In situ observation of a spectral sensitizing dye at a semiconductor electrode-solution interface was attempted by means of fluorescence and laser-induced photoacoustic spectroscopy. Dependences of fluorescence lifetime, fluorescence intensity, photoacoustic signal of the dye, and the sensitized photocurrent on the voltage applied to the Sn02 electrode were measured. Direct evidence that the applied voltage affected a recombination process of electrons in the Sn02 conduction band with the oxidized dye molecules at the electrode surface was obtained.

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The photoacoustic technique was applied to the detection of the transient absorption of a sensitizing dye (eosine Y) adsorbed on ZnO powder dispersed in gelatin. The transient photoacoustic spectrum, which has a maximum around 450 nm, could only be measured when the adsorbed dye on ZnO powder was irradiated with excitation light. The maximum at ca. 450 nm was in good agreement with the literature values observed in a dye solution by use of flash photolysis and pulse radiolysis, the spectrum of which can be attributed to the eosine cation radical. This result seems to give powerful support for the electron-transfer mechanism in the spectral sensitization of semiconductors. The transient ESR signal of the same sample during light exposure confirmed that the excited dye can inject an electron into the conduction band of ZnO, which subsequently reacts with oxygen to form 02".

trolyte-plus-salt solution.2 This leads indeed to a negligible value of 6, less than 1%. If, instead of (2), we propose following values, estimated from experimental data on the investigated polyelectrolyte4J k, = 0.14 kl = 0.40(3) then one obtains 6 = -0.23. Hence, we agree with Dr. Manning to claim that, for the present analysis, a departure from the Debye-Huckel approximation cannot alone explain the massive discrepancy between apparent and effective polyion valences.Another important aspect of our Discussion1 concerns the polymer concentrations used in our experiments. Let us clearly state that, in our work, both polyion transport parameters (u3 and D3) were not obtained in the same concentration regime. Self-diffusion data belong to the dilute regime and electrophoresis data to the semidilute regime (C, = 0.5%), because we were not able to attain, in electrophoretic light scattering, concentrations below Cp*, the concentration at which polymer chains start to overlap. For high-molecular-weight polyelectrolytes (M,,, k lo6), Cp* is very low, typically less or much less than 0.1% (weight percent) depending on polyion ionization and salt concentration. This has two consequences.(i) In the field of high-molecular-weight polyelectrolytes almost all published results on electrophoresis were obtained in the semidilute regime, where transport coefficients are molecular-weight independent, but become concentration dependent,'j a result which reasonably explains the different power laws we observed for the polyion friction coefficients.(ii) The experimental u3 values might be underestimated with respect to their dilute regime values, because of increased friction with the environment, if we refer to experiments performed on neutral polymer^.^ For example, with the polystyrene/toluerie system at 25 "C (M, = 110000) the polymer friction coefficient is increased by a factor of 1.5, when the concentration rises from 0 to 0.5% .'This effect probably contributes to lower u3 and thus z~~P .Finally, let us point out that the observation of low apparent valences could also be due to overestimated polyion self-diffusion coefficients. The spectrum of light scattered by charged macromolecules is indeed complex, and the first theoretical analysis of this difficult subject has been given by Stephen,@ for the model of a rigid rod. However, Stephen's theory is only valid if the Debye shielding length is much larger than the interparticle spacing, a hypothesis which does not apply under excess of salt conditions, for a dilute polymer solution. Thus, a complete theory of the charge effects on light scattering is not yet available. In any case, if quasi-elastic light scattering only measures an apparent self-diffusion coefficient of the polyion, this cannot provide the key explanation for our results, for we also observed very low 2sap values with data published by Nagasawa et ala1O9l1 who used quite different experimental techniques.At the outset of this comment we could agree with Dr. Manning in stating that there is yet no clear-cut ...