AVS 50th International Symposium
    Surface Science Tuesday Sessions
       Session SS1-TuA

Paper SS1-TuA3
Absence of Surface Water Crystallization

Tuesday, November 4, 2003, 2:40 pm, Room 326

Session: Water at Interfaces II: Adsorbed Layers
Presenter: A.W. Kleyn, Leiden University, The Netherlands
Authors: E. Backus, Leiden University, The Netherlands
M. Grecea, Leiden University, The Netherlands
A.W. Kleyn, Leiden University, The Netherlands
M. Bonn, Leiden University, The Netherlands
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For the transition from amorphous solid water to crystalline ice, it has been proposed that the required hydrogen bond rearrangement may occur more readily for the less coordinated water molecules at the surface so that the amorphous-crystalline transition would be expected to nucleate at the solid-air interface. By simultaneously monitoring the phase state of the bulk and the surface passing through this transition, we demonstrate conclusively that crystallization commences in the bulk. In our experiments, water is deposited on a low-temperature Pt substrate by means of a molecular beam in Ultra High Vacuum (UHV), on which it forms a layer of amorphous solid water. By slowly heating the substrate, the water crystallizes. Using RAIRS (reflectance absorption infrared spectroscopy), we can follow the crystallization kinetics real-time, since the RAIRS spectra for crystalline and amorphous ice are very different. Hence by measuring RAIRS spectra while we are heating the crystal, we can deduce the spatially averaged, i.e. bulk fraction of crystalline ice in the water layer throughout the crystallization process. We can also determine surface fraction of crystalline ice in the water layer by monitoring chloroform desorption from the (partially crystallized) ice layer: the desorption of chloroform occurs at temperatures well below the water phase transition, and the desorption temperature from amorphous solid water and crystalline ice differ by 15 K. Therefore chloroform is a good probe for study the crystallinity of the surface of the water layer. In this manner we can correlate surface to bulk crystallinity. We find that the surface crystallinity is always lagging compared to the bulk. We can therefore unequivocally conclude that the crystallization of water nucleates in the bulk and not on the substrate-water interface or on the water-vacuum interface. We further find that initiation of the crystallization process (the nucleation grain) involves ~100 water molecules.