Fig. 2. Schematic setups for the measurement procedures introduced in Fig. 1. (A) Electrically grounded (nearly) neat water LJ with a precisely tuned salt concentration to achieve a field-free condition for gas-phase referencing. (B) Negatively biased LJ used to reveal Ecut in the liquid spectrum for energy referencing; gas- and liquid-phase PE contributions are energetically separated in the field gradient. (C) Same as (B) but for an aqueous solution (here, featuring a surface-active solute). Changes in VIEs can be directly observed. (D) Similar to (A) but with the addition of a metallic reference sample held in electrical contact to and mounted within the vicinity of the LJ. The liquid water spectrum can be referenced to the Fermi edge of a metal sample under field-free conditions. Note that the metallic reference sample surface is probed separately from the LJ in the experiments reported here, and thus is not directly affected by any changes at the surface of the solution.
