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Thermal and nonthermal physiochemical processes in nanoscale films of amorphous solid water.

Amorphous solid water (ASW) is a disordered version of ice created by vapor deposition onto a cold substrate (typically less than 130 K). It has a higher free energy than the crystalline phase of ice, and when heated above its glass transition temperature, it transforms into a metastable supercooled liquid. This unusual form of water exists on earth only in laboratories, after preparation with highly specialized equipment. It is thus fair to ask why there is any interest in studying such an esoteric material. Much of the scientific interest results from the ability to use ASW as a model system for exploring the physical and reactive properties of liquid water and aqueous solutions. ASW is also thought to be the predominant form of water in the extremely cold temperatures of many interstellar and planetary environments. In addition, ASW is a convenient model system for studying the stability of amorphous and glassy materials as well as the properties of highly porous materials. A fundamental understanding of such properties is invaluable in a diverse range of applications, including cryobiology, food science, pharmaceuticals, astrophysics, and nuclear waste storage, among others. Over the past 15 years, we have used molecular beams and surface science techniques to probe the thermal and nonthermal properties of nanoscale films of ASW. In this Account, we present a survey of our research on the properties of ASW using this approach. We use molecular beams to precisely control the deposition conditions (flux, incident energy, and incident angle) and create compositionally tailored, nanoscale films of ASW at low temperatures. To study the transport properties (viscosity and diffusivity), we heat the amorphous films above their glass transition temperature, T(g), at which they transform into deeply supercooled liquids prior to crystallization. The advantage of this approach is that at temperatures near T(g), the viscosity is approximately 15 orders of magnitude larger than that of a normal liquid. As a result, the crystallization kinetics are dramatically slowed, increasing the time available for experiments. For example, near T(g), a water molecule moves less than the distance of a single molecule on a typical laboratory time scale (∼1000 s). For this reason, nanoscale films help to probe the behavior and reactions of supercooled liquids at these low temperatures. ASW films can also be used for investigating the nonthermal reactions relevant to radiolysis.