Photopotential kinetics

Since these voltammograms were obtained in a thin layer electrochemical cell, considerable displacement of the voltammetric peaks can be seen because of the uncompensatable solution resistance. It is nevertheless apparent that in the dark both the H2Q - Q and the Q - H2Q electrolysis are suppressed, while under illumination the rate of oxidation and reduction is enhanced over that observed on the plain gold substrate. In addition, the extent of this kinetic enhancement is controlled by the flux of photons delivered to the electrode surface For this type of modified gold substrate, no apparent photopotential is observed for the quinone/hydroquinone couple. 

As in all potentiostatic techniques, the double layer charging is a parallel process to the faradaic reaction that can substantially attenuate the photocurrent signal at short-time scale (see Section 5.3)" . This element introduces another important difference between fully spectroscopic and electrochemical techniques. Commercially available optical instrumentation can currently deliver time resolution of 50 fs or less for conventional techniques such as transient absorption. On the other hand, the resistance between the two reference electrodes commonly employed in electrochemical measurements at the liquid/liquid interfaces and the interfacial double layer capacitance provide time constants of the order of hundreds of microseconds. Consequently, direct information on the rate of heterogeneous electron injection from/to the excited state is not accessible from photocurrent measurements. These techniques do allow sensitive measurements of the ratio between electron injection and decay of the excited state under pho-tostationary conditions. Other approaches such as photopotential measurements, i.e. relative changes in the Fermi levels in both phases, can provide kinetic information in the nanosecond regime. 

The kinetics of photopotential was studied by Liu and Mauzerall , who explained the decay by a rate constant that exponentially decreases with the distance between the pigment cation and ferricyanide. The other conclusion from their study is that the pigment cation does not transverse the BLM in less than 10 msec. To examine the action spectra of pigmented BLM, a technique known as photoelectrospectrometry has been developed, which is based on the photoelectric effect and optical spectroscopy . 

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