Photoactive bands

In addition to the above mentioned basic requirements, there are a number of secondary factors upon which the success of a particular photoinitiated polymerization may depend. For example, the photoinitiator must possess absorption bands which overlap as nearly as possible the emission bands of the light source employed. It is important, therefore, to consider the emission spectra of various available light sources prior to undertaking the synthesis of a photoinitiator. Judicious placement of chromophors must be made. The introduction of a chromophor at positions remote to the site of the photolysis may not always result in the production of the desired photoactive bands due to inefficient transfer of energy within the molecule. One must consider the absorption characteristics of the photoproducts which result from irradiation of the photoinitiator. In some cases, the absorption of these compounds may overlap that of the photoinitiator and reduce its efficiency. Similarly, the monomers employed in the polymerizations should be as transparent as possible in the region of the spectrum absorbed by the photoinitiator. As a matter of practicality, one must also consider the solubihty of the photoinitiator in any given monomer system and even perhaps incorporate groups into its structure which enhance its... 

No structural studies have been reported on these complexes, but detailed study of their vibrational spectra permits the assignments shown in Table 2.13. Like the rhodium analogues, iridium ammines are photoactive therefore, on excitation of ligand-field bands, solutions of [Ir(NH3)6]3+ or [Ir(NH3)5Cl]+ afford [Ir(NH3)5(H20)]3+. 

To make a breakthrough in household appliances and other consumer product markets UV sensors have to become significantly cheaper while spectral selectivity as a major key feature must be guaranteed. Most of today s UV photodiodes are made from crystalline semiconductor materials. The cheaper materials (Si) lack spectral selectivity, and the wide band gap materials are very expensive. What they all have in common their top performance regarding sensitivity and speed. Crystalline photodiodes have risetimes of often below 1 s. However, the described processes to be sensed here are not faster than some milliseconds or even much slower. In order to obtain a reasonably-priced SiC or GaN photodiode, the photoactive area is often reduced to below 1 mm2 and barely fills the sensor housing. So far, the top sensitivity offered by the semiconductor has been sacrificed for a competitive... 

Of the photocatalytic systems and structures composed of a single active material, eventually coupled with redox catalysts and/or metals, only a wide band gap oxide semiconductor, like Pt/Ti02, requiring UV irradiation, showed some photoactivity for water photosplitting. Water splitting with visible light requires the irradiation of multiple band gap photoelectrochemical cells (PEC) or Z-scheme systems (like the photosynthesis system of plants etc.). 

The major conclusion of the present study, as can be seen in Figures 1 and 2, is that the primary photodissociating states, in both nitrosyl ferrous and ferric heme complexes correspond to the d - d 2 excitations. The calculated energies also indicate that this dissociative channel can be activated independent of the excitation frequency in the range of Q to Soret band energies. This is the same type of excitation previously identified as the photoactive state involved in CO and O2 photodissociation from ferrous heme complexes. 

A number of works are devoted to the electrochemical preparation of ZnO, which may have application in photocatalysis, ceramics, piezoelectric transducers, chemical sensors, photovoltaics, and others. ZnO has the same band-gap energy as Ti02, and the oxygenation capacities for both compounds should be similar. Ya-maguchi et al. [155] prepared photoactive zinc oxide films by anodizing a zinc plate. Such films could decompose gaseous acetaldehyde with the aid of black lights. 

Jeveral aspects of the photolytic behavior of aqueous complex ions have been studied in this laboratory over the past few years. One continually interesting question has been the extent to which the photochemistry of a complex depends on the absorption band irradiated. In the case of Co(III) acidopentamines, such as Co(NH3)5Br+2, we found that irradiation of Ajg —> g) bands showing appreciable charge transfer led to redox and aquation reactions which were competitive. It was reasonable to suppose that the common precursor was the species formed by a prompt heterolytic bond fission (7). The ( Aig —> Tig) band was far less photoactive, and in model cases, irradiation led only to aquation. Each excited state or excited state manifold thus tended to show a distinct photochemistry, which meant that conversion from one excited state to another was not important. 

Fig. 3. Like a photoelectrochemical cell, such a powder includes sites for photo-induced oxidation and reduction, but no external current flow accompanies these transformations. Photoactivity is also maintained as the size of the particle decreases to the colloidal range although the absorption characteristics, the quantum efficiency of charge separation, and the kinetics of interfacial electron transfer may be influenced by the particle size. On sufficiently small particles, for example, the calculated space-charge width necessary for effective band bending may exceed the dimensions of the particle.

Both 4T2g and 2tg states are photoactive. The ratio of 0nh / ncs- is wavelength dependent, the ratios being t5 at 373 nm (quartet band) 22 at... 

However, many ternary systems incorporate a second photoactive center in addition to the [FeO ] octahedra in the present case, [NbO ] octahedra. The interaction between such multiple centers has not previously been investigated. In the present work, interband transitions are observed which appear characteristic of niobium centers, together with other transitions characteristic of the iron centers. Since these are homogeneous, singlephase materials, this result suggests that caution should be exercised when applying the conventional band model to such oxide semiconductors. 

For materials with a single photoactive center, it is generally observed that the optical band gap and flat-band potential are interrelated, so that lower band gaps appear to be accompanied by more positive flat-band potentials. Nevertheless, the... 

Consequently, if multiple photoactive centers can maintain sufficiently independent existence in a single compound, it would be conceivable that significant deviations from the usual correlation of high flat-band potentials with low band-gap energies might occur. 

Before proceeding to ternary oxides with multiple photoactive centers, the effects of composition and structure upon such photo-electronic properties as optical band gap and flat-band potential for a given active center should be considered. It will be seen that composition appears to primarily affect the flat-band potential, whereas the band gap is more sensitive to structure. 

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