Femtosecond Soft-X-ray Absorption Spectroscopy of Liquids with a Water-Window High-Harmonic Source

It is a long-standing quest in science to follow reactions of chemical and biological systems in their natural liquid environment in real time. We demonstrate femtosecond time–resolved soft–X–ray absorption spectroscopy for measurement of valence ionization dynamics of simple organic molecules of ethanol and methanol. Liquid-phase transient-absorption measurements in soft X-ray spectral range rely on a combination of a sub–micrometer–thin flat liquid jet with a high–harmonic table–top source covering the entire water-window range (284 - 538 eV). In a pump-probe scheme, intense 400 nm pump pulses were utilized to kick-off the dynamics via strong-field ionization of the liquid samples and delayed broadband soft X-ray pulse from a water window high harmonic generation source probed the electronic structure with around 30 fs time resolution. We observe differences in rise-time of the induced transient changes in X-ray absorption in methanol and ethanol indicating ultrafast proton dynamics after ionization in those systems. This work enables obtaining a detailed insight on liquid-phase dynamics using the element and site sensitivity of X-ray spectroscopy.

Reference: Smith, Adam D., Balciunas, Tadas,  Chang, Yi-Ping,  Schmidt, Cedric., Zinchenko, Kristina, Nunes, Fernanda B.,  Rossi, Emanuele,  Svoboda, Vít, Yin, Zhong, Wolf, Jean-Pierre, Wörner, Hans Jakob, Journal of Physical Chemistry Letters, 2020, vol. 11, n° 6, p. 1981-1988, (doi: 10.1021/acs.jpclett.9b03559)