Thomas Gaumnitz, postdoctoral fellow in the group of ETH professor Hans Jakob Wörner with the setup that generates the shortest laser pulses in the world.
Credit: ETH Zurich
In order to fully understand the dynamics during a chemical reaction, scientists must be able to study all movements of atoms and molecules on their basic time scale
Breaking down transition states
Starting from an infrared laser, the researchers generate a soft X-ray laser pulse with a very large spectral bandwidth. As a result, various elements including phosphorus and sulphur can be directly observed by exciting their inner-shell electrons. Both elements are present in biomolecules, and it is now possible to observe them with unprecedented time resolution.
But what is the advantage of being able to observe the reaction steps now with even higher resolution? “The faster a charge transfer can take place, the more efficiently a reaction can proceed,” says Prof. Wörner. The human eye for example is very efficient when it comes to converting photons into nerve signals. In rhodopsin, a visual pigment in the retina, the photosensitive molecule retinal is prearranged in such a way that its structure can change extremely fast through the absorption of only a single photon. This enables the visual process even in twilight. A much slower reaction would render vision impossible, because the energy of the photon would be converted to heat in only a few picoseconds.
Attosecond spectroscopy could contribute to the development of more efficient solar cells since it is now for the first time possible to follow the process of excitation through sunlight up to the generation of electricity step by step. A detailed understanding of the charge transfer pathway could help optimizing the efficiency of the next generation of photosensitive elements.
Optical manipulation of the reaction process
Attosecond laser spectroscopy is not only suitable for mere observation, Prof. Wörner explains. Chemical reactions can also be directly manipulated: Using a laser pulse can alter the course of a reaction — even chemical bonds can be broken by stopping the charge shift at a certain location in the molecule. Such targeted interventions in chemical reactions have not been possible until now, since the time scale of electron movement in molecules was previously unreached.
The group of Prof. Wörner is already working on the next generation of even shorter laser pulses. These will make it possible to record even more detailed images, and thanks to a wider X-ray spectrum even more elements can be probed than before. Soon it will be possible to follow the migration of electrons in more complex molecules with an even higher time resolution.
Story Source: Materials provided by ETH Zurich. Note: Content may be edited for style and length.
Thomas Gaumnitz, Arohi Jain, Yoann Pertot, Martin Huppert, Inga Jordan, Fernando Ardana-Lamas, Hans Jakob Wörner. Streaking of 43-attosecond soft-X-ray pulses generated by a passively CEP-stable mid-infrared driver. Optics Express, 2017; 25 (22): 27506 DOI:10.1364/OE.25.027506