RIXS within the coupled-cluster framework reveals the hidden transitions of transient species in ionized liquid water*

X-ray spectroscopies such as resonant inelastic X-ray scattering (RIXS) expand our ability to investigate the chemical structure and dynamics; in particular, when augmented with reliable theoretical tools. Computing the RIXS spectra, however, is challenging; primarily due to the difficulties in computing the underlying core-excited virtual states that are embedded inside the valence-ionization continuum in a converged fashion. We will present a novel electronic structure method within the equation-of-motion coupled cluster framework that facilitates converged calculations of the RIXS spectra for both closed- and open-shell species. We will illustrate the capabilities of our approach by measuring its performance in modeling the RIXS spectrum of the transient aqueous OH radical formed in ionized liquid water against experiments. We will provide orbital characterization of the distinct peaks in the RIXS spectrum of the OH radical for absorption below water’s absorption edge. We will provide ab initio explanation of why the intermolecular charge-transfer transitions of aqueous OH radical, that are dominant in UV-visible spectrum, are suppressed in RIXS revealing the hidden localized valence transition.