RIXS Reveals Hidden Electronic Transitions of the Aqueous OH Radical

We present RIXS spectra of the short-lived hydroxyl radical formed via proton transfer after ionization of pure liquid water [1]. Photoexcitation at the OH-resonance at 526 eV gives rise to an energy loss feature at 4 eV, corresponding to the localized A$\leftarrow$X transition of the OH$(aq)$ radical -- which is hidden by charge transfer transitions in the direct UV absorption spectrum. Theoretical calculations predict relative intensities of localized and deloclized RIXS transitions for OH$(aq)$ and the OH$^-(aq)$ anion. Time-resolved RIXS highlights the localized transitions in the transient OH$(aq)$ radical and may be used to track the electronic state evolution of this chemically aggressive species. [1] Z-H Loh {\it et al.} Science {\bf367}, 179-182 (2020)