Relaxation of Electronically Excited Hydrogen Peroxide in Liquid Water: Insights from Auger-Electron Emission

Autoionization electron spectroscopy is applied to study nonradiative relaxation processes of hydrogen peroxide aqueous solution irradiated by soft X-rays. The high kinetic energy part of the oxygen Is H2O2(aq) Auger-electron spectrum reveals dicationic final states with considerably lower energy than for neat liquid water. Assisted by quantum chemical calculations, it is argued that such lower-energy states arise from two fundamentally different relaxation processes. One is (local) Auger decay, yielding H2O22+(aq) species, and here the low final-state energy arises from charge delocalization across the molecular O-O bond. Alternatively, nonlocal dicationic states can form, corresponding to a charge-separated complex comprising H2O2 and a neighboring water molecule. Different charge-separation mechanisms, depending on whether or not proton dynamics of the core-level excited or ionized H2O2 molecule is involved, are discussed. We also present for the first time the partial electron yield X-ray absorption spectrum of liquid water, which is useful in interpreting the respective spectra from H2O2 in water, especially when identifying solute-specific excitations.