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Electron clouds: seeing is believing

orbitals.jpgThese blobs are images of the electron clouds around a carbon atom. There’s a radially symmetric blob, and a double-lobed blob with a node in the middle – just like the patterns of electron density that the s and p atomic orbitals give rise to.

The snaps come from Igor Mikhailovskij and colleagues at the Kharkov Institute for Physics and Technology, Ukraine, reporting in a paper to be published in Physical Review B. They cool a chain of carbon atoms to 4 kelvin in a vacuum, and apply a voltage so as to create an electric field which draws some electrons away from the atom on the chain’s tip, towards a phosphor screen. (This is called field emission microscopy). The spatial distribution of that image represents the electron density around the atom. If the round blob looks elliptical, that’s perhaps due to interaction with the graphite tip that supports the carbon chain, the researchers think.

More images below the fold…

orbitals2.jpgThere are some mysteries about this experiment – for example, the researchers show an image of two end-chain atoms, one of which suddenly changes from a circle to a dumb-bell blob at constant voltage. They also get overlay blobs, perhaps showing superpositions of s and p orbitals. Still, it’s pretty astounding.

Some blurrier pictures have been taken before of electron density at the atomic level, using the atomic force microscope, a thin tip which essentially feels its way around lumps of electron repulsion on a surface, thus imaging electron density. That usually doesn’t reach sub-atomic resolution, but if the tip itself is atomically thin, you can work out a picture of electron density around it.

Franz Giessibl, at the University of Regensburg, takes the lead here. Using an AFM, he and others (then at the University of Augsburg) spotted in fuzzy form sp3 hybrid orbitals of a silicon atom in 2001 (doi: 10.1126/science.289.5478.422). And here are some clearer 2003 images of electron density (doi:10.1103/PhysRevB.68.045301) representing a 4fz3 hybrid in samarium (the green arrow is pointing to a hole in the silicon surface that the samarium tip is lowered over).


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Images: APS/Kharkov Institute/University of Augsburg