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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