Li, S.; Lu, L.; Bhattacharyya, S.; Pearce, C. I.; Li, K.; Nienhuis,E.; Doumy, G.; Schaller, R. D.; Moeller, S.; Lin, M.-F.; Dakovski, G.; Hoffman, D.J.; Garratt, D.; Larsen, K.; Koralek, J. D.; Hampton, C. Y.; Cesar, D.; Duris, J.; Zhang, Z.; Sudar, N.; Cryan, J. P.; Marinelli, A.; Li, X.; Inhester, L.; Santra, R.; Young, L. Attosecond-pump attosecond-probe x-ray spectroscopy of liquid water. Science. 2024. 383(6687), 1118-1122. DOI: 10.1126/science.adn6059
Attosecond-pump/attosecond-probe experiments have long been sought as the most straightforward method for observing electron dynamics in real time. Although there has been much success with overlapped near-infrared femtosecond and extreme ultraviolet attosecond pulses combined with theory, true attosecond-pump/attosecond-probe experiments have been limited. We used a synchronized attosecond x-ray pulse pair from an x-ray free-electron laser to study the electronic response to valence ionization in liquid water through all x-ray attosecond transient absorption spectroscopy (AX-ATAS). Our analysis showed that the AX-ATAS response is confined to the subfemtosecond timescale, eliminating any hydrogen atom motion and demonstrating experimentally that the 1b1 splitting in the x-ray emission spectrum is related to dynamics and is not evidence of two structural motifs in ambient liquid water.