We report a high-resolution core-level spectroscopy study of the (4X2)-c(8x2) GaAs (100) surface prepared by thermal decapping of surfaces grown by molecular-beam epitaxy. Ga and As 3d core levels are analyzed using very strict parameter conditions, and compared with core levels taken from cleaved GaAs(110). Two surface components are resolved in the Ga spectra while only one surface component is necessary for As. We tentatively assign the two Ga surface components to inequivalent Ga dimers in the unit cell; the As surface component is assigned to the threefold-coordinated second-layer As next to the missing row of Ga dimers. The position of the Fermi level on decapped n-type and p-type substrates is investigated in detail. The characterization of these clean decapped (100) surfaces provides a firm basis for further studies of electrical and chemical properties of metal-semiconductor interfaces.

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The photoelectrochemical behavior of electrochemically etched Si cathodes contacting a nonaqueous redox electrolyte is described. The electrochemically etched Si electrodes exhibit higher energy conversion efficiencies than Si electrodes chemically treated in HF solution. The porous Si layer formed during electrochemical etching plays an important role in suppressing surface recombination and stabilizing the Si electrodes against photocorrosion.

A simple, quantitative relationship between the semiconductor surface recombination velocity (Sr) and the open-circuit photovoltage (Voc) of photoelectrochemical systems was derived and verified experimentally. Experimental results obtained for the n-Si/acetone-FeCp~/~ junction indicate that Voo is controlled by surface recombination. Quantitative analysis of the results using the derived expression yields values of the barrier height and Sr that compare favorably with published data. Application of the equation to other semiconductor/liquid junction cells suggests that the expression may be important in evaluating and understanding the behavior of a wide range of photoelectrochemical systems.

The mean flow field in a tank stirred with a pitched blade turbine was measured using a two-component Laser Doppler Anemometer system (LDA). The effects of impeller clearances and impeller geometries (number of blades, blade angle and blade size) on the mean flow field have been studied. The primary pumping number, induced pumping number of the primary circulation loop and the induced pumping number of the secondary circulation loop, which often has been ignored, are reported. The flow patterns and circulation loops are more complex than those traditional ones, which vary with the geometries of the PTD and the clearances.On a mesure le champ d'ecoulement moyen dans un reservoir agite muni d'une turbine a pales inclinees a I'aide d'un anenometre laser Doppler bicomposantes. Les effets du degagement de la turbine par rapport aux parois et de la geometrie de la turbine (nombre, angle et dimension des pales) sur le champ d'ecoulement moyen ont ete etudies. Le nombre de pompage primaire, le nombre de pompage induit de la boucle de circulation primaire ainsi que le nombre de pompage induit de la boucle de circulation secondaire, qui sont souvent ignores, sont presentes ici. Les profils d'ecoulement et les boucles de circulation sont plus complexes que ceux des cas classiques, qui varient selon la geometrie de la turbine a pales inclinees (TPI) et les degagements par rapport aux parois.Keywords: stirred tank reactors, PTD, flow pattern, pumping number, Laser Doppler Anemometry.echanically agitated reactors are very versatile and M are commonly used in practically all the small and large scale chemical process industries. The stirred tank is almost our 'tool-of-all-work' (Villermaux, 1993), and there are several functions of mechanical agitation such as solid suspension, gas-liquid dispersion, liqui&liquid dispersion, gas-liquid mass transfer, solid-liquid mass transfer and liquidliquid mass transfer. They are achieved by introducing mechanical energy into the tank with the aid of a rotating impeller and converting this energy into hydrodynamic motion. Therefore, detailed understanding of the hydrodynamics of the tanks (velocities and stress fields, turbulence characteristics, etc.) is essential for the confident design of stirred tanks. The characteristics of the flow strongly depend on the types and geometries of the impellers. Recently, pitched blade downflow type turbines (PTD) have been found to be more efficient with respect to liquid phase mixing and solid suspension than other impellers (Pandit and Joshi, 1983;Raghav Rao et al., 1988). The aim of this work is to study the three dimensional, circulating turbulent flow field formed by an impeller in order to be able to predict the flow field formed by a given impeller in the new geometric configuration, and to be able to characterize the important features of the flow field in all parts of the tank. Previous workMost of the available data about flow in stirred tanks is for the standard disc turbine; information for pitched blade turbines has only appeared recently ...