The 1999 Nobel prize in chemistry was awarded to Ahmed Zewail for his studies of chemical reactions using ultrashort laser pulses. Zewail was able to watch the motion of atoms and thus visualize transition states on the molecular level. Watching the dynamics of single electrons was still considered a dream at that time.

Now a research group led by ETH Zurich has for the first time, visualized the motion of electrons during a chemical reaction.  The new techniques of the experiment are of fundamental importance for chemistry and could also assist the design of more efficient solar cells and many other chemistry matters.

Prof. Hans Jakob Wörner from the Laboratory of Physical Chemistry at ETH Zurich, together with colleagues from Canada and France, for the first time was able to record electronic motion during a complete chemical reaction.

The experiment is detailed in the latest issue of Science.

The research team irradiated nitrogen dioxide molecules (NO2) with a very short ultraviolet pulse.  The molecules take up the energy from the pulse, which sets the electrons in motion. The electrons start rearranging themselves, which causes the electron cloud to oscillate between two different shapes for a very short time, before the molecule starts to vibrate and eventually decomposes into nitric oxide and an oxygen atom.

Molecules Observed in Reaction. Click image for more info.

It seems the research team choose nitrogen dioxide because it has a model character with respect to understanding electronic motion. In the NO2 molecule, two states of the electrons can have the same energy for a particular geometry – commonly described as a conical intersection. The conical intersection is very important for photochemistry and frequently occurs in natural chemical processes induced by light.

The conical intersection works like a dip-switch. For example, if the retina of a human eye is irradiated by light, the electrons start moving, and the molecules of the retina (retinal) change their shape, which finally converts the information of light to electrical information for the human brain. The special aspect about conical intersections is that the motion of electrons is transferred to a motion of the atoms very efficiently.

How this works was first described by Hans Jakob Wörner who published how attosecond spectroscopy can be used for watching the motion of electrons. First a weak ultraviolet pulse sets the electrons in motion. The second strong infrared pulse then removes an electron from the molecule, accelerates it and drives it back to the molecule. As a result, an attosecond light pulse is emitted, which carries a snapshot of the electron distribution in the molecule.

Wörner illustrates the principle of attosecond spectroscopy, “The experiment can be compared to photographs, which, for example, image a bullet shot through an apple. The bullet would be too fast for the shutter of a camera, resulting in a blurred image.  Therefore, the shutter is left open and the picture is illuminated with light flashes, which are faster than the bullet. That’s how we get our snap-shot.” Pretty fast stop motion photogaphy.

In Wörner and his colleagues’ experiment the electron returns to the molecule where it releases energy in the form of light.  The team measured the light of the electrons and was therefore able to deduce detailed information on the electron distribution and its evolution with time. This information reveals details of chemical reaction mechanisms that were not accessible to most of previous experimental techniques.

The experiment on NO2 helps understanding fundamental processes in molecules and is an ideal extension of computer simulations of photochemical processes.

Wörner said, “What makes our experiment so important is that it verifies theoretical models.”

The team’s breakthrough or break-in if precise, answers an intense interest photochemical processes.  The photochemical field is aiming at improving solar cells and making artificial photosynthesis possible.

Yet the break-in to the activity of a molecule in reaction offers a huge area of study and deeper understanding of chemical actions.  Just how fast the technology and techniques can adapt to other molecules, compounds and reactions will be a fascinating story with, hopefully new insights on improving the use of chemistry.


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