2. This topic deals with the emission of electrons from electrodes on exposure to light. When a metal electrode is illuminated the cathodic currents are stimulated. These cathodic currents are stimulated by the presence of electron scavengers.
5. If an electron scavenger is present these solvated electrons may react with it irreversibly, then the cathode current is enhanced. If no scavenger is present these electrons will return to the electrode, then the cathode current is small(since the net charge transfer from the electrode is small).
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7. Thus there is a red-limit wavelength above which photoejection is improbable.
8. Fermi level depends on electrode potential, this limit shifts to shorter wavelengths as potential becomes more positive. Hence, for a monochromatic radiation, there is a threshold potential for photoemission and the emission becomes more probable at more negative potentials.
9. Experimental section: Earlier techniques: Irradiation of a DME by chopped light from low -, medium -, and high– pressure mercury lamps. Pulse polarographic monitoring circuitry synchronized to the light chopper was used for measurement of photocurrent. The time scale of a photoemission experiment is inherently very short. The diffusion time for return of the ejected electrons is of the order of 100 ns, hence we use this technique to study study very fast electrode processes. Because we cannot realize this potential with low frequency chopping techniques, the flash illumination techniques are developed.
10. Flash illumination techniques: Light is supplied to a continuously renewed mercury pool electrode by a Q-switched, frequency –doubled ruby laser with a pulse width of ~ 15 ns . The electrode is set initially at any desired potential, whose response was very slow enough that the electrode’s reaction to the flash can be monitored as a coulostatic transient, ΔE (measured with respect to the initial potential) vs. time. The difference in charge with respect to the initial condition is straightforwardly related to ΔE by: Δq = Cd x ΔE ->where Cd is the differential capacitance of the interface ->as ΔE is only a few millivolts, Cd can be treated as a constant to a given experiment Useful information can be obtained for times as short as a few tens of ns after the flash.
11. Applications: The solution chemistry that can be studied by photoemission is largely related to solvated electrons.(This is same as in radiolysis, although the photoemission technique avoids extensive radiolytic damage to the solution components) For electrochemistry, there is a very useful ability to generate near the electrodes, radicals, and other intermediates of electrochemical interest/importance. Purely electrochemical experiments often cannot often provide insight into electrode processes that can be found in this manner. (Ex: processes in which Hradical is involved, it is difficult to isolate the electrochemistry of H radical within the whole discharge process)
