A Coreography for Two Electrons on the Stage

XChem team, together with researchers at Max Planck Institute for Nuclear Physics in Germany, have for the first time observed and controlled the synchronized movement of the two electrons that make up the helium atom. The work opens the way for the production of substances that cannot be synthesized using conventional chemical methods.

Electrons moving around an atom nucleus minimize the effect of mutual repulsion by avoiding each other. This results in a “correlated” or “concerted” movement, in which the behavior of a single electron affects the others. So far, this movement had eluded direct experimental observation of scientists.
In the latest issue of the journal Nature, Spanish and German scientists not only presented the first film of correlated motion of the two electrons that make up the helium atom, they also report having controlled the footsteps of his particular ‘dance’.

To do this they used an advanced version of a technique known as Attosecond Transient Absorption Spectroscopy (ATAS), which combines pulses of visible and ultraviolet light to an ultrafast time scale (about a trillionth of a second).

Experiments consisted on the measure of the transparency of a sample of helium with short flashes of ultraviolet light, depending on the time between the flash and a red pulse generated by a titanium-sapphire laser. The ultraviolet pulse leads the atom to an excited state, where both electrons oscillate in concert. Red light pulse weakens or strengthens the ultraviolet light absorption as a function of the relative position of the two electrons and thus, the time between this pulse and the ultraviolet pulse. From variations in ultraviolet absorption, the movement of both electrons can be reconstructed and thereby generate a motion film. The control of this movement was achieved by varying the intensity of the red pulse.

Since most of the bonds holding atoms –from water molecules to DNA- are formed by a two electron pair, this work opens the door to control the properties of chemical bonds and perhaps to produce compounds that cannot be obtained using standard procedures in Chemistry.

These results were obtained with the help of unprecedented quantum mechanical calculations carried out in the frame of XChem project

Figure: Snapshots of correlated motion of the two electrons of helium. 15.3 femtosecond (fs) after starting the clock, the two electrons are near the nucleus (red area near the center of the image) and then move away from it. Color indicates the probability of finding an electron at the position A (vertical axis) and the second of electrons in the position B (horizontal axis) on a line drawn through the atom (along the direction of laser polarization). A femtosecond later, ie 16.3 femtoseconds after starting the clock, electrons have moved back to its original position.

 
Source: Ott, C., Kaldun, A., Argenti, L., Raith, P., Meyer, K., Laux, M., Zhang, Y., Battermann, A., Hagstotz, S., Ding, T., Heck, R., Madroñero, J., Martín, F. & Pfeifer, T. “Reconstruction and Control of a Time-Dependent Two-Electron Wave Packet”. Nature, 516, 374-378 (2014) December 18/2014 (doi:10.1038/nature14026)
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