  The absolute-scale electronic energetics of liquid water and aqueous
solutions, both in the bulk and at associated interfaces, are the central
determiners of water-based chemistry. However, such information is generally
experimentally inaccessible. We demonstrate that a refined implementation of
the liquid microjet photoelectron spectroscopy (PES) technique can be adopted
to address this. Implementing concepts from condensed matter physics, we
establish novel all-liquid-phase vacuum and equilibrated
solution-metal-electrode Fermi level referencing procedures. This enables the
precise and accurate quantification of previously elusive solute-induced
perturbations of water's electronic energetics and vertical ionization energy
(VIE) definition on an absolute and universal chemical potential scale.
Applying these procedures over a broad range of ionization energies, we
accurately and respectively determine the VIE and oxidative stability of liquid
water as 11.33 $\pm$ 0.02 eV and 6.60 $\pm$ 0.08 eV with respect to its
liquid-vacuum-interface potential and Fermi level. Combining our referencing
schemes, we determine the work function of liquid water as 4.73 $\pm$ 0.09 eV.
Further, applying our novel approach to a pair of exemplary aqueous solutions,
we extract absolute VIEs of aqueous iodide anions, reaffirm the robustness of
water's electronic structure to high bulk salt concentrations, and quantify
reference-level dependent reductions of water's VIE and a 0.48 $\pm$ 0.13 eV
contraction of the solution's work function upon partial hydration of a known
surfactant. Our combined experimental accomplishments mark a major advance in
our ability to quantify electronic-structure interactions and chemical
reactivity in water, which now explicitly extends to the measurement of
absolute-scale bulk and interfacial solution energetics, including those of
relevance to aqueous electrochemical processes.
