X-Ray absorption spectroscopy of H 3 O .

We report the X-ray absorption of isolated H 3 O + cations at the O 1s edge. The molecular ions were prepared in a flowing afterglow ion source which was designed for the production of small water clusters, protonated water clusters, and hydrated ions. Isolated H 2 O + cations have been analyzed for comparison. The spectra show significant differences in resonance energies and widths compared to neutral H 2 O with resonances shifting to higher energies by as much as 10 eV and resonance widths increasing by as much as a factor of 5. The experimental results are supported by time-dependent density functional theory calculations performed for both molecular cations, showing a good agreement with the experimental data. The spectra reported here could enable the identification of the individual molecules in charged small water clusters or liquid water using X-ray absorption spectroscopy.