Resonant Inelastic X-Ray Scattering Reveals Hidden Local Transitions of the Aqueous OH Radical

Resonant inelastic x-ray scattering (RIXS) provides remarkable opportunities to interrogate ultrafast dynamics in liquids. Here we use RIXS to study the fundamentally and practically important hydroxyl radical in liquid water, $\mathrm{OH}(aq)$. Impulsive ionization of pure liquid water produced a short-lived population of $\mathrm{OH}(aq)$, which was probed using femtosecond x-rays from an x-ray free-electron laser. We find that RIXS reveals localized electronic transitions that are masked in the ultraviolet absorption spectrum by strong charge-transfer transitions—thus providing a means to investigate the evolving electronic structure and reactivity of the hydroxyl radical in aqueous and heterogeneous environments. First-principles calculations provide interpretation of the main spectral features.

Figure 1

UV absorption and RIXS energy-loss spectra of $\mathrm{OH}(aq)$ (top) and molecular orbital diagrams (bottom). The electronic configuration of OH is $1s_O^{2}1{\sigma }^{2}2{\sigma }^{2}1{\pi }^3$. The lowest valence transition ($2\sigma \to 1\pi$, marked as ${\mathrm{LT}}_{\mathrm{val}}$ for local transition) has low oscillator strength due to its $p_z\to p_x/p_y$ character; the UV spectrum is dominated by charge-transfer transition (${\mathrm{CT}}_{\mathrm{val}}$) from nearby waters. In resonantly excited OH ($1s_O^{1}1{\sigma }^{2}2{\sigma }^{2}1{\pi }^4$), both $1\pi \to 1s$ and $2\sigma \to 1s$ transitions are bright, giving rise to the elastic peak (zero energy loss) and a feature at $\sim 4\mathrm{eV}$. The gap between the two RIXS peaks corresponds to the $2\sigma \to 1\pi$ energy.

Figure 2

RIXS maps of ionized liquid water (a) before valence ionization and (b) after valence ionization, integrated for time delays between 200–1400 fs. The intensity of the maps is the number of counts on the emission spectrometer (15 054) normalized by the number of XFEL pulses (365 846). Before ionization, a threshold for emission is seen near 534 eV incident photon energy. After ionization, a resonant feature appears at 526 eV due to the transient $\mathrm{OH}(aq)$ radical. The two energy windows used to create the spectra shown in Fig. 3 are marked: 536–542 eV for bulk water and 525.70–526.12 eV for $\mathrm{OH}(aq)$.

Figure 3

Comparison of RIXS from OH and ${\mathrm{OH}}^{-}$. Top: Experimental XES from water for excitation energies between 536–542 eV compared to XES of water excited at 550.1 eV [15]. Middle: RIXS from the 526-eV resonance of $\mathrm{OH}(aq)$ compared with theory (shifted $+1.0\mathrm{eV}$). Bottom: RIXS from ${\mathrm{OH}}^{-}$ excited at 533.5 eV and compared with theory (shifted $-0.2\mathrm{eV}$). Ref. [15] data shifted $-0.7\mathrm{eV}$. Insets show molecular orbital diagrams for the RIXS transitions in OH and ${\mathrm{OH}}^{-}$.