Radiationless decay spectrum of O 1s double core holes in liquid water

What is it about?

We present a combined experimental and theoretical investigation of the radiationless (i.e., Auger) decay spectrum of an O 1s double core hole in liquid water. Our experiments were carried out using liquid-jet electron spectroscopy from cylindrical microjets of normal and deuterated water. The signal of the double-core-hole spectral fingerprints (so-called hypersatellites) of liquid water is clearly identified, with an intensity ratio to Auger decay of singly charged O 1s of 0.0014(5). We observe a significant isotope effect between liquid H2O and D2O. For theoretical modeling, the Auger electron spectrum of the central water molecule in a water pentamer was calculated using an electronic-structure toolkit combined with molecular-dynamics simulations to capture the influence of molecular rearrangement within the ultrashort lifetime of the double core hole. We obtained the static and dynamic Auger spectra for H2O, (H2O)5, D2O, and (D2O)5, instantaneous Auger spectra at selected times after core-level ionization, and the symmetrized oxygen-hydrogen distance as a function of time after double core ionization for all four prototypical systems. We consider this observation of liquid-water double core holes as a new tool to study ultrafast nuclear dynamics.

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Photo by Daniele Levis Pelusi on Unsplash

Photo by Daniele Levis Pelusi on Unsplash

Why is it important?

This manuscript presents the first-ever double-core-hole study in the liquid phase. In particular, we investigated, experimentally and theoretically, the radiationless (i.e., Auger) decay spectrum of O 1s double core holes in liquid water. The experiments using liquid-jet electron spectroscopy from cylindrical microjets of normal and deuterated water are state-of-the-art. Although the double-core-hole signal is a factor of ~700 smaller than the single-core-hole signal, we could clearly identify the spectral fingerprints (so-called hypersatellites). We present a significant isotope effect between liquid H2O and D2O. The theoretical modeling is state-of-the-art as well; the Auger electron spectrum of the central water molecule in a water pentamer was calculated using an electronic-structure toolkit combined with molecular dynamics simulations to capture the influence of molecular rearrangement on the ultrashort lifetime of the double core hole (DCH). We consider the present observation of liquid-water double core holes as a new tool to study ultrafast nuclear dynamics. Due to polarization of the surrounding water molecules in the final state, we find the kinetic energy of the DCH feature blue-shifted by 12.5 eV compared to its gas-phase counterpart. Despite the lifetime of an oxygen DCH being around 1.5 fs, nuclear dynamics within this time window is evidenced by a noticeable isotope effect between H2O and D2O. Our calculations allowed to identify a symmetric stretch of the molecule as the main driver of nuclear dynamics and shed light on the temporal evolution of the Auger spectrum. How will the manuscript advance the field of chemical physics? (1) We show that nuclear dynamics play a role even on ultrafast timescales of <1.5 fs. Double-core-hole spectroscopy of liquid samples can enhance our understanding of such processes. Extending studies to DCHs will extend the range of parameters under which ultrafast nuclear dynamics can be probed. (2) At X-ray free-electron lasers, (multi-photon) DCHs are observed on a regular basis and with rather high probability due to the extremely intense X-ray pulses. Our (single-photon) synchrotron study paves the way for future liquid-phase DCH experiments at XFEL light sources. (3) We want to point out that nowadays measuring time-resolved Auger spectra is in reach. So, our calculations reported here are interesting not only regarding the high-energy tail, but also the energy shift. We envision time-resolved Auger spectra using intense XFELs and, e.g., self-referenced attosecond streaking of more complex systems than noble gases in the future. (4) Double-core-hole spectroscopy of anions in the gas phase has revealed a factor of ~10 stronger signal as compared to neutral molecular systems. Of course, our study also paves the way for DCH studies of anions in aqueous solution.

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