Charge recombination electric effect

The Mataga-Kakitani (M-K) theory is based on the rather general observation that e.t. processes which show the M.I.R. are mostly charge recombinations and charge shifts, whereas the photo-induced charge separations which start from neutral reactants follow Rehm-Weller behaviour. It is then suggested that the difference is due to the electric field which acts on the solvent in the field of ions or ion pairs, partial dielectric saturation of polar solvents would be reached, and this would restrict solvent motion. No such dielectric saturation effect would exist in the solvent shell of neutral reactants, so that solvent motion remains unhindered. 

Fig. 7 Effect of membrane electrical potential on the photocurrent under polychromatic visible light excitation and rate constant for the charge recombination reaction, observed during the sensitization of Ti02 membrane films by RUL3 complex.

G. Feher, T. R. Amo, and M. Y. Okamura The effect of an electric field on the charge recombination rate of D Qa DQa in reaction centers from Rhodobacter sphaeroides R-26, in "The Photosynthetic Bacterial Reaction Center" J. Breton and A. Vermeglio, eds.. Plenum, New York, 271 (1988). 

Y. Blatt, A. Gopher, M. Montal, and G. Feher, Photovoltages from reaction centers incorporated in interfacial lipid layers, Biophys. J. 41 A121 (1983) A. Gopher, Y. Blatt, M. Schonfeld, M.Y. Okamura, G. Feher, and M. Montal, The effect of an applied electric field on the rate of charge recombination in reaction centers incorporated in planar lipid bilayers, Biophys. J. 48 311 (1985). 

Semiconductor - Electrolyte Interlace The electric field in the space charge region that may develop at the semiconductor electrolyte interface can help to separate photogenerated e /h 1 couples, effectively suppressing recombination. When a semiconductor is brought into contact with an electrolyte, the electrochemical potential of the semiconductor (corresponding to the Fermi level, Ey of the solid [50]) and of the redox couple (A/A ) in solution equilibrate. When an n-type semiconductor is considered, before contact the Ey of the solid is in the band gap, near the conduction band edge. After contact and equilibration the Ey will... 

Light absorption causes formation of an electron/hole (e h ) pair in the interfacial region of the solid and, in the presence of an electric field (e. g. when the solid is held in an electrolyte), the electrons migrate inwards towards the bulk of the solid and the holes move towards the surface and react with the FeOH groups, i.e. the charges separate. The surface reaction is, Fe-OH + hye Fe(OH)s where s = surface and hvB is a hole. A feature of the iron oxides is electron/hole pair recombination - many electrons recombine with the holes and are neutralized - which decreases the photo-activity of the solid. The extent of recombination depends to some extent on the pH of the solution and its effect on the proportion of FeOH groups at the surface (see Chap. 10 and Zhang et al., 1993). 

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