Equivalent surface states present

Mathematical treatments for equivalent circuit of this type for a single level and a continuum of surface state levels have been presented elsewhere (e.g., 1 0, 20, 21, 22). The admittance of the reduced circuit after removing the influence of R., . 

Fig. 3.12 Equivalent circuit diagrams for photoelectrochemical cells, (a) Extensive circuit that contains contributions from the semiconductor/back contact interface (Rcomact. Ccontact). the semiconductor bulk resistance (Rbuik). the space charge region (Rsc. Csc). surface states (Rss, Css), the Helmholtz layer (Rh. Ch), and the electrolyte resistance (Rsoi)- (h) Simplest circuit possible in which only the space charge capacitance and an overall series resistance is present, (c) Circuit that is often encountered in practice for photoelectrodes...

In practice, the electrochemical behavior of semiconductor-electrolyte interfaces is far more complex than that described above (for a good review, see Boddy [1965]). One of the complications arises because the semiconductor surface at the electrolyte-semiconductor interface is not equivalent to that in the bulk. In particular, the energy states localized at the surface for holes and/or electrons are different than those present in the bulk. These surface states may arise in several ways—for example through pretreatment (etching, polishing, etc.) of the semiconductor surface before immersion in the electrolyte. The surface states can be detrimental to the PESC efhciency if they increase the recombination of the electron-hole pairs in the semiconductor, thus reducing the number of holes (electrons for p-type material) available for chemical reaction with the redox species in solution. 

CASSCF calculations have found Jahn-Teller geometries present in the photodissociation pathways for these systems, with tetrahedral symmetry for singlet Fe (CO)4, and a Dsh trigonal bipyramid for singlet Cr(CO)s and doublet Mn(CO)5. The topology on the lower surface around the point of intersection has a number of symmetry equivalent minima separated by equivalent transition states (see Fig. 3). In the case of Cr(CO)s and Mn(CO)5 the surfaces are very similar involving three... 

The sorption of water by excipients derived from cellulose and starch has been considered by numerous workers, with at least three thermodynamic states having been identified [82]. Water may be directly and tightly bound at a 1 1 stoichiometry per anhydroglucose unit, unrestricted water having properties almost equivalent to bulk water, or water having properties intermediate between these two extremes. The water sorption characteristics of potato starch and microcrystalline cellulose have been determined, and comparison of these is found in Fig. 11. While starch freely adsorbs water at essentially all relative humidity values, microcrystalline cellulose only does so at elevated humidity values. These trends have been interpreted in terms of the degree of available cellulosic hydroxy groups on the surfaces, and as a function of the amount of amorphous material present [83]. 

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