Photoassisted diffusion

Localized electrons can also be considered as temporary negative ion states. The electron can leave its trap either by absorption of phonons (thermal activation) or by absorption of photons (photoassisted diffusion). The latter process requires an energy of the order of 1 eV, as can be deduced from the optical absorption spectrum of localized electrons in n-hexane. The process of photoassisted diffusion has been demonstrated experimentally by Balal et al. (Balakin et al., 1981 Balakin and Yakovlev, 1979). Electrons were produced by photoionization of anthracene in n-hexane or 2,2,4-trimethylpentane by a laser pulse of 347 nm. Subsequent illumination of the solution with a pulse of 694 nm led to a temporary increase in the photocurrent due to the liberation of trapped electrons. The process responsible for this increase can be envisaged as follows  

The wavelength dependence of the photoinduced diffusion coefficient was theoretically compared to the optical absorption spectrum of the trapped electron by Funabashi (1982). It was assumed that the absorption spectrum of the electron is due to electron-transfer transitions with a distribution of transfer distances. At constant light intensity, the photodiffusion coefficient exhibits a peak at shorter wavelengths than the absorption peak. 

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Fig. 6. Schematic illustration of a photoassisted corrosion phenomenon during the post CMP cleaning of damascene interconnections. At the electrode connected to the p-side of the junction, the metal is corroded by the oxidation reaction M - M" + n.e , while the produced soluble M"" species diffuse to the other electrode where the opposite reaction can occur.

Fig. 9. Photochemical cells used for the photoassisted electrolysis of H20. In (a) the two electrode compartments are separated by a fine glass frit to prevent diffusion of the 1 N H2SQ4 on the Pt side and 1 M NaOH on the Ti02 side. In (b) applied potentials of > 0.2 V are required to observe efficient photocurrents in the homogeneous electrolyte (from Ref.1891)...

In the case of Ti02, ranged from 0.5 to 300. Quantum yields in excess of one provide strong evidence for a free-radical chain mechanism. These quantum yields are relatively high compared to tther photoassisted semiconductor-catalyzed redox reactions thus some contribution to the overall rate may, in fact, he due to a homogeneous free-radical chain pathway with a very low concentra-lion of freely diffusing initiator radicals (i.e., SOj, SO4, OH). 

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