Partially Hydrated Electrons at the Air/Water Interface Observed by UV-Excited Time-Resolved Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy

Abstract

Hydrated electrons are the most fundamental anion species, consisting only of electrons and surrounding water molecules. Although hydrated electrons have been extensively studied in the bulk aqueous solutions, even their existence is still controversial at the water surface. Here, we report the observation and characterization of hydrated electrons at the air/water interface using new time-resolved interface-selective nonlinear vibrational spectroscopy. With the generation of electrons at the air/water interface by ultraviolet photoirradiation, we observed the appearance of a strong transient band in the OH stretch region by heterodyne-detected vibrational sum-frequency generation. Through the comparison with the time-resolved spectra at the air/indole solution interface, the transient band was assigned to the vibration of water molecules that solvate electrons at the interface. The analysis of the frequency and decay of the observed transient band indicated that the electrons are only partially hydrated at the water surface, and that they escape into the bulk within 100 ps.

Plasma physics of liquids—A focused review

“…157 Surface electrons with a partial solvation shell were also reported in a more recent investigation, yet with a shorter lifetime. 292 Note in this regard that the stability and structure of surface electrons might be significantly altered in the presence of an external electric field, in analogy with the thought experiment of Fig. 8.…”

“…III C, also solvated electrons can temporarily reside near the gas phase. 157,[291][292][293] Accordingly, it seems justified, in general, to model the upper edge of the air-water interface as a Stern layer, in analogy to the double layer theory for solid-liquid interaction. Yet, more detailed research is required to elucidate the solvation structure of specific ions at the liquid surface, i.e., whether they can break out of their solvation shell and to which degree they are exposed to the gas phase.…”

“…157,292,293,330 While in some studies, no excess electrons are detected at the surface, 330 other reports mention them with conflicting data on their stability and solvation structure. 157,292,293 According to the experiments in Ref. 293, surface electrons are completely surrounded by a solvation shell, with a lifetime exceeding 750 ps.…”

Charge Transfer to Solvent Dynamics at the Ambient Water/Air Interface

“…39 Nevertheless, the proximity to the vapour phase may have important consequences on its chemical properties as recently implicated in plasma chemistry at the water/air interface. 40 A very recent sumfrequency generation study has also shown a distinctive vibrational spectrum for water interacting with the surface e -(aq) , 41 which resembles the vibrational spectrum of an excess electron in water clusters but not e -(aq) in the bulk, suggesting the binding site at the interface may be more distinctive than previously thought. These sum-frequency generation experiments are only comparable to the present experiments after ~50 ps where e -(aq) remains at the surface, but may reflect our suggestion that the e -(aq) is solvated within a reduced density water environment at the interface, where the hydrogen-bonding network differs to bulk water and may be more closely related to water clusters.…”

Selectivity in Electron Attachment to Water Clusters

“…[1][2][3][4][5][6] While generally considered as a bulk solute, many of the roles of the hydrated electron are in fact associated with interfacial processes, but much less is known about the structure, let alone the reactivity, of excess electrons at aqueous interfaces. [7][8][9][10][11][12][13][14][15][16] To gain a fundamental understanding of electron solvation in water, isolated water cluster anions, (H2O)n − , have received much attention from both experiment 7,17-23 and theory. 9,[24][25][26][27][28][29] In these, the excess electron can adopt a number of binding motifs.…”