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Attosecond spectroscopy of liquid water

August 20, 2020

To understand how chemical reactions begin, chemists have been using super-​slow motion experiments for years to study the very first moments of a reaction. These days, measurements with a resolution of a few dozen attoseconds are possible. An attosecond is 1x10-18 of a second, i.e. a millionth of a millionth of a millionth of a second.

“In these first few dozen attoseconds of a reaction, you can already observe how electrons shift within molecules,” explains Hans Jakob Wörner, Professor at the Laboratory of Physical Chemistry at ETH Zurich. “Later, in the course of about 10,000 attoseconds or 10 femtoseconds, chemical reactions result in movements of atoms up to and including the breaking of chemical bonds.”

Five years ago, the ETH professor was one of the first scientists to be able to detect electron movements in molecules on the attosecond scale. However, up to now such measurements could be carried out only on molecules in gaseous form because they take place in a high-​vacuum chamber.

After building novel measuring equipment, Wörner and his colleagues have now succeeded in detecting such movements in liquids. To this end, the researchers made use of photoemission in water: they irradiated water molecules with light, causing them to emit electrons that the scientists could then measure. “We chose to use this process for our investigation because it is possible to start it with high temporal precision using laser pulses,” Wörner says.

The new measurements also took place in high vacuum. Wörner and his team injected a 25-​micrometre-thin water microjet into the measuring chamber. This allowed them to discover that electrons are emitted from water molecules in liquid form 50–70 attoseconds later than from water molecules in vapour form. The time difference is due to the fact that the molecules in liquid form are surrounded by other water molecules, which has a measurable delay effect on individual molecules.

Fig. 3 Physical mechanisms of attosecond interferometry in liquid water. In the gas phase, the XUV- and IR-induced interactions are both localized to the same molecule. In the condensed phase, we distinguish “local” pathways [(1) and (2)], followed by additional scattering events without exchange of photons, from “nonlocal” pathways [(3) and (4)], consisting of ionization followed by one laser-assisted scattering event (including exchange of one photon with the IR field) among n + n′ non–laser-assisted collisions. Along the local pathways, photoelectron wave packets with central momenta kq are launched. The nonlocal pathways correspond to the launch of wave packets with central momenta kq–1 and kq+1 that are converted to a central momentum kq through a remote LAES interaction.

For more info see: ETH Zürich news,
Reference: Jordan, I., Huppert, M., Rattenbacher, D., Peper, M., Jelovina, D., Perry, C., von Conta, A., Schild, A., and Wörner, H.J. (2020). Attosecond spectroscopy of liquid water. Science 369, 974-979 (10.1126/science.abb0979)

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