Proton transport between acids and bases in aqueous solution involves the exchange of protons, rearrangements of hydrogen bonds, and the changes in electron charge distributions of all molecules involved. Progress in the development of ultrafast structurally resolving techniques have enabled to follow in real-time the elementary individual steps in proton transfer that occur on femto- to picosecond time scales. Ultrafast probing in the mid-infrared has provided a wealth of information on hydrogen bond dynamics and proton transfer time scales. Recent progress in soft X-ray spectroscopy of solution phase acids and bases using novel flatjet technology has now made possible to locally probe electronic structure. I will showcase this with examples on proton hydration, and photoacid molecular systems, and indicate how further advances in ultrafast soft X-ray spectroscopy of solution phase proton transfer may be made.
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