >From there, the integrals come easily by stating that you need to add up scattering contributions from all parts of the atom, which to a first approximation is well described as a spherical symmetric gaussian distribution of charge. Finally, the integrals are replaced by an experimentally determined scattering curve, where the angular components are "integrated out", leaving only a radial component, which is consulted via "table-lookup". Don't know if it's a better description, but it fast-forwards to the result which is most important in a crystallography context, and teleports the students across the hermeneutic circle ;-) Cheers, Morten -- Morten Kjeldgaard, asc. professor, MSc, PhD BiRC - Bioinformatics Research Center, Aarhus University C. F. Møllers Alle, Building 1110, DK-8000 Aarhus C, Denmark. Lab +45 8942 3130 * Fax +45 8942 3077 * Home +45 8618 8180 Mobile +45 5186 0147 * http://www.bioxray.au.dk/~mok
