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Atomic Orbitals Imaged

Posted by Roblimo on from the fun-with-science dept.

joshv writes "Ever think that physics professor was smokin dope as he described those mysteriously shaped clouds of electron probability floating around atoms? Here's proof. Someone has managed to image atomic orbitals using X-ray crsytallography." The story's from Scientific American; very cool stuff. This may be old hat to physics grads, but it's interesting to us laypeople. ;-)

2 of 53 comments (clear)

  1. The article doesn't inspire confidence... by teraflop+user · · Score: 5
    X-rays (in crystallography experiments) are not scattered by nuclei, they are scattered by the electron density in the crystal, just like the electron beams. If you want to image nuclei, you have to go to neutron scattering.

    The X-ray and electron beams are not combined, they are collected separately, and the information is combined.

    Moreover, the principle difficulty has been completely ignored. Electron beams can be focused to produce a direct image. X-rays cannot be focused for imaging purposes (although crude focussing to concentrate a beam is just possible). As a result, you only get a diffraction pattern with no phase information. The image must be reconstructed by Fourier transformation, which needs the phases. (There is a strong analogy with optical holography, in which a reference beam must be interfered with the diffracted beam to obtain phase information, but with x-rays the coherence length is too short to get a reference beam).

    The trick is to use the phases from the low resolution electron image, and some mathematical relationships to reconstruct the missing phases in the high resolution image, which will show your electron orbitals. The problem is unless the statistics are treated very carefully, all you get is an image which confirms the assumtions of the model you used to get the relationships with which you reconstruct the phases.

    The mathematical techniques were just coming on line in X-ray crystallography in 1996 and there was still considerable debate back then over their correct application. So there is a fair possibility that these results are correct, but I would suspend judgement until they have been scrutinised for a year or two.

  2. Atoms in Molecules (AIM) by mvw · · Score: 5
    I was excited recently to learn that there exists a (more) rigorous derivation of the empirical chemical wisdom that molecules can be described fairly good by considering them built from smaller components (functional groups and atoms of course).

    That such a treatment is possible, is not obvious, nature could have been that way, that one had to treat the whole system (like solving the wave equation for all particles at once) in order to make any useful statement at all.

    Read this article on Atoms in Molecules by Richard Bader to find out more.

    Bader claims that the study of the Laplacian (2nd spatial derivative) of the electron distribution leads to a natural spatial decomposition of a molecule.

    Have a look at these great pictures for some simulations based on that AIM theory.

    I am still surprised, that during my physics studies, I heard nothing about that treatment. You get exposed to Feynman of course, but Schwinger's formulation of quantumn electro dynamics I knew only as possible but not practical alternative. Very interesting to see Schwingers approach at the heart of this AIM theory.