Fig. Energy level diagram for a gas phase (right hand side) and liquid jet of aqueous solution (left hand side) photoelectron spectroscopy experiment. Abbreviations: E vac , vacuum level; E F , Fermi energy; f, work function; ana, analyzer; KE, kinetic energy; IE, ionization energy. In the gas phase experiment the vacuum level of the gas is equilibrated with that of the analyzer, whereas in the aqueous solution experiment the vacuum level of the gas is pinned to the vacuum levels of the liquid and of the analyzer, which results in an electric field between the two (depicted as sloped energy levels of the gas). Zero kinetic energy is defined for both experiments as the vacuum level of the analyzer. to the ejected electrons with zero kinetic energy) in the liquid jet is higher than the vacuum level of an isolated molecule in gas phase due to the presence of the field created by the water surface.