ABSTRACT
The redox potentials measured by electrochemical experiment can be regarded as relative adiabatic ionization energies. Photoemission spectroscopy (PES) also probes the ionization of species, but the measured energy differences are absolute vertical ionization energies referred to vacuum. Correlating these two complementary ionization energies has been a long-standing ideal of fundamental physical electrochemistry.1-4 Such comparisons have become much more precise due to recent progress in PES techniques for liquids and solutions.5,6 Of special interest are small aqueous anions, such as hydroxide and halide anions. Work by the Berlin/Heidelberg group5 has confirmed the ideas of Delahay about the ionization of these elementary aqueous anions.1-3 Vertical electron detachment levels lie much deeper than the adiabatic redox level. The difference in the case of OH− is as much as 2.9 eV. In fact, at –9.2 eV relative to vacuum, the OH− vertical detachment level is separated by only 0.7 eV from the valence band maximum (VBM) of liquid water, as inferred from the PES threshold of liquid water (9.9 eV).5
