Surface fast states

Many of the fiindamental physical and chemical processes at surfaces and interfaces occur on extremely fast time scales. For example, atomic and molecular motions take place on time scales as short as 100 fs, while surface electronic states may have lifetimes as short as 10 fs. With the dramatic recent advances in laser tecluiology, however, such time scales have become increasingly accessible. Surface nonlinear optics provides an attractive approach to capture such events directly in the time domain. Some examples of application of the method include probing the dynamics of melting on the time scale of phonon vibrations [82], photoisomerization of molecules [88], molecular dynamics of adsorbates [89, 90], interfacial solvent dynamics [91], transient band-flattening in semiconductors [92] and laser-induced desorption [93]. A review article discussing such time-resolved studies in metals can be found in... 

To obtain fast LC photoresponse, a new guest/host system was developed, in which ferroelectric LCs (FLCs) were used as a host LC. FLCs exhibit spontaneous polarization (Ps) and show microsecond responses to change in applied electric field (flip of polarization) in a surface-stabilized state.1261 If a flip of polarization of FLC molecules in the surface-stabilized state can be induced by light in the presence of an applied electric field, photoresponse in the microsecond time region might be achievable. 

An interesting avenue for investigation is to examine the adsorption characteristics on single crystals concurrently with electrical measurements. Thus, any relationship which possibly exists between the slow states and the chemisorption might be positively revealed. Examination of the adsorption characteristics of reduced germanium crystals and the effect of the fast states would also be of interest. These studies have been initiated. It remains clear at this time, however, that the semiconductor properties of the germanium influence the surface properties of the thin oxide films supported thereon. The influence is clear in the case of propanol adsorption and the differences are even more dramatic in the case of water adsorption. 

Furthermore, femtosecond diffuse reflectance spectroscopy with a white continuum probe pulse has been applied to detect the dynamics of hole transfer from photoexcited TiC>2 to adsorbed reactant molecules. As shown in Figure 18, at pH < 7 of the TiC>2 aqueous suspension with KSCN, ultrafast hole transfer takes place in less than 1 ps (Furube et al., 2001b). Subsequent structure stabilization of dimer anion radicals, (SCN)2, within a few picoseconds and slow hole transfer with a time constant of a few hundred picoseconds are clearly observed (Furube et al., 2001b). Fast hole transfer is caused by a surface-trapped state interacting strongly with adsorbed molecules. Slow hole transfer observed at pH values >7 is caused by deep trapped states with a Boltzmann distribution... 

It has recently been discussed [61-63] whether the diffusional barrier at the intestinal surface can be accounted for solely by an unstirred water layer. It has been proposed that the mucus layer overlying the enterocytes should be regarded as an important diffusion barrier for uptake of lipid solutes from the luminal contents. The mucus adherent to the rat duodenal wall has been found to be approximately 80 jam thick in the fasted state [64]. The intestinal mucus coat is formed by proteoglycans produced by goblet cells, but so far very little is known about the molecular structure of the mucus layer [65]. The possible interaction between mucus constituents and luminal lipid solutes needs to be investigated in detail, since it might reveal key factors which constitute the diffusional barrier of the small intestine. 

Increased uptake of glucose into muscle and adipose tissue. This is effected by recruitment to the cell surface of glucose transporters that are in intracellular vesicles in the fasting state. 

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