Researchers observe electrons in the molecule

New findings could make solar cells more effective

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For the first time, a research team has made the movement of electrons completely visible during a chemical reaction. The findings from the experiment are fundamental to photochemistry and could also help make solar cells more effective, the scientists report in the journal "Science".

In 1999, Ahmed Zewail received the Nobel Prize in Chemistry for his investigation of chemical reactions with ultrashort laser pulses. Zewail was able to observe the motion of atoms and thereby visualize transient states at the molecular level. To be able to observe the movement of individual electrons at that time still seemed to be a dream of the future. Thanks to new laser technology and intensive research in the field of attosecond spectroscopy - an attosecond equivalent to 10-18 seconds - this field of research has developed rapidly.

Complete tracking of movement of electrons

Researchers led by Professor Hans Jakob Wörner from the Laboratory of Physical Chemistry at ETH Zurich, together with colleagues from Canada and France, have now succeeded in completely tracking the movement of electrons during a chemical reaction. To this end, the team of scientists irradiated nitrogen dioxide (NO2) with a very short ultraviolet laser pulse.

The molecule absorbs the energy contained in this pulse and sets the electrons in motion. The electrons then begin to disperse differently, with the electron cloud for a short time in two different forms, the researchers say. The molecule then vibrates and eventually breaks down into nitric oxide and an oxygen atom.

Conical overlap

Nitrogen dioxide has a model character in terms of electron movement. The molecule has a balanced angled geometry. It is also clear that in the NO2 molecule two states of the electrons can have the same energy - one speaks of a conical overlap. The conical overlap is central to photochemistry and often occurs in nature in chemical processes triggered by light, the researchers say. display

The conical overlap works like a toggle switch. For example, if light strikes the retina, the electrons move there, and the retinal molecules ("retinal") "flip", ultimately transforming the information of light into electrical information for the brain. The special feature of conical overlapping is that the electron movement very efficiently passes into a movement of the atoms.

Snapshot of the electron

Wörner has already shown in an earlier study how the movement of electrons could be observed with attosecond spectroscopy. The first weak ultraviolet pulse stimulates electrons to move. A second strong infrared laser pulse removes an electron from the molecule, accelerates it and leads it back into the molecule. In this process, an attosecond pulse is emitted, which contains a snapshot of the electron distribution in the molecule.

You could compare that to photos where, for example, a bullet smashes through an apple. The ball is too fast for the shutter, so you leave the shutter completely open and exposed with flashes that are faster than the ball. This is how the snapshot is created, W rner illustrates the principle of attosecond spectroscopy.

From experiment to solar cell

When the electron returns to the molecule, it gives off energy in the form of light. In the experiment W rner and his colleagues measured the light of the electrons and thereby obtained detailed information about the electron distribution and its temporal evolution. This information reveals details of chemical reaction mechanisms that previously could not be captured.

The NO2 experiment, according to the researchers, helps to better understand fundamental processes in molecules and is an ideal complement to computer simulations of photochemical processes. Our experiment is so important because it puts theoretical models to the test. The findings can be applied, inter alia, in photochemistry, "says W rner. The immense interest in photochemical processes is not surprising, because this research should, among other things, improve solar cells or one day allow artificial photosynthesis. (Science, 2011; doi: 10.1126 / science.1208664)

(Swiss Federal Institute of Technology Z rich (ETH Zurich), 18.10.2011 - DLO)