Photoelectron Angular Distributions from Liquid Water: Effects of Electron Scattering

Abstract

Photoelectron angular distributions (PADs) from the liquid-water surface and from bulk liquid water are reported for water oxygen-1s ionization. Although less so than for the gas phase, the measured PADs from the liquid are remarkably anisotropic, even at electron kinetic energies lower than 100 eV, when elastic scattering cross sections for the outgoing electrons with other water molecules are large. The PADs reveal that theoretical estimates of the inelastic mean free path are likely too long at low kinetic energies, and hence the electron probing depth in water, near threshold ionization, appears to be considerably smaller than so far assumed.

Specific cation effects at aqueous solution−vapor interfaces: Surfactant-like behavior of Li + revealed by experiments and simulations

“…To confirm that our findings are not affected by electron emission anisotropy, in a separate experiment, we measured the depth profile of 2.0 M LiI in comparison with 2.0 M NaI solutions at the magic angle (54.7°) where electron emission anisotropy is eliminated. In this case, only the orbital photoionization cross-sections are needed to obtain the ion concentration ratios (29,38). The data measured at the magic angle for 2.0 M NaI and LiI solutions (SI Appendix, Fig.…”

“…1 A-C provides a measure of the concentration of each ion at a particular probe depth of the experiment. To this end, the spectral peaks are first normalized by their respective photoionization cross-section, photon flux, detection angle, and the electron transmission of the analyzer, as described in detail in Materials and Methods (29,37,38). To obtain ion concentrations, the normalized ion spectral peak areas are divided by the normalized water O1s peak area at the same photoelectron KE.…”

Introducing Ionic-Current Detection for X-ray Absorption Spectroscopy in Liquid Cells

“…Soft X-rays can penetrate typical liquid-cell membranes and reach up to ~ 1 µm into liquid samples, while the photoelectrons excited by X-ray absorption travel only a few nanometers in samples due to their short inelastic-mean-free-path (IMFP). 21 The photoelectrons that are created in the bulk solution cannot reach either of the electrodes and therefore do not contribute to the current measurements. However, the photoelectrons created in the vicinity of the front electrode (10-nm-thick Au film) can be collected by this electrode, which may result in an electric current detectable by the ammeter.…”

Advances in liquid phase soft-x-ray photoemission spectroscopy: A new experimental setup at BESSY II

“…Photoelectron angular distributions from aqueous solution provide insight into the effect on hydrogen-bonding on the orbital structure, and also reveal information on electron scattering in solution. 20 Specifically, the broadening of the PAD, and hence decrease of the anisotropy parameter β, with respect to the gas-phase distribution is directly connected with the electron elastic-to-inelastic scattering cross-section ratio. For sufficiently small electron kinetic energies, <100 eV, for which a significant anisotropy in the PAD remains, information about the orbitals can be extracted from the PE spectra.…”

“…16,18,19 Here one exploits the electron-energy dependent travel length, the electron elastic and inelastic mean free paths in aqueous solutions to tune the sensitivity of this method from an essentially surface-sensitive to a rather bulk-solution sensitive measurement. Less explored, but of large importance for understanding the effect of hydrogen bonding on solute electronic structure, and also for the characterization of molecular orientation at the solution interface, are photoelectron angular distributions (PADs), 20 which will be described below in more detail.…”