Laser pulses lasting less than 150 attoseconds have been used to observe, in real time, the motion of electrons in the outermost (“valence”) shell of ionised krypton atoms. This technical achievement, reported in Nature 466(7307), 739–742, lays the groundwork for observations in more complex systems, which should allow a detailed examination of the fundamental processes underlying the making and breaking of chemical bonds.

“Pump–probe” spectroscopy, in which optical pulses separated in time are used to investigate a chemical reaction, is now a well established technique at timescales of the order of femtoseconds. This timescale is sufficiently fast to follow changes in molecular structure, but the motion of the valence electrons that ultimately drive chemical reactions occurs on faster, sub-femtosecond timescales.

Ferenc Krausz and colleagues have extended the pump–probe technique to these faster timescales, reporting the successful observation of the sub-femtosecond motion of valence electrons over a multi-femtosecond timespan. The authors use an infrared “pump” pulse to ionise a collection of krypton atoms, which is followed by an extreme-ultraviolet probe pulse, lasting less than 150 attoseconds, to take a snapshot of the resulting electron vacancy, or “hole”. By varying the delay between pump and probe, the authors are able to reconstruct the motion of the valence electron wave-packet.

In an accompanying News and Views article, Olga Smirnova delves into the quantum-mechanical underpinnings of the authors’ experiment, and explains how the technique may uncover new mechanisms of chemical reactivity.

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