Low-Energy Photoelectron Spectroscopy and Scattering from Aqueous Solutions and the Role of Solute Surface Activity.

Experimental insights into low-kinetic-energy electron scattering in aqueous solutions are essential for an improved understanding of electron-driven chemistry and radiobiology, and the development and informed application of aqueous-phase electron-based spectroscopy and dichroism methods. Generally, in aqueous environments and for electron kinetic energies below 12-15 eV, significant and, thus far, incompletely understood low-energy-transfer inelastic electron scattering with solvent molecules preponderates. This leads to cascades of tens-of-meV kinetic-energy losses that distort nascent photoelectron spectra, prevent direct and accurate electron-binding-energy measurements, and limit possibilities to determine electron-scattering cross sections at especially low electron kinetic energies. Here, we quantify aqueous-phase inelastic-scattering-based energy losses using 1-30 eV kinetic energy photoelectrons and liquid-jet photoemission spectroscopy, specifically by photoionizing an exemplary surface-active solute and comparing the results with those from the homogeneously distributed aqueous solvent. Thereby, we identify a general ≳17 eV electron-kinetic-energy requirement for the direct and accurate measurement of aqueous-phase electron binding energies, irrespective of interfacial concentration profiles. Further, at electron kinetic energies from 10 eV down to a few-eV above the ionization threshold, we observe and quantify lower degrees of scattering for photoelectrons generated from surface-active solutes, allowing moderately distorted surface-active-solute photoemission peaks to be resolved down to just few-eV electron kinetic energies. These results demonstrate that liquid-jet photoemission spectroscopy can be used to probe interfacial surface-active-solute dynamics and dichroism effects close to ionization thresholds, in stark contrast to similar experiments on homogeneously distributed solution components. Furthermore, they offer novel insights into low-electron-kinetic-energy scattering in aqueous environments, thereby addressing the current lack of reliable experimental data in this critical energy range.