Liquid microjet photoelectron spectroscopy is fast becoming a standard tool for solution-phase photoelectron spectroscopy, especially at x-ray wavelengths, but absolute ionization energies in the liquid phase can be difficult to ascertain due to scattering of the outgoing photoelectron, streaming potentials at the liquid surface, and other complicating factors. Questions also arise as to whether photoelectron spectroscopy probes the bulk liquid environment or merely the liquid–vapor interface. To address these issues, we have developed a quantitative protocol to compute vertical ionization energies (VIEs) in aqueous solution, in bulk water and at the air–water interface. Slow convergence of the VIE with respect to the number of water molecules is addressed by means of nonequilibrium continuum boundary conditions. Our formalism also naturally incorporates anisotropic dielectric boundaries, as encountered at the liquid–vapor interface. Our protocol, combining classical molecular dynamics with electronic structure calculations, reproduces experimental ionization energies to an accuracy of ∼0.2 eV for a set of inorganic anions (including polyvalent ions), as well as for the hydrated electron, making it the state-of-the-art for ionization energies in complex environments.  In most cases, the VIE is insensitive to the presence of the air–water interface. Surprisingly, structural and energetic analysis of the calculations reveals that first-shell anion–water hydrogen bonding is remarkably similar in the bulk and interfacial environments. This suggests that ion-induced shifts in the O–H lineshape, which are well documented in surface-sensitive vibrational spectroscopy, are subtle effects of ion-induced changes to water–water hydrogen bonding, rather than a direct result of ion–water hydrogen bonding.

Quantitative protocol for aqueous-phase ionization energies affords insight into anion–water hydrogen bonding at the air–water interface

ACS Spring 2022

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