1b1 splitting in the X-ray emission spectrum of liquid water is dominated by ultrafast dissociation

The 1b1 splitting observed in the K-edge non-resonant X-ray emission spectrum (XES) of liquid water has been a subject of intense debate. Some argue that it represents two distinct structural motifs, while others suggest that it is evidence of ultrafast dissociation within the core-hole lifetime. In this study, we use a theoretical approach to provide a nearly quantitative description of both non-resonant and pre-edge resonant XES. Our theoretical predictions indicate that the 1b1 splitting arises due to ultrafast dissociation, with the dynamics of this process dependent on the X-ray absorption energy. For non-resonant radiation, hydrogen is ejected as H+, while for pre-edge resonant radiation it is ejected as a neutral atom. This leads to near-dissociation for non-resonant radiation, as ~ 20% of all core-ionized molecules do not dissociate and dissociated protons still remain in close proximity to the parent hydroxyl fragment. In contrast, for pre-edge radiation, all core-excited molecules dissociate and the neutral H atom moves further away from the hydroxyl fragment, completely leaving the first solvation shell in most cases. Our simulations also predict the observed isotope effects in both non-resonant and pre-edge resonant XES and reveal that the so-called lower-energy 1b1 peak in the non-resonant XES is primarily of 3a1 symmetry. Additionally, we found that hydrogen bonding plays a significant role in explaining the XES behavior across different temperatures and phases.