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.
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