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Attosecond analysis illuminates a watery mystery

Attosecond analysis illuminates a watery mystery

22 February 2024

With powerful x-ray free-electron lasers, researchers are making great strides in ultrafast spectroscopy—with lessons about how molecules arrange themselves at rest.

Two closely spaced x-ray pulses approach a water molecule in an artist's depiction.
Two closely spaced x-ray pulses, represented in this artist’s depiction as red and green flashes, pump and probe a sample of liquid water. The first pulse kicks electrons (blue) out of some of the water molecules, and the second pulse measures the energy levels of the ionized molecules. Credit: Courtesy of Stacy Huang

While atomic and molecular motions are on a time scale of femtoseconds, electrons move on the scale of attoseconds. As highlighted by last year’s Nobel Prize in Physics, electron dynamics can be probed by attosecond light pulses derived from ordinary tabletop lasers. The technique has yielded plentiful results, but one long-sought capability is largely missing: the ability to excite a sample with one attosecond pulse and probe it with another after a controllable attosecond-scale delay.

X-ray free-electron lasers—the handful of large-scale facilities dedicated to producing intense x-ray pulses—have also entered the attosecond game, and they have plenty of power to produce pulse pairs. Now, working at SLAC’s Linac Coherent Light Source, researchers led by Linda Young, Robin Santra, and Xiaosong Li have performed attosecond pump–probe spectroscopy on liquid water. The results help resolve a debate, not about water’s ultrafast dynamics, but about its resting structure.

Previously, researchers have used x-ray emission spectroscopy to investigate how water molecules arrange themselves. As shown in panel a below, the technique removes an electron from a low-energy state and measures the energies of the photons emitted when electrons from valence states tumble down to fill the hole. Weirdly, the emission from the 1b1 state, which should be a single spectral line, is split in two, as shown in panel b. One explanation is that water contains globs of molecules arranged in two different ways—perhaps a remnant of the distinct liquid-water phases observed in the supercooled regime.

Graphs showing some of the energy transitions in water molecules.
(a) Attosecond pump–probe spectroscopy (green) and x-ray emission spectroscopy (blue) probe the same energy transitions in water, but in reverse order. (b) In the x-ray emission spectrum, the peak from the 1b1 state is split in two, but the splitting could be due to molecular dynamics. In attosecond pump–probe spectroscopy, the measurement is completed quickly enough to rule out dynamics, and the peak is unsplit. Credit: Adapted from S. Li et al., Science (2024), doi:10.1126/science.adn6059

But in x-ray emission spectroscopy, the time between ionization and emission is about 4 fs, which is enough time for atoms to move around. So that technique can’t rule out the possibility that the split peak is due to molecular motion and isn’t a feature of water’s resting structure at all.

Attosecond pump–probe spectroscopy can resolve the ambiguity. In their new experiment, the researchers measure the same energy transitions as in x-ray emission spectroscopy, but in reverse order: The first pulse removes an electron from a valence orbital, and the second excites an electron from a low-energy state. The pulses are delayed by a mere 700 attoseconds—not enough time for molecules to move—and the researchers see a single unsplit 1b1 peak. Water may be just the first of many systems in which ultrafast measurements yield new insights into the system at rest. (S. Li et al., Science, 2024, doi:10.1126/science.adn6059.)

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