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The deposition and the isothermal crystallization kinetics of thin amorphous solid water (ASW) films on both Ru(0001) and CO-precovered Ru(0001) have been investigated by real-time studies employing helium atom scattering, infrared reflection absorption spectroscopy and isothermal temperature-programmed desorption, simultaneously. During ASW deposition, the interaction between water and the substrate exhibits a significant difference depending on the amount of pre-adsorbed CO. However, the mechanism and kinetics of the ASW (~50 layer films) crystallization were found to be independent of the amount of pre-adsorbed CO. We conclusively demonstrate that crystallization mechanism consists of random nucleation events in the bulk of the material, followed by homogeneous growth, for both substrates. The morphological change accompanying the formation of the 3D-grain crystalline ice has been found to results in the exposure of the water monolayer just above the substrate to the vacuum during the crystallization process on both substrates.
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