Probing charge correlation and coherence across hydrogen bonds using computational X-ray pump X-ray probe spectroscopy

Hydrogen-bonded systems are ubiquitous in chemistry and biology, yet a detailed understanding of valence charge correlation and electronic coherence across the hydrogen bond remains challenging. The UV-induced excited-state photodynamics of 10-hydroxybenzo(h)quinoline (HBQ), a hydrogen-bonded molecule undergoing excited-state intramolecular proton transfer, have been investigated by several groups - including transient X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) studies by our group. However, the electronic coupling between the proton donor (oxygen) and acceptor (nitrogen) atoms are not directly observed with these experimental techniques. Here, we present a computational study of two-color attosecond X-ray Pump–X-ray Probe (XPXP) spectroscopy as a nonlinear X-ray probe of charge correlation across the hydrogen bond. By tuning the X-ray energy to locally excite a core electron at either the nitrogen or oxygen atom (pump) and probing the other site (probe), we reveal how ionization at one atom perturbs the local electronic structure at the other, providing direct insight into charge correlation and coupling. These simulations lay the groundwork for future attosecond XPXP experiments at LCLS-II.