UV-induced protonation of molecules adsorbed on ice surfaces at low temperature

UV-Induced Formation of Ice XI Observed Using an Ultra-High Vacuum Cryogenic Transmission Electron Microscope and its Implications for Planetary Science

The occurrence of hydrogen atom-ordered form of ice Ih, ice XI, in the outer Solar System has been discussed based on laboratory experiments because its ferroelectricity influences the physical processes in the outer Solar System. However, the formation of ice XI in that region is still unknown due to a lack of formation conditions at temperatures higher than 72 K and the effect of UV-rays on the phase transition from ice I to ice XI. As a result, we observed the UV-irradiation process on ice Ih and ice Ic using a newly developed ultra-high vacuum cryogenic transmission electron microscope. We found that ice Ih transformed to ice XI at temperatures between 75 and 140 K with a relatively small UV dose. Although ice Ic partially transformed to ice XI at 83 K, the rate of transformation was slower than for ice Ih. These findings point to the formation of ice XI at temperatures greater than 72 K via UV irradiation of ice I crystals in the Solar System; icy grains and the surfaces of icy satellites in the Jovian and Saturnian regions.

Proton Transport and Related Chemical Processes of Ice

Excess protons play a key role in the chemical reactions of ice because of their exceptional mobility, even when the diffusion of atoms and molecules is suppressed in ice at low temperatures. This article reviews the current state of knowledge on the properties of excess protons in ice, with a focus on the involvement of protons in chemical reactions. The mechanism of efficient proton transport in ice, which involves a proton-hopping relay along the hydrogen-bond ice network and the reorientation of water, is discussed and compared with the inefficient transport of hydroxide in ice. Distinctly different properties of protons residing in the ice interior and on the ice surface are emphasized. Recent observations of the spontaneous occurrence of reactions in ice at low temperatures, which include the dissociation of protic acids and the hydrolysis of acidic oxides, are discussed with regard to the kinetic and thermodynamic effects of mobile protons on the promotion of unique chemical processes of ice.

Some fundamental properties and reactions of ice surfaces at low temperatures

Ice surfaces offer a unique chemical environment in which reactions occur quite differently from those in liquid water or gas phases. In this article, we examine the basic properties of ice surfaces below the surface premelting temperature and discuss some of the recent investigations carried out on reactions at the ice surfaces. The static and dynamic properties of an ice surface as a reaction medium, such as its structure, molecule diffusion and proton transfer dynamics, and the surface preference of hydronium and hydroxide ions, are discussed in relation to the reactivity of the surface.