ABSTRACT
At the present time, it is clear that the traditional PC [549551,555] also arises due to hydrogen atoms, detached under the action of light from hydrogen-containing molecules (water and organics), that have been uncontrollably adsorbed on the films during preparation or from the ambient [558,559] (the “wild” PC). Obviously, this effect is small compared with the ‘‘cultivated’’ PIH where specially selected hydrogen-donor molecules are adsorbed in great quantities on the oxide surface [557,560-585], which makes it possible to insert approximately the same amounts of hydrogen atoms as by the EC. The PC arising with the PIH is a unique phenomenon. It is not connected with intrinsic defects of solids but the defects being injected from outside, which makes it possible to achieve a recordbreaking value of color centers using the PIH. When hydrogen atoms are inserted into the TMO HxMyOz are formed, which are under intensive long-time investigations due to their various interesting properties [586-590 and references therein]. These bronzes can be prepared by various methods, and it has been reported that x can reach the maximum value of ~0.5 in WO3 and of ~2 in MoO3and V2O5 which corresponds to the concentration of the injected hydrogen atoms within the limits 1021-1022 cm-3 [24,552]. In this chapter the main basic principles and ideas important for understanding of the state of art in the PIH are considered; the details of which can be found in the references. The PIH involves various aspects, including the mechanisms of hydrogen transfer between the adsorbed molecules and the oxide surface, the nature of the drastic changes of the optical parameters in the oxide films, the factors influencing the efficiency of the PIH. The PIH is the crossing point of hydrogen science; it is related to production of atomic photochemical hydrogen, its diffusion, reactions, and behavior inside solids. For this reason it makes to elucidate the PIH here, since the issues to be discussed have much in common to those in other hydrogen processes covered above. First the mechanism of the PIH will be elucidated since the aims of ongoing investigations, besides the PC as such, involve also the production, storage, and transport of hydrogen for different functions, for example, as a fuel, dopant, or catalyst, which may find applications in science and technology, especially, in hydrogen energetics, biology, and medicine.
