Behavior of Illuminated Semiconductors

When photons are absorbed in a metal the ensemble of electrons are excited and some of the electrons are promoted to higher energy levels. The excited state is preserved in the metal for only a short time, and the system rapidly returns to its original state. When the photon energy hv is higher than the metal s electron work function, (M,E) in the solution at a given potential, individual excited electrons can be emitted into the solution. Usually the quantum yields of this process are low, for example, around 10 , and depend on the depth of the layer in which the photons are absorbed. 

Photoemission produces dry (nonsolvated) electrons leaving the metal surface with a high initial velocity. An excess photon energy of 0.05 eV is enough to impart a velocity of about 10 m/s to the electron. Already in the first layers of solution the electrons are strongly decelerated and thermalized, that is, their kinetic energy is reduced to values typical for thermal motion at the given temperature. 

Several picoseconds after photon adsorption, the electrons end up in a localized solvated state at distances of l-5nm from the electrode surface. The electron s hydration energy in aqueous solutions is about 1.5-1.6eV. 

The further fate of the solvated electrons depends on solution composition. When the solution contains no substances with which the solvated electrons could react quickly, they diffuse back and are recaptured by the electrode. A steady state is attained after about 1 ns at this time the rate of oxidation has become equal to the rate of emission. 

Semiconductor electrodes exhibit electron photoemission into the solution, like metal electrodes, but in addition they show further photoelectrochemical effects following excitation of the electrode under illumination. In 1955, W.H. Brattain and Ch.G.B. Garrett published a paper in which they established the connection between the photoelectrochemical properties of single-crystal semiconductors and their electronic structure. 

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