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Online Laboratory Magazine
09/24/2021

10/30/2014

Watching the interplay of molecules

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Imagine a soccer season when no one watches the matches - f ans only see results and league tables. It doesn't sound like much fun, right? It wouldn't give much insight into how the players perform, how their coaches intervene, or how the referees judge. You would never see how your team turned being down by two at halftime into a 3 - 2 victory in the final five minutes. You would only know the outcome.

This is how chemistry has been understood until recently. Chemists had been pouring things together and taking notes about the outcomes without being able to see the actual development. Scientists could only make clever guesses about the involved intermediate steps - the movements of the electrons and the changes of the molecular structure - because the reactions themselves happen far too quickly and are also too complex. But to fully understand chemical reactions, you have to understand what is going on in between - the throwing-ins, the moves, and the fouls.

Femtochemistry - as fast as chemistry can get

This is where femtochemistry enters the stage. Femtochemistry studies extremely short processes, about a few tens of 10-15 seconds. Those are femtoseconds, hence the name femtochemistry. In one second, light travels from the Earth to the moon. In femtoseconds, it travels not even a human hair's width.

Femtoseconds - that is as short as time can get in chemistry. Some scientists take the vibrational motion of a hydrogen molecule, the lightest molecule in the universe, as a lower limit for the fastest chemically relevant motions. At 14 femtoseconds per cycle, it gives a good idea of how quickly things can happen in the molecular world. Heavier atoms and molecules move more slowly, but chemical reactivity in general commences as fast as those few tens of femtoseconds.

In the world around us, such ultrafast processes are thus commonplace. They rule processes in the chemical industry, in the manufacture of pharmaceuticals, in many other areas of everyday production, and especially in nature itself (for example, as in photosynthesis).

While femtochemistry started with the investigation of regular molecular vibrations, the scientists' dreams have become much bigger. "We are no longer interested in molecular vibrations of molecules alone", says Christian Bressler, leader of the group Femtosecond X-ray Experiments (FXE) at the European XFEL. "Now we want to understand the evolution of excited molecules, from the reactant to the product."

FXE was founded in late 2009, when scientists Andreas Galler and Wojciech Gawelda joined the team. The group's objective is ambitious. They not only want to commentate on ultrafast chemical matches, but they want to become coaches and referees, too, improving the molecules' performance while the chemical game is still on. Scientists call this ability to control the course of a chemical reaction "coherent control", and the FXE group is thinking of including such tools into their portfolio of ultrafast techniques. At the European XFEL, the group is constructing a scientific instrument which they refer to as high speed molecular camera.

Nobel beginnings

Femtochemistry - an expression coined by Egyptian-American Nobel laureate Ahmed Zewail - is still a young discipline, although it has meanwhile become mainstream in chemistry departments throughout the world. Getting down to the necessary time scale did not become routine until the 1980s, when technological advances were made in the generation of femtosecond laser pulses. Zewail used the technological opportunity to study the vibrations of sodium iodide, a heavy cousin of table salt, and their influence on the dissociation of this molecule. The research earned him a Nobel Prize in Chemistry in 1999.

You are forgiven if oscillating table salt does not get you too excited, but Zewail's research opened a completely new chapter in chemistry: the advent of experiments in real-time, which are the first step towards the old dream to make a molecular movie of an evolving ultrafast reaction. Nowadays, nearly every chemical research institute has a few femtosecond laser setups at their disposal.

Source: European XFEL GmbH