22 October 2026

Photo: Nature 630, 109 (2024), CC-BY-4.0-licence

Snapshots of the electronic current generated in chiral molecules by a pump UV light pulse, show the molecules for two distinct orientations Ri (a). Photoionization is enhanced for orientation R1.This leads to a calculated forward/backward fragment asymmetry along the laser propagation axis during the dissociative ionization (b).

Imagine being able to direct chemical reactions by steering electrons in real time. Thanks to groundbreaking experiments using ultrashort laser pulses – flashes of light lasting just billionths of a billionth of a second – scientists can now watch and control electron motion. The text from our magazine, „Advanced Imaging of Matter – Seven Years CUI in Focus“, describes how our understanding of photochemical processes fundamentally changes.

When we think about chemical reactions, we usually picture atoms rearranging themselves over a relatively long period of time. But what if the fate of a reaction could be changed from its earliest stages, before the atomic nuclei start to move, by controlling the flow of electrons themselves? Attosecond science, a rapidly advancing field recognized by the 2023 Nobel Prize in Physics, enables doing exactly that. Scientists are now using ultrashort bursts of extreme ultraviolet (XUV) light, lasting just attoseconds, to visualize and manipulate electron motion within atoms, molecules, and solids. (To put an attosecond in perspective: the ratio between an attosecond and a second is equivalent to comparing a single second to 31.7 billion years – more than twice the age of the universe.)

One particularly exciting phenomenon identified using such ultrashort pulses is charge migration, an ultrafast process in which electrons redistribute across a molecule immediately after excitation, even before atoms start to move. The Attosecond Science group led by CUI spokesperson Francesca Calegari has recently pushed this idea further, not only capturing electron-driven phenomena within biomolecules but also revealing unexpected ways to control chemical reactions at their most fundamental level.

Catching electrons at play in DNA building blocks

Understanding how radiation damages DNA is essential for the development of new protection strategies against harmful exposure. In pioneering work, the CUI team focused on adenine, a basic building block of DNA. Using ultrashort XUV pulses – each lasting less than 300 attoseconds – with photon energies high enough to potentially cause biological radiation damage, the researchers observed how electrons rapidly rearrange after ionization. Remarkably, they found that within an extremely narrow time window of only 2,300 attoseconds, the electron motion can be exploited to create a stable doubly charged molecule. How? With carefully timed near-infrared pulses to precisely interfere with the electron rearrangement, the researchers effectively stabilized adenine against photoinduced fragmentation. This groundbreaking observation opens up exciting new perspectives for the manipulation of biochemical photoreactions through controlled electron dynamics.

Twisted currents: unveiling electronic chirality

The Attosecond Science group has demonstrated the ability to steer electron dynamics also in another important class of compounds, chiral molecules. Chirality – the molecular property responsible for handedness – is usually defined by the spatial arrangement of atoms. But recent work by the CUI researchers revealed something remarkable: chirality can also be significantly influenced by ultrafast electron dynamics alone. Using time-resolved photoelectron circular dichroism (PECD), a technique involving circularly polarized laser pulses, the team studied the chiral molecule methyl lactate. Unexpectedly, they found that when photoexcited by one of the shortest UV light pulses ever produced, the direction of emitted PECD electrons can be completely reversed within just 10 femtoseconds (one femtosecond is a thousand attoseconds), well before the atoms begin to rearrange. Theoretical models confirmed that these ultrafast changes were caused by coherent electron oscillations, creating “chiral currents” at unprecedented speed.

The experiment also showed that these chiral currents can be selectively manipulated, suggesting powerful new means to control molecular reactivity in chiral systems. This discovery paves the way for exciting novel approaches in chemistry, allowing scientists to dictate the outcomes of reactions by precisely steering electron motion, for example by controlling the fragmentation of chiral molecules during photolysis (see image). Text: Francesca Calegari (DESY, UHH), Vincent Wanie (DESY)

Literature

• F. Calegari et al., Science 346, 336 (2014).
• E. P. Månsson et al., Communications Chemistry 4, 73 (2021).
• V. Wanie et al., Nature 630, 109 (2024).