Light absorption deterioration

Free radicals are generated at the wood surface during irradiation. The rate of free-radical formation is enhanced when moisture content increases from 0 to 6.3%. Electron spin resonance and UV light absorption studies on the behavior of generated free radicals and their interactions with oxygen molecules to form hydroperoxides have revealed that free radicals and singlet oxygen play important roles in discoloration and deterioration reactions of wood surfaces. 

The TERS strategy is not suitable for IR absorption spectroscopy (IRAS). This is because a broadband light source is used in IRAS and it is difficult to create a constant enhancement over the entire spectral range. Another problem stems from the much longer wavelength of IR compared to visible light. In other words, the far-field excitation area will be much larger than in the case of TERS, and the SNR will deteriorate. In order to overcome these problems, s-SNOM is commonly used for near-field IRAS [32]. 

Since protection of electrodes against corrosion in the photoelectrolysis cells is a question of vital importance, many attempts have been made to use protective films of different nature (metals, conductive polymers, or stable semiconductors, eg., oxides). Of these, semiconductive films are less effective since they often cause deterioration in the characteristics of the electrode to be protected (laying aside heterojunction photoelectrodes specially formed with semiconducting layers of different nature [42]). When metals are used as continuous protecting film (and not catalytical "islands" discussed above), a Schottky barrier is formed at the metal/semiconductor interface. The other interface, i.e., metal/electrolyte solution is as if connected in series to the former and is feeded with photocurrent produced in the Schottky diode upon illuminating the semiconductor (through the metal film). So, the structure under discussion is but a combination of the "solar cell" and "electrolyzer" within the photoelectrode Unfortunately, light is partly lost due to absorption by the metal film. 

This shows that the surface deterioration advances approximately in proportion to exposure time, and that the progress of deterioration depends upon the surface oxygen concentration and the rate constant of photo-oxidation, though it depends,of course, linearly on the intensity of incident light. The surface deterioration can be observed in IR absorption spectra and in the morphological change in SEM, and further in the change of material properties. 

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