The charge at the surface of water and the resultant surface voltage play an important role in many natural phenomena and technological applications. However, the relationship between surface charge and the interfacial distribution of H + and OH − ions remains unclear. We measured the surface voltage produced by an ionized acid or a base at the surface of amorphous solid water (ASW) using a Kelvin work-function probe and studied the depth distributions of H + and OH − ions. H + ions were distributed over a thicker region from the surface than OH − ions, although both ions reside preferentially at the surface. This difference led to the formation of opposite surface charges in the presence of the acid or base. The deeper penetration of H + ions is attributed to efficient proton transport dynamics in the lattice and the resultant dynamic delocalization of protons. The study demonstrates that the asymmetric H + and OH − distributions may be important to understand the electrical and acid−base properties of ASW and crystalline ice surfaces and, possibly, those of the liquid water surface as well.

The interaction of sulfur dioxide (SO 2 ) gas with a crystalline ice surface at low temperature was studied by analyzing the surface species with low energy sputtering (LES) and reactive ion scattering methods and the desorbing gases with temperatureprogrammed desorption mass spectrometry. The study gives direct evidence for the occurrence of efficient hydrolysis of SO 2 with low energy barriers on the ice surface. Adsorbed SO 2 molecules react with the ice surface at temperatures above ∼90 K to form anionic molecular species, which can be detected by OH − , SO 2 − , and HSO 3 − emission signals in the LES experiments. Heating the sample above ∼120 K causes the desorption of SO 2 gas from the surface-bound hydrolysis products. As a result, the hydrolysis of SO 2 on an ice surface is most efficient at 100− 120 K. The surface products formed at these temperatures correspond to metastable states, which are kinetically isolated on the cold surface. Quantum mechanical calculations of a model ice system suggest plausible mechanistic pathways for how physisorbed SO 2 is transformed into chemisorbed HSO 3 − species. HSO 3 − is formed either by direct conversion of physisorbed SO 2 or through the formation of a stable H 2 SO 3 surface complex, both involving proton transfer on the ice surface with low energy barriers. These findings suggest the possibility that thermal reactions of SO 2 occur efficiently on the ice surface of Jovian satellites even without bombardment by high-energy radiation.

We studied the adsorption and reaction of nitrogen dioxide gas on the surface of an ice film at temperatures of 100−170 K under ultrahigh vacuum (UHV) conditions. Cs + reactive ion scattering (RIS) and low-energy sputtering (LES) techniques were used to identify and quantify the reactants and products on the surface of the ice film, in conjunction with the use of temperature-programmed desorption (TPD) to monitor the species desorbed. Temperature-ramping experiments were performed to examine the changes in the populations of these species as a function of temperature. Adsorption of NO 2 gas on the ice film at <110 K produced physisorbed species that may possibly possess negative charge character (NO 2 δ-), as deduced from the NO 2 and NO 2 − signals in the RIS and LES experiments. At 110−130 K, NO 2 δ-species were either desorbed as NO 2 gas or converted to nitrous acid (HONO), NO 3 − , and H 3 O + on the surface. Nitrous acid gas was desorbed at 140−160 K. The efficiency of conversion of NO 2 to surface nitrous acid was about 40%, and that to nitrous acid gas was about 7%. The efficiency of the reaction of NO 2 on the ice surface may be higher than that at the gas/liquid water interface. The reaction efficiency increased with a decrease of the NO 2 coverage and was inversely correlated with the N 2 O 4 coverage, which favors the mechanistic interpretation that an isolated NO 2 molecule reacts with water. However, NO 2 can diffuse on the ice surface to form clusters at ≥120 K. Under these conditions, the possibility that dimerization of NO 2 contributes to the hydrolysis reaction of NO 2 may not be excluded.

We studied the interactions between low-energy (1–10 eV) electrons and a crystalline ice film in a low fluence condition, where incident electrons interacted mostly with the pristine ice lattice. The electron beams were irradiated onto an ice film sample of large thickness (>100 monolayers) at 95 K. The kinetic energy and flux of incident electrons were maintained constant by applying an offset bias potential to the sample to compensate the charging voltage of an ice film developed by electron trapping. A Kelvin work-function probe was used to measure the charging voltage and estimate the electron trapping efficiency. The measurements for ice films with different thicknesses showed that low energy electrons transmitted through the ice films very efficiently. When electrons were trapped in the ice films, the trapping occurred preferentially at the surface of ice films. The electron trapping cross-section for the ice surface was estimated to be ≤(2.6 ± 0.3) × 10–23 m2 at the incident electron energy of 1–10 eV. The electron trapping cross-section for the ice interior was below ∼10–25 m2. In comparison, adsorbed SO2 and CFCl3 molecules on the ice surface were able to trap incident electrons much more efficiently than the surface water molecules, with corresponding cross sections of (1.1 ± 0.4) × 10–21 m2 and (2.8 ± 0.3) × 10–21 m2, respectively.