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Seemingly simple yet surprisingly difficult to probe, excess protons in water constitute complex quantum objects with strong interactions with the extended and dynamically changing hydrogen-bonding network of the liquid. Proton hydration plays pivotal roles in energy transport in hydrogen fuel cells and signal transduction in transmembrane proteins. While geometries and stoichiometry have been widely addressed in both experiment and theory, the electronic structure of these specific hydrated proton complexes has remained elusive. Here we show, layer by layer, how utilizing novel flatjet technology for accurate x-ray spectroscopic measurements and combining infrared spectral analysis and calculations, we find orbital-specific markers that distinguish two main electronic-structure effects: Local orbital interactions determine covalent bonding between the proton and neigbouring water molecules, while orbital-energy shifts measure the strength of the extended electric field of the proton. © 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.

Field ionization of vapor deposited water on a platinum field emitter was studied over a temperature range of 103-150 K. Water adlayers that were 50 - 3600 A thick were grown under field-free conditions by exposure of a cryogenically cooled emitter tip to water vapor in ultrahigh vacuum. Field ionization was probed by ramped field desorption (RFD) in which desorption of ionic species (hydrated protons) is measured while increasing the applied electric field linearly in time. The dependence of the field required for ionization onset as a function of temperature and water thickness is presented and discussed. In the limit of thin layers the onset field decreased from 0.5 to 0.2 V/A as temperature increased from 105 to 150K. An activation barrier of 0.7 eV (16 kcal/mol) for ionization of amorphous and crystalline water was estimated from the temperature dependence of the onset field. This is in excellent agreement with the 0.74 eV (17 kcal/mol) required to produce a pair of ions from a pair of solvated water molecules., Prepared for publication in Surface Science.