![]() ![]() ![]() It allows for a controlled layer-by-layer deposition of ordered molecular films and has been used to create model biological membranes for studies under controlled conditions. On the way to biological complexity of membrane-type structures, we decided to first investigate a bilayer of fatty acids deposited on a hydrophobic surface substrate, invoking the well known Langmuir-Blodgett (LB) technique. ![]() The issues of interest are the coexistence of these structures, their different dynamics, and the time scales for energy transfer and disruption of the hydrogen bond network. We note that for the hydrophilic substrate studied, water adjacent to the surface is mostly crystalline, while that away from the surface is polycrystalline. The change in the structure of I c has different temporal behaviors, reflecting the distinct difference in the transfer of energy to polycrystalline and crystallite I c. The interface is dominated by polycrystalline I c, but coexisting in this phase are crystallite structures, not adjacent to the surface of the substrate. The structural dynamics are also different. On the hydrophobic surface, the structure is still cubic but very different in order. On the hydrophilic surface substrate, the structure is cubic (I c), not hexagonal (I h), and structural dynamics are distinctive. The temporal evolution of interfacial water and layered ice after the temperature jump was studied with monolayer sensitivity. We identified the interfacial and ordered (crystalline) structure from the Bragg diffraction and the layered and disordered (polycrystalline) structure from the Debye-Scherrer rings. Interfacial water was formed on a hydrophilic surface (silicon, chlorine-terminated) or hydrophobic surface (silicon, hydrogen terminated) under controlled ultrahigh vacuum (UHV) conditions. We reported using UEC the determination of the structural dynamics of interfacial water following substrate infrared temperature jump. Therefore, it is essential to elucidate the nature of these structures and the time scales for their equilibration. However, the transformation from ordered to disordered structure and their coexistence critically depends on the time scales for the movements of atoms locally and at long range. Structurally, the nature of water on a substrate is determined by forces of orientation at the interface and by the net charge density, which establishes the hydrophilic or hydrophobic character of the substrate. The directional molecular features of hydrogen bonding and the different structures possible, from amorphous to crystalline, make the interfacial collective assembly of water, on the mesoscopic scale, much less understood. Molecular assemblies were our next target, and water at interfaces is perhaps the most challenging chemical structure. The structural change is a phase transition to the liquid-like state. Because we can vary the fluence of the initiating pulse, we also studied the structural changes involved in phase transitions when the temperature of the lattice is sufficiently high to cause large amplitude disorder. Similarly, we studied surface and bulk crystals of silicon, with and without adsorbates. Structural dynamics were classified into three regimes, and we are still exploring with this system because of its richness and some unexpected results. These results were compared with those of nonthermal fs optical probing reported by Mazur's group, and the agreement for the temperature response from the fluence dependence of the dielectric function is impressive. The ‘transient temperature’ reaches its maximum value (1565 K) in 7 ps. From the change of Bragg diffraction (shift, width, and intensity), we showed the change in non-equilibrium structure as the temperature rises and the restructuring at longer times. Determination of surface structural dynamics, using frame referencing, was achieved for crystalline solids (GaAs), following the temperature rise of the crystal. Molecules were studied as adsorbates on the surface either as physisorbed or chemically functionalized entities, and thin crystals were studied in the reflection or transmission mode.įirst we studied the substrate crystals. In our UEC apparatus, which includes three interconnected UHV chambers, the crystal is mounted on a computer-controlled goniometer for high-precision (0.005°) angular rotation, and the substrate can be cooled to low temperatures and characterized with LEED and Auger spectroscopy. Zewail, in Femtochemistry VII, 2006 3.2 Ultrafast Electron Crystallography (UEC) ![]()
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